a next generation light source
TRANSCRIPT
a next generation light sourcea transformative tool for energy science
Proposal for approval of Conceptual Design (CD-0)Submitted to the U.S. Department of EnergyOffice of Basic Energy Sciences
December 2010
(TOC continued)
Cover — from a concept by Greg Engel
Disclaimer
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CSO 21173-2
Table of contentsTable of contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiScientific and technical contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv1 Needs for a next generation light source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Outline of the Current Proposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Overview of revolutionary X-ray science tools at NGLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1 Multi-dimensional X-ray Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2 Ultrafast Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 Coherent Scattering and Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3 NGLS – science drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.1 Photosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.2 Fundamental Energy and Charge Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.3 Advanced Combustion Science. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.4 Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.5 Nanoscale Materials Nucleation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.6 Dynamical Nanoscale Heterogeneity in Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.7 Quantum Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563.8 Spin and Magnetization at the Nanoscale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713.9 Biological Systems: Imaging Dynamics and Function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4 New techniques enabled by NGLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914.1 Imaging structure and function in heterogeneous ensembles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914.2 X-ray Imaging: From High Resolution to High Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1014.3 Multidimensional X-ray Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
5 Proposed facility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1095.1 Capability Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
5.1.1 Requirements for the NGLS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1095.1.2 Capabilities of Present Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110
5.2 Alternate Approaches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1115.2.1 Conventional Pulsed Linacs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1125.2.2 Energy Recovery Linacs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1145.2.3 Third- and Fourth-Generation Storage Rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1145.2.4 HHG Laser Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .115
5.3 NGLS: A Transformative Tool for X-Ray Science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1155.3.1 Machine Overview and Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1155.3.2 Layout, Conventional Facilities, and Utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .118
5.4 Design Considerations and Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1195.4.1 Overview of FEL Physics and Technology Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1195.4.2 Injector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1215.4.3 Linac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1275.4.4 Beam Spreader . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1335.4.5 FEL Beamlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1345.4.6 Beam Dumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1445.4.7 Timing and Synchronization Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1455.4.8 Instrumentation and Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1465.4.9 Radiation Protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
6 Experimental systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1516.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1516.2 Overall Beamline Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1516.3 Mirror Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1536.4 Split and Delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1536.5 Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1546.6 Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
7 Future upgrades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1578 Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
8.1 Cost Estimate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1598.2 Schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1608.3 Risk Management and R&D. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1608.4 Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1618.5 Environment, Safety and Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163Appendix 1 – X-ray Interactions and Non-Disruptive Probing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163Appendix 2 – Workshops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164Appendix 3 – List of Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168
i
Scientific and technical contributors
Peter Abbamonte38
Paul Adams14
Musa Ahmed14
Caroline Ajo-Franklin14
A.P. Alivisatos14
Elke Arenholz14
Brian Austin23
William Bachalo4
Sam Bader2
Jill Banfield14
Ken Baptiste14
Ali Belkacem14
Alexis Bell14
James Berger30
Robert Bergman30
Uwe Bergmann25
Nora Berrah42
Jean-Yves Bigot12
Hendrik Bluhm14
Mike Bogan25
Axel Brunger26
Phillip Bucksbaum 25
John Byrd14
Jamie Cate30
Andrea Cavalleri7
Lorenz Cederbaum37
Henry Chapman7
Andrew Charman14
Lin Chen2
Yulin Chen14
Majed Chergui9
Yi-De Chuang14
C.L. Cocke13
Paul Corkum17
John Corlett14
Tanja Cuk14
Peter Denes14
Dan Dessau36
Thomas Devereaux25
Jim DeYoreo14
Lou DiMauro18
Larry Doolittle14
Hermann Durr25
Thomas Earnest14
Wolfgang Eberhardt10
Paul Evans29
Charles Fadley31
Roger Falcone14
Daniele Filippetto14
Peter Fischer14
Jim Floyd14
Steve Fournier14
Jonathan Frank22
Heinz Frei14
Bruce Gates31
Oliver Gessner14
Ben Gilbert14
Mary Gilles14
Steve Gourlay14
Michael Grass14
Chris Greene36
Jinghua Guo14
Joe Harkins14
M. Zahid Hasan20
Franz Himpsel29
Axel Hoffmann2
James Holton14
Malcolm Howells14
Greg Hura14
Nils Huse14
Zahid Hussain14
Enrique Iglesia14
Richard Jared14
Peter Johnson5
Chris Jozwiak14
Robert Kaindl14
Chi-Chang Kao25
Cheryl Kerfeld14
Steve Kevan40
Janos Kirz14
Chris Kliewer22
Alessandra Lanzara14
Wei-sheng Lee26
Dung-Hai Lee14
Steve Leone14
Nate Lewis6
Derun Li14
Mark Linne8
Zhi Liu14
Robert Lucht21
Jon Marangos11
Todd Martinez25
C.W. McCurdy14
Keith Moffat35
Joel Moore14
Shaul Mukamel32
Keith Nelson15
Anders Nilsson25
Dan Nocera15
Joe Orenstein14
David Osborn22
Abbas Ourmazd29
Howard Padmore14
C. Papadopoulos14
Chris Pappas14
Fulvio Parmigiani24
Claudio Pellegrini33
Gregg Penn14
Massimo Placidi14
Soren Prestemon14
Donald Prosnitz14
Ji Quiang14
R. Ramesh14
Theo Rasing28
Alex Ratti14
Kenneth Raymond30
Doug Rees6
Matthias Reinsch14
Eli Rotenberg14
Sujoy Roy14
Dilano Saldin29
Annette Salmeen14
Miquel Salmeron14
Fernando Sannibale14
Robin Santra7
Ross Schlueter14
Robert Schoenlein14
Andreas Scholl14
Andrew Sessler14
Zhi-Xun Shen25
Oleg Shpyrko34
Volker Sick39
Steve Singer14
Gabor Somorjai30
John Spence3
John Staples14
Jo Stohr25
Albert Stolow17
Craig Taatjes22
John Tainer14
Lou Terminello19
Neil Thomson23
Joachim Ullrich16
Marco Venturini14
Angela Violi39
Marc Vrakking1
Hai Wang41
Glenn Waychunas14
Russell Wells14
Russell Wilcox14
Kevin Wilson14
L. Andrew Wray14
Jonathan Wurtele14
Wilfred Wurth27
Vittal Yachandra14
Peidong Yang30
Junko Yano14
Linda Young2
A.A. Zholents2
Shuyun Zhou14
Max Zolotorev14
Peter Zwart14
ii
1AMOLF2Argonne National Laboratory3Arizona State University4ARTIUM Tech5Brookhaven National Laboratory6California Institute of Technology7CFEL DESY8Chalmers University9EPF Lausanne 10Helmholtz-Zentrum Berlin11Imperial College London12IPCM, Strasbourg13Kansas State University 14Lawrence Berkeley National Laboratory
15Massachusetts Institute of Technology16Max-Planck-Institut für Kernphysik 17National Research Council of Canada18Ohio State University,19Pacific Northwest National Laboratory20Princeton University ,21Purdue University,22Sandia National Laboratories,23Science and Technology Facilities Council, UK24Sinchrotrone Trieste25SLAC National Accelerator Laboratory26Stanford University27University of Hamburg28University of Radboud
29University of Wisconsin30University of California, Berkeley31University of California, Davis32University of California, Irvine33University of California, Los Angeles34University of California, San Diego35University of Chicago36University of Colorado37University of Heidelberg38University of Illinois39University of Michigan40University of Oregon41University of Southern California42Western Michigan University
1 Needs for a next generation light source
TheNextGenerationLightSource(NGLS)willbea
transformativetoolforenergyscience.Thishighrepeti-
tionrate,highbrightnessX-raylaserwillenablecinematic
imagingofdynamics,determinationofthestructureof
heterogeneoussystems,anddevelopmentofnovelnon-
linearX-rayspectroscopies.Theseuniquecapabilitieswill
leadtoanewunderstandingofhowelectronicandnucle-
armotionsinmoleculesandsolidsarecoupled,andhow
functionalsystemsperformandevolveinsitu.
NGLSwilldramaticallyimpactawiderangeofenergy
applications:fromnaturalandartificialphotosynthesis,
tocatalysts,batteries,superconductors,carbonseques-
tration, and biofuels. Solving the complex long-term
energy challenges facing the nation, and the world,
is the subject of a wide-ranging set of reports
produced by the scientific community together
with DOE’s Office of Basic Energy Sciences (BES)
(http://www .er .doe .gov/bes/reports/list .html) .These
reportshighlighttheurgentneedfordeeperunderstand-
ingofthebasicscienceunderpinningenergytechnolo-
giesinordertoensureasafeandsecureenergyfuture.
TheNGLS—withitscombinationofhighaveragepower,
ultrashortpulsesandcoherence— isa revolutionary
observational tool thatwillbridge thecriticalgaps in
ourunderstanding.
SincetheirfirstdiscoverybyRoentgen,X-rayshave
beenexploitedbyscientiststoanswerfundamentalques-
tionsaboutmoleculesandmaterials.Assourcesevolved
from X-ray tubes to synchrotron storage rings, three
broadclassesofX-rayexperimentshaveemerged:imag-
ing,structuraldetermination,andspectroscopy.NGLS
willtransformallthreeofthesetechniques,allowingusto
observe,inwaysneverbeforepossible,hownaturaland
artificialsystemsfunction—onmultipletimescalesand
downtonano-spatialscales.
Overthepast40years,DOE’slightsourcefacilities—
electron-storage-ring-basedX-raysynchrotrons,operated
acrossthenationbyBES—haveprovidedresourcesfor
ten thousand scientists annually, from universities,
nationallabs,andindustry.Researchershavereliedon
these facilities to answer fundamental questions in
diversefieldsofscience,andaddresscriticaltechnology
problemsinareasincludinghumanhealth,electronics
and informationprocessing,andenergy.Synchrotron
X-raylightsourceshaveenabledscientiststounravelthe
structuresofbiologicalmacromolecules,essentialforthe
designofnewdrugs;theyhaverevealedthepropertiesof
electronicmaterialsfordevicesthatunderlietheinforma-
tiontechnologyrevolution;andtheyhaveprovidedthe
firstglimpseofhowenergyconversionsystemsworkat
theatomiclevel.Whiletheseadvanceshavebeendra-
matic,thereismuchmoretolearn,andthearrayofX-ray
lasersatNGLSwillprovideafoundationformajorscien-
tificadvancesinthe21stcentury.
DOEhasbuiltuponits40-yearlegacyofX-raylight
sources,continuouslyupgradingexistingsynchrotron
facilitiestokeepthematthefrontier.Recently,aremark-
ablenewtool,theworld’sfirsthardX-raylaser—the
LinacCoherentLightSource(LCLS)attheSLACNational
AcceleratorLaboratory—hasstartedoperations.Ithas
exceededexpectations in termsofperformance,and
hascrackedopenthedoortotheX-raylaserera.While
earlyexperiments from theLCLSare illustrating the
promiseofX-raylasers,andestablishingastronguser
communityforthem,itisalsoalreadyclearthatanext
generationX-raylaserwillbeneededtorealizethefull
potentialofthisnewtool.Anextgenerationsource,built
usingamodernsuperconductinglinearaccelerator,and
takingadvantageofthelatestlaserseedingtechnolo-
gies,willhavethehighrepetitionrateandhighaverage
2
1 . NEEDS FOR A NEXT GENERATION LIGHTSOURCE
• tunabilityandpolarizationcontrol
• multicolorX-raypump-probeexperiments
• synchronizationtosub-femtosecondtimescales
• moderatepeakpower,highaveragepower,andthus
highpulserepetitionrate
To maintain global leadership in X-ray discovery
science — and the technologies enabled by those
discoveries— theUSmust remainat the frontierof
X-raylightsources.TheNGLSdesignisuniqueinbeing
abletomeettheseneeds.
1.1 OutlineoftheCurrentProposalInSection2,weprovideanoverviewoftherevolu-
tionarycapabilitiesofanextgenerationlightsource—
anX-raylaserthatproducesatrainofultrashortpulses
athighrepetitionrateandunprecedentedcoherentpower.
The capabilities we envision for the NGLS can be
viewedwithinasetofthreeoverarchingthemes:
• Multidimensional spectroscopy: Thisthemerefersto
aclassofmeasurementcapabilitiesthatincorporate
atime-orderedsequenceofpulsestoprepareand
probeevolvingcorrelatedstatesofsolid,liquid,and
gas-phasesystems.Thesetechniquesallowtheiden-
tificationofdynamic,chemicallyspecificinformation,
e.g.,ontheflowofenergyandcharge.Includedin
thisthemeareexperimentsthatutilizethehighpeak
andaveragepoweroftheNGLSX-raypulsesfornon-
lineartechniques.Also,thecoherenceoftheNGLS,
byitsnarrowbandwidthcapabilitiesinlong-pulse
operation,willallowunprecedentedhigh-resolution
spectroscopy,tounderstandimportantlow-energy
modesoffunctionalmaterials.
• Ultrafast dynamics:This theme refers to a class
ofcapabilities thatwillallowthemeasurementof
processes on timescales extending from those
of chemical reactions that might take seconds to
complete,downto the fundamental timescalesof
electron correlation that determine the behavior
ofpairsofelectronsinmaterialslikesuperconduc-
tors.ThenewestexistingX-raysourcescanprobesys-
tems on picosecond (10-12 second) or potentially
femtosecond(10-15second)timescales,whicharerel-
evanttochemicalreactions,asdeterminedbythe
ratiooftypicalatomicspacingstoatomicvelocities.
The NGLS will extend this capability to systems
evolving on the hundred-attosecond timescale
coherentpowerneededtogobeyondtheinitialstageof
X-raylasers,andenablescientiststoanswerfundamen-
talquestionsinawiderangeofdisciplines.Theadventof
X-raylasershasledtohundredsofscientistspublishing
importantworkfromLCLSandothersources,interna-
tionally.Thishasfocusedglobal interest,andset the
stageforthenextgeneration.
Thenecessityfornewobservationaltoolshasbeen
citedinseveralBESreports:
• Directing Matter and Energy: Five Challenges for
Science and the Imagination (2007) noted that
answering thecallof thegrandchallengeswould
necessitate“athree-foldattack:newapproachesto
trainingandfunding,development of instruments
more precise and flexible than those used up to now
for observational science,andcreationofnewtheories
andconceptsbeyondthosewecurrentlypossess.”
• New Science for a Secure and Sustainable Energy
Future(2008)describedacomprehensivesetofscien-
tificresearchthemes,andidentifiednewimplementa-
tionstrategiesandtoolsrequiredtoaccomplishthe
sciencedescribedinthetenBESBasicResearchNeeds
WorkshopsandintheGrandChallengesReport.These
included“…characterization tools probing the ultra-
fast and the ultrasmall…,”andthedevelopmentof
advancedtheoryandsimulationsforwhichexperi-
mentswouldprovidecriticalvalidation.
• Next-Generation Photon Sources for Grand
Challenges in Science and Energy(2008)identified
connections between new research opportunities
andthecapabilitiesofnextgenerationoflightsources,
with emphasis on energy-related research.
Itnoted that“…femtosecond time resolution and
high peak brilliance are required for following chemi-
cal reactions in real time, but lower peak brilliance
and high repetition rate are needed to avoid radiation
damage in high-resolution spatial imaging . . .”
The futureneedsof thescientificand technological
communitythatutilizesX-raylightcannotbemetsolelyby
upgradingexistinglightsources—asimportantasthose
sourceswillcontinuetobeoverthenextdecade.Scientific
andtechnologicalchallengesnowrequirenewcoherent
X-raysources—X-raylasers—tomeettherequirements
ofthemostincisiveexperiments.AfutureX-raylaserfacil-
itymustincorporatetechnologythatallows:
• simultaneousoperationofmultipleexperiments
• abroadrangeoftemporalandspectralproperties
3
1 . NEEDS FOR A NEXT GENERATION LIGHTSOURCE
• Spin and magnetism at the nanoscale,tounderstand
thefundamentalmechanismsofspinandmagne-
tism,andtodeterminetheultimatespeedandperfor-
manceofmagneticsystems
• Biological systems: imaging dynamics and function,
utilizingnovelmethods in the rapidlydeveloping
fieldofcoherentimagingofbiologicalsystemsunder
physiologicallyrelevantconditions
InSection4,werelatethesecapabilitiesandscientific
driverswithasummaryoftherevolutionarytechniques
thatwillbeenabledbyNGLS:
• Cinematic3Dchemicalimagingatthehighestspatial
resolution
• Imaging structureand function inheterogeneous
ensembles
• Multidimensional X-ray spectroscopy in rapidly
evolvingsystems
InSections5-6,therequirementsforanextgeneration
lightsourcearederivedfromthescientificneedsoutlined
inSection3,fromDOEworkshopsandresultingreports,
andfromLBNLworkshopsandreports.Wethendescribe
theproposedfacility,whichrespondstotheserequire-
ments,andcompareitwithothersources.
Thescientificchallengesdescribedcallforcapabilities
beyond those foundatanyexistingorplannedX-ray
source.Theyinclude:
• Higheraveragepowerwithanevenly-spaced,high-
repetition-ratetrainofcoherentpulses(torevealsub-
tleeffectsinawiderangeofcomplexmaterials)
• Shorterpulsedurations(toprobetherelevanttime-
scalesofphysical,chemical,andbiologicalfunction)
• Narrower bandwidths (to examine the important,
lowest-energymodesofcomplexsystems)
Addressingabroadrangeofscientificapplications,
andservingalargescientificcommunityrequiresmulti-
ple instruments—operatingsimultaneously—with
flexible means of delivering X-rays tailored to each
instrument and experiment. Synchronization of the
X-raypulseswithadditionalsources(THz,IR,oroptical),
aswellaslongitudinal(temporalphase)andtransverse
coherence,tunability,polarizationcontrol,andstability
areallneeded.
NGLSwillmeettheseneeds.
NGLSisamultiple-beamX-raylaser.Itutilizesahigh-
current(upto1mA)superconductingelectronaccelera-
tor (nominally 1.8 GeV energy) to produce a train of
(1attosecond=10-18second),theatomictimescale
determinedbytheratioofelectronorbitsizetoelec-
tronvelocity.
• Coherent scattering and imaging:Thisthemecap-
turestheabilityoftheNGLStorevealstructureand
dynamicsatthenanoscale,througheithercoherent
X-rayscatteringordiffractiveimaging.Thehighrep-
etitionrateandhighaveragepowerofNGLSwillnot
onlyallowimagingofthestructureofsystemswith
long-rangeorderorhomogeneoussamples,butits
highpulserepetitionrate,whencoupledwithhigh-
speedreadoutdetectorsandadvancedcomputational
techniques,opensthepossibilityofacquiringand
processingbillionsofimages,inordertounderstand
heterogeneousand/orfluctuatingmicroscopicsys-
tems (e.g., evolving nanoscale catalytic particles
underfunctionalconditionsorchangingproteincon-
formationsintheirnativeenvironment).
InSection3,wedescribethescientificdriversfora
nextgenerationlightsource.Wedetailaprospectiveset
ofninescientificchallengesforwhichNGLSwillsingu-
larlyaddresscriticalknowledgegaps:
• Photosynthesis,tounderstandallofthestepsofnat-
ural photosynthetic processes, and to guide the
designofartificialdevicesforconvertingsolarenergy
tofuel
• Fundamental charge dynamics,todevelopanewlan-
guagetoaccuratelydescribeandpredictchargeand
energytransferinmolecularsystems
• Advanced combustion science,tounderstandspatial-
ly,chemically,andtemporallydependentphenomena
inawidevarietyofburningfuels,inordertooptimize
combustionefficiencyandtovalidatecomputational
modelsofcombustion
• Improved catalysis,toenhanceefficiencyandselec-
tivitybyinvestigatingin-situprocessesoffunctioning
catalyticsystemsonmultipletimeandlengthscales
• Nanoscale materials nucleation, to observe and
controlthekineticsofnano-materialformationand
self-assembly
• Dynamical nanoscale heterogeneity in materials,to
understandspontaneousfluctuationsspanningmulti-
pletimeandlengthscales,theevolutionofnanoscale
morphology,andtheirrelationshiptotheproperties
andfunctionalityofcomplexmaterials
• Quantum materials,todirectlyprobethenatureof
correlatedelectronsystems
4
1 . NEEDS FOR A NEXT GENERATION LIGHTSOURCE
InSection7webrieflydescribehow the facility is
designedtobeupgradable:expandingcapacitybyadd-
ingadditional simultaneouslyoperating free-electron
lasers(FELs),andexpandingcapabilitybyextendingthe
energyrangetobothlower(100eV)andhigher(10keV)
photonenergies.
Section8providesaproposedNGLSmanagement
structure,cost,andtimeline.
IntheAppendicesweprovide:(1)ashortdescription
ofthepotentialforperturbationofsamplesbytheX-ray
pulses,andtherationaleforlimitingthenumberofpho-
tonsperpulse;and(2)alistofrelevantworkshopsheld
atLawrenceBerkeleyNationalLaboratory(LBNL).
electronbunches(at1MHzandultimatelysignificantly
higher repetitionrates),whicharesequentially fed to
multipleundulators,whichinturndeliverindependent,
simultaneousX-raylaserbeamsintoend-stationinstru-
mentsformultipleusers.
Eachexperimentalend-stationinstrumentattheNGLS
facilitywillreceiveabeamofX-raypulseswithhighrepeti-
tionrate(typically100kHzormore).Initially,theX-raypho-
tonenergyrangewillextendfrom280eVto1200eV,and
thepulsedurationfrom250asto250fs,withpulseshav-
ingbetween108and1012photons.Harmonicsoftheundu-
latoroutputwillproducephotonenergiesextendingto
3keVandabove,albeitwithfewerphotonsperpulse.
2 Overview of revolutionary X-ray science tools at NGLS
Thescienceprogramat theNextGenerationLight
SourcewillbebasedonX-raymeasurementtoolswith
spatial, temporal, and energy resolution that are far
beyondwhatcanbeachievedwithpresentsources.Most
importantly,thisnewsciencewillexploitentirelynew
X-raymeasurementcapabilitiesandapproachesthatare
qualitativelydifferentfromanythingavailablefromcur-
rentX-raysources,orfromanyotherX-raysourceinthe
foreseeablefuture.
TheNextGenerationLightSourcewillrevolutionize
X-ray science by providing unprecedented coherent
power(upto~100W)inacontinuoustunabletrain(ulti-
matelyupto100MHz)ofultrafast(femtosecondorless)
pulses.Muchaspassivemode-lockingofthecontinuous-
wavelaserinthe20thcenturyusheredintheeraofnon-
linear optical spectroscopy and ultrafast science, a
versatile X-ray laser facility combining high average
power,highrepetitionrate,andtunableultrafastpulses
willusherinaneweraofX-rayscienceforthe21stcentury.
Followingisabriefintroductionofthenewscientific
toolsenabledbysuchanX-raylaser.Section3presents
examplesofthescientificimperativesforanextgeneration
lightsource,andillustrateshowthenewmeasurement
capabilitiesofNGLSwillenablenewareasofscience.
Section4discussesthesekeycapabilitiesindetail,and
providessomecomparisonwithexistingapproaches.
2.1 Multi-dimensionalX-raySpectroscopy
Multi-dimensional X-ray spec-
troscopyreferstoabroadclassof
measurementcapabilities incorporating time-ordered
sequencesofX-raypulsestogenerateasignalthatisa
functionofmultipletimedelaysand/orphotonenergies.
ThesearenonlinearX-raytechniques,andinsomecases
coherentwave-mixing,inwhichX-raypulsesareusedas
bothapump,topreparespecificnear-equilibriumstates
ofmatter,andasaprobeoftheseevolvingstates.These
newtoolsrelyonsimultaneouscombinationsof:high
peakpower,highaveragepower(highrepetitionrate),
spatialcoherence,temporalcoherence,andtunability.
IntheX-rayregion,thetremendouspromiseofmulti-
dimensionalspectroscopyliesinthecapabilitytofollow
coherentchargeflowandenergyrelaxationonfunda-
mental (attosecond to femtosecond) timescaleswith
accesstothefullrangeofvalencestates(unrestrictedby
dipoleselectionrules).Importantly,theelementsensitivi-
typrovidedbyX-rays(tunedtocore-levelabsorptions)
willenableusforthefirsttimetofollowchargeandener-
gyflowbetweenconstituentatomsinmaterials.These
essentialcapabilitiesarenotattainableusinginfraredor
visiblelaserpulses,andwillprovidecriticalinsighttocor-
relatedelectronsystems,andmolecularcomplexeswith
strongcouplingbetweenelectronicandnucleardynamics.
Theanalogoustechniqueofnuclearmagneticreso-
nance(NMR)illustratesthetremendouspotentialimpact
ofmulti-dimensionalX-rayspectroscopy.NMRincorpo-
ratessequencesofradio-frequencypulsestogeneratea
two-dimensional signal-map that is a function of the
Fouriertransformofthetimeintervalsbetweendifferent
pulsepairs.NMRsignal-mapsarefingerprintsofspecific
chemicalstructures,andtheirrelativepositions,withina
molecular complex.The scientific significance is evi-
dencedbythe1991NobelPrizeinChemistrywhichwas
6
2 . OVERVIEW OF REVOLUTIONARY X-RAY SCIENCE TOOLS AT NGLSULTRAFAST DYNAMICS
interest,~meVformanyimportantscienceapplications.
Inaddition tosacrificingphotons that lieoutside the
bandwidthofinterest,therelativelypoorefficiencyof
X-rayopticsresultsinadditionallossofphotonswithin
thebandwidthofinterest.Incontrast,thebandwidthgen-
eratedbyanX-raylasercanbedirectlycontrolledbythe
seedingprocess.NGLSwillultimatelybecapableofgen-
eratingpulsesupto500fsdurationFWHMwithaband-
widthof~10meV(neartheFouriertransformlimit).The
averagefluxavailableinthisbandwidthwillbemany
orders of magnitude beyond any present or planned
source,andwilldrivedramaticadvancesinhigh-resolu-
tionX-rayspectroscopy.
2.2 UltrafastDynamics
NGLSwillprovideimportantnew
capabilitiesforinvestigatingfun-
damentaldynamicsofchargeandenergyflowinmatter:
on the attosecond and few femtosecond time scales
(characteristic of electron correlations and coherent
charge-transferprocesses),andonthe10-100femtosec-
ondtimescale(characteristicofatomicmotionandvibra-
tionalmodes).Dynamicstudieswillbeindispensiblefor
separatingcoupledphenomenainthetimedomain,such
ascollectiveelectronicexcitationsinmaterials,andcou-
pledelectronicandnuclearmotioninreactingmolecules.
While theattosecond frontierhasbeenopenedby
high-orderlaserharmonicsourcesatthe10-100µWaver-
agepowerlevelsandkHzrepetitionrates,NGLSwillenable
X-raypump/X-rayprobeattosecondresearchat100kHz
rateswithinitialtunabilityfrom280eVto1.2keV,and
averagepowerof~1mW.Upgradepathsarealreadyidenti-
fiedtoreachWattlevelaveragepowerinpulsesofafewfs
duration,withspectralrangeextendingtothehardX-rays.
TheflexibledesignofNGLScanreadilyincorporatenew
developmentsinseedlaserstoenhancetheX-raylaser
performance.Thecombinationoflaserseedingandtiming
stabilityprovidedbyacontinuous-wavesuperconducting
RFlinacwillallowforsynchronizationtoexternallasersources
atthefewfemtosecondlevelforsampleexcitationwith
ultrafastpulsesintheUV,visible,near-IR,andTHzregions.
awardedtoR.Ernstforhisdevelopmentofmulti-dimen-
sionalNMR.Inadramaticadvance,vibrationalmultidi-
mensional spectroscopy was demonstrated nearly a
decadeago,usingsequencesofultrafastinfraredlaser
pulses.Theinfraredsignal-mapsprovideafingerprintof
thecouplingbetweendifferentvibrationalmodesina
molecule,therebyrevealingnewinsighttothemolecular
structure and its evolution on the femtosecond time
scale.Thedevelopmentofelectronicmultidimensional
spectroscopynowprovidesanapproachtoexploitultra-
fastvisiblepulsestomapthedynamiccouplingbetween
electronicstates.Overthepastseveralyears,thistech-
nique has become invaluable for following quantum
coherencesandcharge relaxationbetweenelectronic
statesinsystemsrangingfromchlorophyll(responsible
forlightharvestinginphotosynthesis)toexcitonicstates
insemiconductors.
Multi-dimensionalX-rayspectroscopyandnonlinear
X-raysciencewillbehallmarksofNGLSastheyrequire
capabilitiesthatarenotavailablefromanyotherX-ray
source.Highpeak-powerX-raypulsesarejustoneofsev-
eralessentialrequirements.Equallyimportantistheabil-
ity to control the degree of X-ray nonlinearity while
resolvingsmallsignalswithhighfidelity.Highrepetition
rate isabsolutelyessential toachieve this inorder to
avoiddisruptingtheelectronicstates(orothersample
attributes) that are being investigated.An important
benchmarktorecognizeisthatthescientificimpactof
multi-dimensional laser techniqueswas realizedonly
after thedevelopmentofmulti-kHzandMHzultrafast
laser sources.These laserscombinedbothhighpeak
powerandhighaveragepowertoenableextremelysen-
sitivemeasurementsofcontrollednear-equilibriuminter-
actionsoflaserpulsesequenceswithmatter.
InadditiontononlinearX-rayspectroscopy,high-reso-
lutionspectroscopywillalsobetransformedbythecapa-
bilities of NGLS. A fundamental limit of present
synchrotronsources(andSASEFELs)forhigh-resolution
spectroscopyistheirlackoflongitudinal(temporalphase)
coherence.ThegeneratedX-raysareinherentlybroad-
band(typicallyseveral10’sofeVat1keV),*andasacon-
sequence, high-resolution measurements must use
monochromatorsinordertofilteroutthebandwidthof
*Forsynchrotronsources,thefractionalbandwidthinthecentralconefromanundulatorscalesasΔλ/λ~1/NuwhereNuisthenumberofundulatorperi-ods.Presentcapabilities(typicallyNu~100,or1%fractionalbandwidth)aremanyordersofmagnitudebeyondthemeVresolutionofscientificinterest,andarefundamentallylimitedbye-beamemittance(whicheventuallydegradesthecoherentsuperpositionofradiationoverthelengthoftheundulator)andpracticallylimitedbythemaximumundulatorlengthsinastoragering(~10m).
7
2 . OVERVIEW OF REVOLUTIONARY X-RAY SCIENCE TOOLS AT NGLSCOHERENT SCATTERING AND IMAGING
X-ray cinematic imagingusestomographictechniques
withultrafastpulsesandhighrepetitionratestocreate3D
tomographic movies of fluid dynamics coupled with
chemistry.NGLSwillapplyX-raycinematicimagingto
understandcombustiondynamicsand reactive flows
withanunprecedentedcombinationofvolumetricreal-
timeprobingandchemicalspeciation.
Coherent X-ray scattering(X-rayphotoncorrelation
spectroscopy)islaserspeckleintheX-rayregime.Whilevis-
iblelaserspeckle,ordynamiclightscattering,probesdif-
fusionaldynamicsinsoftmatter(e.g.polymers,solution
suspensions,glasses)onthemicronscale,coherentX-ray
scatteringprobesspatialcorrelationsanddynamichetero-
geneityonthenanometerscale,withthechemicalsensi-
tivityandmagneticcontrastmechanismsprovidedby
tunablesoftX-rays.NGLSwillapplycoherentX-rayscat-
tering(at100kHzrepetitionrates,withultrafastpulses)to
understanddynamicnanoscaleheterogeneityinmaterials,
including:(1)transientnanoscalefluctuationsofchargeand
orbitalorderingphenomenainsolids;(2)vortexdynamics
inhighTcsuperconductors;and(3)protein-proteininter-
actionsinphysiological conditions.
Coherent diffractive imagingusesiterativephase-recon-
structionalgorithmstoinvertcoherentX-rayscatteringpat-
ternsandtherebyreconstructthree-dimensionalimagesof
objectsatthenanoscale,avoidingtheresolutionlimitations
imposedbyX-rayoptics.NGLSwillapplycoherentdiffrac-
tiveimaging(at100kHzrepetitionrates,withultrafastpuls-
es) to capture and image: (1) the earliest events in
nanoparticlenucleationandsynthesis;(2)ultrafaststruc-
turalchangesinsupramolecularcatalystsin operation;and
(3)heterogeneousbiologicalsystemsrangingfrommultiple
conformationsanddynamicsofbio-molecules,tomolecu-
larmachines,towholecells—all in native environments.
Thesecapabilitiesfordynamicstudiesrepresentadra-
maticadvanceoverexistingandplannedX-rayFELsources.
Finally,thetemporalcoherenceandversatilityofNGLS
willbeexploitedtotailorthepulseduration(timeresolution)
andbandwidth(energyresolution)forspecificexperiments.
2.3 CoherentScatteringandImaging
MHz ultrafast X-ray lasers at
NGLSwillrevolutionizeourabili-
tytoimagethenanoscalestruc-
tureinmatterwithunprecedented
detail.Structuralheterogeneity, rare transientnano-
structures,spontaneousfluctuations,anddynamicevo-
lutionatthenanoscalewillberevealedusingadvanced
coherentX-rayscatteringanddiffractionmicroscopy
techniquesthatcannotbeachievedwithpresentX-ray
sources.Importantly,thecapabilitiesofNGLSwilllever-
agestate-of-the-artcomputationalapproachesandhigh-
speeddetectorsnowundergoingrapiddevelopment.
X-ray scattering and imaging represent powerful
scientifictoolsthathavebeendevelopedoverthepast
century, based primarily on incoherent (or partially
coherent)Xraysources.Importantly,synchrotronsourc-
esprovidelimitedspatialcoherence,enablingtheemer-
gence of coherent X-ray scattering and diffractive
imagingtechniques.Thesetechniqueswillconvergeand
reachtheirfullpotentialwithcoherentX-raysources.
Spatial and temporal (longitudinal) coherence at
MHzrepetitionrateswillenableNGLSX-raylasersto
revolutionizethecharacterizationofnanoscalestructure
anddynamics.
NGLS – science drivers3
thataccomplishtheconversionofcarbondioxideand
watertocarbohydratesinasingleintegratedsystem.This
motivatesthedevelopmentofartificialphotosynthetic
systemsforthesingularpurposeofgeneratingadesired
fuelonalargescale.
Whileconsiderableprogresshasbeenmadetoward
thisgoal,andsomeartificialsolarfuelsystemsareeffi-
cient,4theycontainrarematerials,arenotdurable,orrely
onsyntheticprocessesthatarenotscalable.5Conversely,
partialorcompletesystemsmadeofabundantmaterials
areinefficientandoftennotrobust.Bridgingthescientific
3.1 Photosynthesis
Research on Solar Fuel Generating Systems at the NGLS
Therisingdemandforenergy,thediminishingsupply
ofoil andnaturalgas, and theenvironmental conse-
quencesfromtheuseoffossilfuels,allhighlighttheneed
forrenewableenergysources.Ofthemanyalternatives,
solarenergyisbyfarthemostabundantandinherently
cleanenergysource.1,2,3Thegoalofgeneratingsolar
fuelsbydirectconversionoflightenergytofuelmole-
culesisinspiredbynature’sphotosyntheticorganisms
Detailed understanding of the processes that comprise photosynthesis — the set of reactions that use solar energy to convert water and carbon dioxide into organic compounds and oxygen — will have both fundamental and applied importance. Fundamental studies can be traced back hundreds of years, with modern investiga-tions including the Nobel prize winning work of Melvin Calvin on critical carbon pathways, and the recent work of Graham Fleming on the quantum coherence underlying photosynthetic reactions. Today, artificial photosyn-thetic systems carry the promise of sustainable energy by producing fuels from sunlight.
The unique capabilities of the NLGS will provide a far greater understanding of natural photosynthetic reactions. Ultrafast pulses will reveal the chemical dynamics that occur on time-scales ranging from the fastest quantum mechanical transfer of electronic charge across molecules, to the relatively gradual regulation of reactions asso-ciated with changing solar flux. Wavelength-tunable, high-repetition-rate X-ray pulses will for the first time allow interrogation of specific photosynthetic molecules and charge states in a time-ordered and non-perturbing manner. These capabilities will also help identify the optimal pathways for efficient conversion of sunlight to fuels in arti-ficial photosystems.
10
3 . SCIENCE DRIVERSPHOTOSYNTHESIS
3.1.1 Natural Photosynthesis — Critical Knowledge Gaps
3 .1 .1 .1 Scientific Gaps
Themostcriticalreactionduringphotosynthesisisthe
photo-inducedoxidationofwater(Figure2).Wateroxida-
tioninnaturalphotosynthesisconsistsoffouroxidation
stepsdrivenbythesuccessiveabsorptionoffourpho-
tonsbythePhotosystemII(PSII)reactioncenter(Figure
3).2Thetimescalesofelectrontransportfromonepig-
menttoanotherspanaboutnineordersofmagnitude.
Afterexcitationoftheantennasystemthelightenergyis
transferredtotheprimarydonorChlorophyllcomplex
P680,locatedinthereactioncenterofPSII,andthensub-
sequentlytootheracceptorpigments.TheoxidizedP680
returnstothegroundstatebyreceivinganelectronthatis
extractedfromtheoxygen-evolvingcomplex(OEC)by
thewateroxidationreaction.Namely,PSIIcombinesone
photochemical reaction at the P680 and four electron
redoxchemistryat theOECtocompleteonecatalytic
cycle.Throughthiscyclicprocess,thecentralMn4Caclus-
terstoresfouroxidizingequivalents,whichareusedto
extractfourprotonsandfourelectronsfromtwowater
gapsfordevelopingviablesolarfuelgeneratorsisafor-
midableyethighlypromisingtaskthatwillbedramati-
cally accelerated by the spectroscopic capabilities of
NGLSX-raylasers.Essentialcapabilitiesincludetimeres-
olution,chemicalspecificityandbonding,andhighrepe-
titionratetoproberaretransientstates,intheiroperating
environment,withoutdisruptingthem.
Naturehasdevisedaremarkablydiversesetofpath-
waystoconvertsolarphotonsintochemicalfuelsthrough
acomplexmechanismoperatingintheleafofaplant.
Simplyspeaking,thephotosyntheticreactioncenterina
plantleafcaptureslight,createsandtransportselectrons
andpositivechargesthat,withthehelpofanaturally
occurringcatalyst,oxidizeswaterandproducesoxygen
moleculesandhydrogenions.Inlaterstagesofthepro-
cess,electronsareusedtoreducecarbondioxideandfix
carbonbycombiningcarbonwithhydrogenionsinthe
formationofasugarthatcanlaterbeconvertedtoalcohol
fuel.Whilethenaturalsystemdealswithacomplexsetof
tasksthatgoeswellbeyondproducingsugarmolecules,
artificialphotosynthesisfocusesonthesingletaskofgen-
eratingafuelfromwater,carbondioxideandsunlight.
CapabilitiesoftheNGLSX-raylaserswillbecrucialto
advanceourunderstandingofkeyenergytransfer,charge
transportandchemicalprocessesofnaturalphotosyn-
thesis,andtodevelopfundamentalprinciplesfortarget-
eddesignofartificialsystemsthatareefficient,durable,
andmanufacturablefromearth-abundantmaterialsusing
scalableprocesses.
Photocathodematerial
Photoanodematerial
Surface-boundcatalyst for O2evolution
Surface-boundcatalyst for H2evolution
2H2O
O2 + 4H+
4H+
2H2
4e–
4e–
H+ permeablemembrane
H+
H+
H+
H+
H+
H+
Sunlight
Carbon fixingreactions
NADP+
non-cyclice- transport
ADP + Pi ATP
CF1
CF0
C14 = 50 – 100 rpm
C14
Lhca
3Lh
ca2
Lhca
4Lh
ca1
A
KH
B
D EFd
JIOG
QKAQKBFx
PC
PC
L C
osep
H+ H+
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½PQH2
PQ-pool
GQ
QH22Fe-2S
A
N F
A
FeCytbH
Qi
Qo
CytbL
M NLQ
cyclic e-
transport
IVcytb6
Stroma
b
Lheb1+2+3Lheb4Lheb6
Psb29(biogenesis)Psb29
extrinsic
Lheb5
ThylakoidMembran
(5nm)
Lumen
16 nm11 nm
Y YD
SC J K Z N X H
B
DD2
AD1 E F
WY MLTc
Cn2+?
Cn2+
Chla
ISPb
Mn MnMn Mn(on D1)Psb27
(repair)P
O
Q
RTn
P680
Phe
QA
cytƒ Fe
e-
e- e-
e-
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RieskeISP
1/2 H2O 1/4 O2+C (CP43)(CP47) B
a
dF6Fe FB
A0 A0
FA
P700
Figure1Left: natural photosynthetic system. Right: conceptual design of an artificial photosynthetic system (Figure courtesy of N. Lewis).
2 H2O 2 H2 + O2
4H + + 4 e– 2 H2
2 O 2- O2 + 4 e–
Figure2Steps in photo-induced oxidation of water.
11
3 . SCIENCE DRIVERSPHOTOSYNTHESIS
es themforasufficientperiodnecessary forsplitting
water,therefore,remainsasacentralquestion.
3 .1 .1 .3 Capabilities Lacking
For a complete under-
standingofthephotocata-
lyt ic react ions, i t is
necessary tostudycritical
stepssuchasthetransient
S4state,and thechemical
dynamics that govern the
directionalityofthecatalytic
reaction cycle. A detailed
molecularpicturerequires
time-resolved measure-
mentsunderambientcon-
ditions.However,forbiologicalcatalystsliketheOECthat
usuallyfunctioninadiluteaqueousenvironment,this
hasbeenachallengeduetohighsusceptibilitytoradia-
tiondamageevenatcryogenictemperatures.9Forthis
areaofscience,NGLSX-raylaserswillbridgeseveralsig-
nificantcapabilitygapsofmodernsynchrotrons,namely:
(1)anabilitytoprobetheseprocessesonthefundamental
timescalesandatambientconditionswherecatalytic
reactions,bondformation,andchargetransferprocesses
occur;(2)acapabilityfortunabletwo-colorX-raypump,
X-rayprobe,andmultidimensionalX-rayspectroscopy
techniques for following the flow of valence charges
betweendifferentatomicsites;(3)provisionoftherequi-
siteaveragebrightness(unachievablefromsynchrotron
sources)thatwilldirectlyrevealthechargecorrelations
molecules,catalyzingtheformationofoxygen,andeven-
tuallyreleasingO2atarateofnearly500molecules/sec.
The electrons on the acceptors are available for CO2
reduction,orforthereductionofH+toH2.
Despitemanydecadesofstudy,essentialcomponents
ofthewateroxidationprocessremainpoorlyunderstood.
Acompleteunderstandingofthefundamentalelectron
dynamicsthatcontrolthiscomplexreactionisagrand
challengeofscience.
3 .1 .1 .2 Current Understanding from Synchrotron-Based
Experiments
ThedetailedchemistryoftheOEChasemergedslowly,
but critical design aspects remain to be elucidated.
Synchrotron-basedexperimentssuchasX-raydiffrac-
tion6,7 and X-ray absorption methods have revealed
insightabouttheproteinscaffoldandtheoverallgeome-
tryoftheMn4Cacluster.8X-rayemissionandabsorption
spectroscopyhavealsoprovidedinformationaboutthe
electronicstructureofsomecryo-trappedintermediate
states(S0toS3).TheMn4Cacomplexplaysakeyrole
owingtotheversatiled-electronorbitalsandtheirmanip-
ulationvia ligand fields, fromwhichemerge: charge-
transferstates,acapabilitytochangeoxidationstateat
relativelysmallenergycost,andahighlevelofcatalytic
activity.Whileitiswidelyacceptedthatallofthemanga-
neseionsareinhighoxidationstatesandbridgedbyoxy-
gen,theexactoxidationstateofeachmanganeseatom,
thenatureofthebondingwithoxygen,theirevolution,
andtheroleofcalciumremainlargelyunknown.Howthe
OECaccumulatesfouroxidizingequivalentsandstabiliz-
Light absorptionand
charge separation
2H2O
Pheo
O2+4H+
OEC
QA
YZ
QB
hν
20-300 ns
~ms
ps
<400 ps
OOEC
P680*
P680
O2
2H2O
S0
Water oxidation
S4
S3
e–, 2H+
e–, H+
e–, H+
e–
S1
S2
Tyr 161
Asp 170His 190
Gln 165
Glu 189Mn
Mn Mn
Mn
Ca
Glu 333
His 332
His 337
CP43
Glu 354
Ala 344c-term
Asp 342
Figure3 Right: The absorption of photons by the PS II reaction cen-ter P680 and the subsequent elec-tron transfer steps that trigger the water oxidation chemistry in the OEC. Left Sequential states, Si, of the Mn4Ca cluster following indi-vidual photon absorption (hν), where i=0-4 indicates the number of oxidizing equivalents stored in the cluster. Inset: One of the pro-posed structures of Mn4Ca cluster.1
UV-visible-THz pump, X-ray probe
Time-resolved XAS, XES, XANES, EXAFS
Time-resolved ambient-pressure XPS
Native environments
Sample replacement between pulses
12
3 . �SCIENCE�DRIVERSPHOTOSYNTHESIS
ates and products interact-
ing with a solid catalyst
surface.13 These techniques
open up monitoring of het-
erogeneous photocatalytic
processes by probing bond-
ing interactions of water,
carbon dioxide, or reaction
intermediates on the cata-
lyst surface. Concurrent
monitoring of transition
metal L-edge absorption and
ligand K-edge spectra of
surface metal centers, using
grazing incidence to enhance surface sensitivity, provides
complementary electronic structure information on the
participating catalytic centers. However, present sources
lack the critical capability to follow these processes on
their natural time scales. Measurements of photosynthetic
systems on time scales faster than ~100 ms are beyond
our reach due to the limited flux and unfavorable time
structure of storage rings.
Time resolved X-ray absorption spectroscopy
(TR-XAS) has been used for understanding the dynamic
structural changes of ligand environments upon excita-
tion of organometallic light absorbers. For example,
Della-Longa et al., have shown an expansion of the por-
phyrin ring of a nickel porphyrin chromophore in the
excited state upon absorption of light, along with detailed
information on electronic structural changes.14 Khalil et
that are thought to play a key role in important catalytic
processes; (4) requisite pulse spacing to allow for sample
replacement, preparation (pump), and probe — on each
pulse, and in native (liquid) environments.
3.1.2� �Artificial�Photosynthesis�—��Critical�Knowledge�Gaps
3.1.2.1 ScientificGaps
Breakthroughs in present thermodynamic and quantum
efficiency limits of artificial photosynthetic systems will
require new understanding of: (1) adequately matched
redox potentials of light absorbers, charge separators,
and catalysts, which are essential prerequisites for con-
verting a maximum fraction of the solar photon energy to
chemical energy of the fuel (thermodynamic efficiency);
(2) efficient and durable contacts for directed charge
transport between components — typically either mole-
cule-solid or solid-solid interfaces (high quantum effi-
ciencies require fast directed charge transport in order to
compete with undesired pathways); (3) robust catalysts
for water oxidation, or proton or carbon dioxide reduc-
tion that operate at sufficiently fast rates for the catalysis
to keep up with the photon flux at high solar intensity;
(4) efficient coupling of the fuel-generating and water-
oxidation half reactions across a proton permeable, prod-
uct impermeable membrane that affords separation of
fuel molecules from evolving oxygen. While some these
scientific barriers are overcome by using components
made of non-scalable materials such as noble metals, an
overarching challenge is to bridge these scientific gaps
with components made of abundant, robust materials.
3.1.2.2 CurrentExperimentalCapabilities
andLimitations
Various X-ray spectroscopy techniques are now being
applied to understand the time-averaged geometric and
electronic structure of artificial photosynthetic systems.
For example, cobalt EXAFS measurements revealed the
atomic structure of a recently discovered cobalt contain-
ing electrocatalytic film for water oxidation10,11 (Figure 4).
EXAFS spectroscopy of multiple metal edges have been
applied to investigate structural relationships of polynu-
clear light absorbers — catalyst assemblies in nanopo-
rous silica supports.12 In situ X-ray emission spectroscopy
of oxygen and carbon K-edges provides atom-specific
details on the electronic structure of reactants, intermedi-
00 2
ExperimentFit
4
R (Å)
6 8
5
10
15Co–O
Co
Co Co
O
Figure4��Co-Pi�EXAFS�spectrum11.
Two-color�X-ray�probe
High-resolution�RIXS
X-ray�pump,�X-ray�probe
Stimulated�X-ray�Raman�(CXRS)�–�wave�mixing
Core-hole�correlation�–�wave�mixing
see�Section�4.3
13
3 . SCIENCE DRIVERSPHOTOSYNTHESIS
3 .1 .2 .3 Capabilities Lacking
Acriticalcapabilityfordevelopingefficientartificial
photosyntheticsystemsmadeofviablematerialsisto
observeandunderstandthesequenceofelementarypro-
cessesfromabsorptionofsolarphotonstothereleaseof
fuelandoxygenmolecules.Thereiscompellingevidence
thatenergeticsandstructuralaspectsoflightabsorbers,
chargeseparatinginterfaces,catalyticcomponentsand
linkagesbetweenhalf reactionsaremutuallyaffected
throughoutthesequenceofenergytransfer,chargetrans-
portandcatalyticprocesses.Therefore,thedesignofeffi-
cient solar fuel generators depends on the ability to
understand the electronic properties and structural
changesofactivesites,ontheirnaturaltimescales,under
operatingconditions,andacrossthecompletesystem.
Current synchrotron-based X-ray spectroscopies
describedabovedonotprovidethecapabilitytofollow
changesinelectronicpropertiesandstructureonrelevant
timescales,andwithrequiredsensitivity.Additionally,
currentX-raysourcesarepractically limited tosingle
probewavelengths,whichpreventsmonitoringofpro-
cessesatmorethanonemetalcenteror ligand,orof
morethanonechemicalspeciesatatime.Importantly,no
X-raysourceexiststhathasatimeresolutionoffemto-
seconds,sufficientpulseenergy,andtherequiredhigh
pulserepetitionratetomakesimultaneousmonitoringof
multipleabsorptionedgesfeasible.
al.,havedetectedultrafastbondlengthchangesinorga-
noiron-basedlightabsorbers(spin-crossovercomplex)
duringtheinitialphototriggeredevents.15Veryrecently,
TR-XAShasbeenappliedforthefirsttimetoL-edges,
identifyingdetailedchangesinbondingconfigurationof
thehybridizedmetal-ligandorbitalsinthetransienthigh-
spincrossovercomplex(Figure5).16Thesestudiesare
substantiallylimited(bypresentsourcecapabilities)to
coarsetimeresolution,andsimplemolecularsystems
withrelativelylargetransientsignals.Thesimultaneous
monitoringofnuclearandelectronicmovementsduring
photon-inducedenergyandchargetransferprocesses,
howeverlimited,demonstratethetremendouspotential
ofultrafastX-rayspectroscopyforobtainingdynamic
informationwithhighspatialandtemporalresolutionin
thevicinityofspecificelements.
BeyondTR-XAS,ResonantInelasticX-rayScattering
(RIXS)isanincisiveX-raytoolforprobingchargetrans-
fer,andotherlow-energyexcitationssuchas d-dtransi-
tionsandprotonenergytransfer.RIXS,incombination
withXAS,hasdemonstratedthemetaltoligandcharge
transferoccurringbetweenaConanoparticlecatalystand
surfactantligandsonitssurface.17Forexample,Figure6
showsthechargetransferpeakinCo3O4nanoclusters
grown in silica nanopores18 that act as efficient and
robustcatalystsforwateroxidation.
–1.5 –1.0 –5 0 5Energy (eV)
35% reductionof CT in nanos
CT
dd
SBA (Nanostructured Co3O4 in nanoporous silica)
Co3O4
Inte
nsity
(a. u
.)
Co3O4
Co Co 2H2O
O2+ 2H+
O
SiSi
O OOO
Si SiO O O
Figure6 RIXS spectra of Co3O4 microcrystalline powder and nano-structured Co3O4 in nanoporous silica (SBA15).
Abso
rban
ce/m
OD
40
0
80
704 708
A
Ground State (Exp.)
[Fe(tren)(py)3]2+
712 716 720 724Energy /eV
1.7 eV
L3
L2
Low Spin
High Spin
90ps delay
Multiplet Theory
Multiplet Theory
Figure5 Photoinduced changes in Fe(II) L-edge spectra.16
14
3 . SCIENCE DRIVERSPHOTOSYNTHESIS
Secondly,thequantumefficiency(productevolution
perphotonenergyabsorbed)ofagivenprocess—beit
electrontransferbetweencatalystandlightabsorberor
theelectrontransferstakingplaceonthecatalyticsurface
—isdeterminedbynotonlythoseintermediatesthat
leadtoproductevolution,butisfurtherinfluencedbyall
thecompetingpathways.Sincetherewillbeavarietyof
competingpathways,thesignalduetoeachwillbesmall.
TransientRIXSisanexperimentthatcannotbedoneat
currentX-raysourcesandwillbeidealforidentifyingrel-
evantchargetransfersbetweenboundreactantsandthe
catalyticsurfaceorbetweenthelightabsorberandcatalyst.
Innaturalphotosynthesis, themanipulationof the
ligandfieldsoftheMnd-electronorbitalsintheMn4Ca
complexmodulatesthecharge-transferstatesandthe
chargedensityofthemetal/ligands,whicharecriticalfor
catalyticactivity.Thesameideaisapplicabletothecata-
lysts(water-splitting,hydrogenproduction,orCO2reduc-
tion) that are embedded in the artificial systems. In
addition,directionalityofthereaction,whichiscontrolled
bythemetal-ligand,catalyst-linker,andabsorber-linker
interfaces,willalsoperturbthefunctionofthecatalysts.
Suchelectronic structural changescanbestudiedby
3.1.3 NGLS: New Capabilities for Research in Natural and Artificial Photosynthesis
Thesub-femtosecondpulseduration,highpulseenergy,
and100kHz–1MHzpulse repetition frequencyof the
NGLSwillopenuptime-resolvedX-rayabsorption,emis-
sionandRIXSexperimentsonnaturalandartificialpho-
tosynthetic systems that will lead to a new level of
mechanisticunderstandingbeyondthereachofexisting
experimental tools. Inparticular, simultaneousmulti-
wavelength,time-resolvedX-rayprobingofmetalcen-
tersandcoordinationenvironmentsoflightabsorbers,
catalystsandinterfacesacrosscompletephotosynthetic
assembliesuponexcitationwithultrafastlightpulseswill
revealtheinterplayofenergy,chargemovement,and
chemical transformationsunderoperatingconditions.
Thehighspatialresolutioncombinedwithtime-resolved
capability from ultrafast to milliseconds will allow
3Dmappingofchargeflowacrossthecomplexheteroge-
neousstructuresofasolar fuelsystem.Theresulting
understandingofhowdynamicandreactiveeventsaffect
energeticsandelectronic structureofallpartsof the
assembly,underreactionconditions,willprovideinsight
forimprovingsolartofuelefficiencythatiscurrentlynot
available.
Outlined below are examples of experimental
approachesopenedupbytheNGLSforunderstanding
bothnaturalandartificialphotosyntheticsystems.
3 .1 .3 .1 Photon Demanding Experiments
Time-resolvedX-rayabsorptionexperimentsatsyn-
chrotronsourceshaveverylimitedcapabilitytofollow
theelectronicandstructuralconfigurationofphoto-excit-
edmoleculesandbulkphasetransitions(owingtolimits
intimeresolutionandaveragefluxavailable).Inaddition,
criticalforthedesignofhighefficiencyartificialphoto-
syntheticsystemsismonitoringtheevolutionofinsitu
photo-driven catalysts (similarly limited by available
averageflux).Experimentsevenatmstimescalesarea
significant challenge, and key experiments requiring
ultrafasttimeresolutionareimpossible.Theseinclude
transientdelocalizationor localizationoforbitals,and
evolutionofnewbondingconfigurationsthatprecede
anddirecttheformationofthosecatalyticintermediate
states.Allrequirehighaveragephotonfluxandhightime
resolutionsincethefasterintermediatescannotbeaccu-
mulatedovertime.
Transition metalK-edge
Transition metalL-edge
M M
Continuum
Valencelevel LUMO
HOMOLUMOHOMO
3p 3p
2p 2p
1s
Abso
rptio
n
Emis
sion
1s
Continuum
Abso
rptio
n
Emis
sion
Figure7 Left: The energy level diagrams showing the hard-X-ray Kß emission lines that probe the charge density and electronic structure of the ligands of Mn. Right: The soft X-ray emission ener-gy level diagram that are sensitive to d-d transitions and charge transfer states that are probed by soft X-ray emission and RIXS.
15
3 . SCIENCE DRIVERSPHOTOSYNTHESIS
FEL’sfallshortoftheserequirementsbymanyordersof
magnitude.
3 .1 .3 .2 Multi-color Time-Resolved Experiments
TheNGLSopensupthepossibilityofsimultaneous
measurementswithX-raysoftwoormoredifferentcol-
ors.Thiscapabilityhasaprofoundinfluenceonourability
tounderstandinteractionsbetweendifferentelements
and/orspatiallyseparatedunits.
AfundamentalmysteryoftheOECinnaturalphoto-
synthesisisthedistributionofchargeassociatedwiththe
fouroxidizingequivalents.Howarethechargesdistribut-
edanddoescoherentcorrelationamongthechargespro-
videforenhancedstability?Inartificialphotosynthetic
systems, in which multi-metals are involved as light
absorbersandoxidationandreductioncatalysts,probing
differentmetalsitessimultaneouslyovermanydecades
oftimeprovidesapowerfultoolforstudyinghoweach
activecomponentissynchronizedwithothers.Element
specificity of X-ray spectroscopy with time-resolved
detectionwillbeessential foraddressingtheseques-
tions.Ahigh-repetition-rateX-raylaserwillprovidepow-
erfulprobesincludingRIXS(spontaneousX-rayRaman
scattering)andmoreadvancedmulti-colorapproaches
suchasstimulatedX-rayRaman(CXRS)andX-raywave
mixing(asdescribedinSection4.3).
BothhardandsoftX-rayRIXSspectroscopyprovide
detailedelectronicstructuralchangesatthecatalyticsites
orlightabsorbersbysimultaneouslyprobingtwoX-ray
photonfrequencies,i.e.incoming(absorption)andscat-
tered(emission)photons(seeFigure7).StimulatedX-ray
Ramanspectroscopy(Section4.3)furtheraddstime-sen-
sitivity toRIXS,andprovides informationofvalence-
excited-statedynamicsbyfollowingthechargeflowfrom
oneatomicsitetoanother.Intheartificialsystem,for
example,a localizedvalence-excitedstateat the light
absorberatomiscreatedbyapumppulsetunedtoaspe-
cificcore-leveltransition(impulsiveX-rayRamanexcita-
tion,seeFigure74),andtheevolutionofsuchastateis
followedbyacontrolleddelaytimewhenasecondprobe
pulseinteractswithacatalyticsite.Thus,onecanfollow
thechangeofsiteAinresponsetothechangeofsiteB,
anddetectcoherentcouplingbetweenthem.Thecapabil-
ities of NGLS will enable such multi-color multi-
dimensional pump-probe experiments in the light
absorber—catalystpair,ordifferentelementsiteswithin
thecatalyst.ThisapproachisanalogoustoNMRtech-
time-resolvedKβemission(Figure7, left)orsoftX-ray
emission/RIXSspectroscopies(Figure7,right).However,
these much more photon-demanding experiments
require sensitivity to detect weak signals that probe
valencetocoretransitions,d-d transitions,andcharge-
transferstateswhicharelessprobable(lessintense),yet
highlysensitivetothechemistry.Thesewillonlybepos-
sibleattheNGLSwithhighrepetitionrate(100kHzto
1MHz),moderatefluxperpulsetoavoiddisruptingthe
statesbeingprobed,andtimestructuretoallowforsam-
plereplacement/recoverybetweenpulses.
Time-resolvedhardX-ray
XES studies of ligand-to-
metaltransitionsareessen-
tial for understanding the
water oxidation chemistry,
as these measurements
directlyprobetheligandsof
the metal (Kβ). However,
such transitions are much
weaker(~10timesforKβ1,3
andKβ’,~500timesforKβ2,5
andKβ”)thantheKαemis-
sion signals making them
impossible without NGLS
capabilities.
Time-resolvedsoftX-ray
absorption/emissionstudies
arealsokeytounderstanding
thecomplexchemistryofthe
catalytic function as they
directlyprobethetransient
metal electronic states
throughtransitionsfromthespin-orbitsplitmetal-2plevels.
Furthermore,softX-rayRIXSallowsdirectmeasurementof
chargetransferprocessesthatarenotpossiblebyanyother
knownmethod.Additionally,itisnearlyimpossibleto
collectsoftX-rayspectroscopydatafrombiologicalcata-
lystsatsynchrotronfacilitiesunderambientcondition
withinthetimescaleofradiationdamage.X-raysource
requirementsinclude:(1)tunabilityacrossthemetalL-
andligandK-edges;(2)temporalresolutionof~50fsor
better;(3)averagesourcefluxupto1015ph/s/(0.1%BW);
and(4)timingstructuretoenablerapidsamplereplace-
mentbetweenmeasurements(e.g.flowingliquidjets).
TheserequirementsarewellmatchedtoNGLS,while
present3rdgenerationsynchrotronsandsoft/hardX-ray
UV-visible-THz pump, X-ray probe
Two-color X-ray probe
Time-resolved XAS, XES, XANES, EXAFS
Native environments
Sample replacement between pulses
High-resolution RIXS
X-ray pump, X-ray probe
Stimulated X-ray Raman (CXRS) – wave mixing
Core-hole correlation – wave mixing
see Section 4.3
16
3 . SCIENCE DRIVERSPHOTOSYNTHESIS
towardshighefficiencysystemsisexpectedtotakemany
decadesintheabsenceofmajornewtools(forcompari-
son,considerthe65yearsofresearchonsolarphotovol-
taicmaterialsforsunlighttoelectricityconversionsince
theirfirstappearance).OneanticipatedimpactofNGLSis
theshorteningofprogresstowardshighefficiencyartifi-
cialphotosyntheticsystemsbydecades.Thisisinaddi-
tiontoanticipatedmajorfundamentalbreakthroughsin
thefieldofexcitation,chargetransport,andchemical
transformationincomplexheterogeneoussystems.
Beamlines for Photosynthesis Research
Visible-pump,X-ray-probespectroscopyexperiments
onnaturalandartificialphotosyntheticsystemswillrely
primarily on the seeded NGLS beamlines 1 and 2 as
describedinSection5(Table2).Theseexperimentswill
useone-color(andinsomecasestwo-color)Xrayprobes
tofollowvalencechargedynamicsviaXASandXESat
transition-metalL-edgesandligand(e.g.O,N)K-edgesin
thesoftX-rayrange.EXAFSprobesoflocalstructural
dynamicswillrelyonhardX-raysatthe3rdand5thhar-
monicstoprobetransition-metalK-edges(andbeyondfor
hardX-rayXES).Multi-dimensionalspectroscopyexperi-
mentswillrequirethetwo-colorsub-femtosecondcapa-
bilitiesofbeamline2.SoftX-rayRIXSexperimentswill
relyonthehighenergyresolution(andhighaverageflux)
of NGLS beamline 1 in long-pulse seeded operation
(<50meVresolutionwithoutamonochromator,andhigher
resolutionwithamonochromatoratsomelossofflux).
Manyoftheseexperimentswilluseflowingsamples
(orsamplereplacementorrastering)inordertoprovide
forphysiologicalconditionsandtopreparethesameini-
tialstateforeachprobepulse.Thissetsalimitof~100kHz
ontheusablerepetitionrate.Amaximumfluenceper
pulse in themJ/cm2 range is anticipated, inorder to
insurethattheX-rayprobedoesnotdisrupttheelectronic
statesbeingmeasured(seeAppendix1).
niques which resolve chemical-specific nuclear spin
coherences.MultidimensionalX-raytechniques(Section
4)willenable2Denergymappingofelement-specific
valencechargecoherencesforthefirst time.Withthe
short-pulsecapabilitiesofNGLS,wewillbeabletofollow
theevolutionofsuchcoherencesonthesub-femtosec-
ondtimescale.
3 .1 .3 .3 Ambient, Real Time Measurements
UltrafastX-raylasersoperatingwithuniformlyspaced
pulsesat10μsintervals(100kHz)willenableanimpor-
tantnewexperimentalapproachfortime-resolvedX-ray
spectroscopymeasurements,particularlyforthenatural
photosyntheticsystem.Inthisscheme,thesampleisnot
frozen,butremainsatambienttemperatureinsolution,
consistentwiththenaturalenvironmentinwhichphoto-
synthesisoccurs,andthesampleisreplaced(e.g.via
flowing jet)oneachpulse.Thus,eachcombinationof
laser-pumppulse(s)initiates,fromafreshsample,the
formationofaparticularstate(S1,S2,..Sn)storinganum-
berofoxidizingequivalents,andanultrafastX-rayprobe
pulseinterrogatestheformationandevolutionofthat
statewithunprecedenteddetail.Suchanapproachisnot
practicalatpresentsynchrotronsourcesowingtothe
ultrashort(~2ns)intervalbetweenpulsesthatisusedin
ordertoachievethehighestpossibleaverageflux.
Impact of NGLS on Solar Fuel Generator Development
Thelevelofmechanisticunderstandingofcomplete
integratedsolarfuelsystemsunderreactionconditions
thatwillemergefromresearchenabledbyNGLSX-ray
laserswilldramaticallyacceleratethedevelopmentof
highly-efficientsolarfuelgeneratingsystems.Withfirst
prototype systems performing at modest efficiencies
expectedtobedevelopedinthenextfewyears,progress
17
3 . SCIENCE DRIVERSPHOTOSYNTHESIS
Time-resolved XES Experiments — Photosynthesis
Thefollowingestimatesillustratethescientificneedforahigh-repetition-rateultrafastXraylaserforphotosyn-
thesisresearch:Required integrated flux on the sample: ~1017 photons for 100 time points
ph/pulse (usable) Rep . rate [Hz]Time to do experiment
Time resolution
StorageRing 105[3] 105[1] 100 days 100ps
PulsedFEL 108[2] 102 100 days ~fs
NGLS 108[2] 105[1] 3 hrs ~fs
[1] Rate limit:~105Hz,determinedbysamplereplacementrestrictions
-physiologicalconditions—liquidjets
-storagerings(multibunch)presentlylimitedto
cryogenicallytrappedstates
[2] Fluence limit:~1mJ/cm2toavoiddisruptionofelectronicproperties;100meVBW
(e.g.1keV,108ph/pulse,50μmfocalspot⇒1mJ/cm2)
[3] Bandwidth limit:100meVBWand~10xlossesfrommonochromatoroptics
Nominal Storage Ring Source:
Flux ~5x1015ph/s/0.1%BW@1keV(withoutmonochromatorlosses)
Rep.rate 5x108Hz
Pulseduration 100ps
Nominal Storage Ring Source with Bunch Tilting:
Flux ~6x1012ph/s/0.1%BW@1keV(~106ph/pulse/0.1%BW)(withoutmonochromatorlosses)
Rep.rate 6x106Hz
Pulseduration ~1ps
18
3 . SCIENCE DRIVERSPHOTOSYNTHESIS
References:
1. Yano, J., et al., Where Water Is Oxidized to Dioxygen: Structure of the
Photosynthetic Mn4Ca Cluster. Science, 2006. 314(5800): p. 821-825.
2. Kok, B., Forbush, B., and McGloin, M., Photochem. Photobiol., 1970. 11: p.
457-476.
3. Lewis, N.S. and D.G. Nocera, Powering the planet: Chemical challenges
in solar energy utilization. Proceedings of the National Academy of
Sciences of the United States of America, 2006. 103(43): p. 15729-15735.
4. Khaselev, O. and J.A. Turner, A Monolithic Photovoltaic-
Photoelectrochemical Device for Hydrogen Production via Water
Splitting. Science, 1998. 280(5362): p. 425-427.
5. Office of Science Workshop Report: Basic Research Needs for Solar
Utilization. 2005, U.S. Dept. of Energy.
6. Ferreira, K.N., et al., Architecture of the photosynthetic oxygen-evolving
center. Science, 2004. 303(5665): p. 1831-1838.
7. Loll, B., et al., Towards complete cofactor arrangement in the 3.0 angstrom
resolution structure of photosystem II. Nature, 2005. 438(7070): p. 1040-1044.
8. Yano, J. and V.K. Yachandra, Where Water Is Oxidized to Dioxygen:
Structure of the Photosynthetic Mn4Ca Cluster from X-ray Spectroscopy.
Inorganic Chemistry, 2008. 47(6): p. 1711-1726.
9. Yano, J., et al., X-ray damage to the Mn4Ca complex in single crystals of
photosystem II: A case study for metalloprotein crystallography.
Proceedings of the National Academy of Sciences of the United States
of America, 2005. 102(34): p. 12047-12052.
10. Risch, M., et al., Cobalt-Oxo Core of a Water-Oxidizing Catalyst Film.
Journal of the American Chemical Society, 2009.131(20): p. 6936-6937.
11. Kanan, M.W., et al., Structure and Valency of a Cobalt-Phosphate Water
Oxidation Catalyst Determined by in Situ X-ray Spectroscopy. Journal of
the American Chemical Society, 2010. 132(39): p. 13692-13701.
12. Weare, W.W., et al., Visible Light-Induced Electron Transfer from Di-μ-
oxo-Bridged Dinuclear Mn Complexes to Cr Centers in Silica Nanopores.
Journal of the American Chemical Society, 2008.130(34): p. 11355-11363.
13. Nilsson, A. and L.G.M. Pettersson, Chemical bonding on surfaces probed
by X-ray emission spectroscopy and density functional theory. Surface
Science Reports, 2004. 55(2-5): p. 49-167.
14. Della-Longa, S., et al., Direct Deconvolution of Two-State Pump-Probe
X-ray Absorption Spectra and the Structural Changes in a 100 ps
Transient of Ni(II)-tetramesitylporphyrin. Inorganic Chemistry, 2009.
48(9): p. 3934-3942.
15. Khalil, M., et al., Picosecond X-ray absorption spectroscopy of a photo-
induced iron(II) spin crossover reaction in solution. Journal of Physical
Chemistry A, 2006. 110(1): p. 38-44.
16. Huse, N., et al., Photo-Induced Spin-State Conversion in Solvated
Transition Metal Complexes Probed via Time-Resolved Soft X-ray
Spectroscopy. Journal of the American Chemical Society, 2010. 132(19):
p. 6809-6816.
17. Liu, H., et al., Nano Lett., 2007. 7: p. 1919-1922.
18. Jiao, F. and H. Frei, Nanostructured Cobalt Oxide Clusters in Mesoporous
Silica as Efficient Oxygen-Evolving Catalysts. Angewandte Chemie, 2009.
121(10): p. 1873-1876.
19
3 . SCIENCE DRIVERSFUNDAMENTAL ENERGY AND CHARGE DYNAMICS
areseparable from thoseof thenuclei inamolecule
based on their disparate time responses (Born-
Oppenheimerapproximation).Thisassumptionbecomes
increasinglyinvalidandmisleadingwhenattemptingto
understandphotochemicalprocessesinlargemolecules.
Thus,asecondandcrucialstepinpresenttheoretical
descriptionsistorepairthisassumptionwhenitbreaks
downnearspecificpointsinthecourseofaphotochemical
reaction.Theseareconicalintersections(seamsofinter-
sectionbetweenthepotentialenergysurfacesofdifferent
electronicstatesofamolecule)thatattempttodescribe
viaquantumchemistrytheregionswhereelectronicand
nucleardynamicsare closely coupled (and theBorn-
Oppenheimerapproximationisnotapplicable).
Importantly,theconversionofenergyfromlightinto
chemicalenergyincomplexmoleculesoftenproceeds
throughmultiplebreakdownsoftheBorn-Oppenheimer
approximation,withprofound influenceonthecourseof
thereactionpathway.Presently,however,weareunable
toobservethisfundamentalphenomenoninunambigu-
ousdetail—eveninsimplemolecules.Toadvanceour
fundamentalunderstandingofphotochemistryandsolar
Theflowofenergyandelectricchargeinmolecules
arecentraltobothnaturalandman-mademolecularsys-
temsthatconvertsunlightintofuelsordirectlyintoelec-
tricity.Understandingandcontrollingtheseprocesses
remainsafundamentalsciencechallenge,inlargepart
becausewelacktherequisitetoolstoprobethesepro-
cesses—simultaneouslyattheatomiclevelandonnatural
timescales.NGLSwillprovidequalitativelynewprobes
ofenergyandchargeflowandhowtheyworkinsimple
andcomplexmolecularsystems.
How is Electronic Energy from the Absorption of Visible or Ultraviolet Light Converted into Chemical Energy in Molecular Systems?
Energyfromtheabsorptionofvisiblelightbyelec-
tronsinamoleculeisconvertedtochemicalenergyvia
couplingtonuclearmotionandbondingbetweenatoms.
Today,muchofourunderstandingofthiscentralprocess
insolarenergyconversionandphotochemistryisbased
onaninitialassumption:namelythattheelectrondynamics
When visible or ultraviolet light interacts with a molecule, the energy is initially absorbed by the electrons that are also responsible for molecular bonding, and then couples to motion of the atoms. Unless this excess energy is rapidly channeled into a coordinated rearrangement of chemical bonds and/or migration of electrical charge, it is quickly converted into heat and lost. For example, a key feature of light harvesting complexes which produce chemical fuels or electrical current, is that they are designed to rapidly channel the energy of electronic motion into specific and useful molecular pathways, by breaking and forming particular chemical bonds, or moving elec-trical charge to specific molecular locations.
The fundamental mechanisms of energy and charge migration in molecular systems are still only partially under-stood, and modeled only by limiting approximations. Direct observation of the intramolecular machinery remains difficult because measurements must be made on the ultrafast time scales of electron motion, bond breaking and formation, and subtle nuclear motion. NGLS will provide ultrafast pulses of extreme ultraviolet and X-ray radiation at high repetition rates that will allow us to probe the details of these fundamental processes of energy and charge migration on the relevant time-scales of attoseconds to femtoseconds. These experiments will bridge a critical gap in our understanding, and will facilitate bottom-up molecular design principles for the development of new energy producing systems.
3.2 FundamentalEnergyandChargeDynamics
20
3 . SCIENCE DRIVERSFUNDAMENTAL ENERGY AND CHARGE DYNAMICS
energysurfaces.Wecurrentlyhaveonlyarudimentary
understandingofthiscentralprocess,eveninsmallmol-
ecules,andverylittleabilitytoobserveitonanatomic
scaleinmoleculesofanysize.NGLSwillenableanarrayof
experimentstofollowchargemigrationinbothlargeand
smallmoleculeswithatomicresolutionbytargetingspe-
cificatomic“reporter”sitesforX-raydynamicalinvesti-
gationsonfemtosecondandevenattosecondtime-scales.
3.2.1 NGLS: Probing and Visualizing Coupled Electronic and Nuclear Dynamics in Molecules — Motion Through Conical Intersections
3 .2 .1 .1 The Conversion of Electronic Energy to
Chemical Energy
Biologicalsystemsandlightharvestingcomplexesare
drivenintoelectronicallyexcitednon-stationarystatesby
theabsorptionofvisibleorUVphotonsorthroughcharge
exchange with neighboring molecular systems.The
resulting coherent superposition of excited states (a
quantumwavepacket)rapidlyevolvesundertheinflu-
enceofthecoupledelectronicandnucleardegreesof
freedom.Inthiswaytheenergyofelectronicexcitationis
converted into nuclear motion and chemical energy.
Manytheoreticalandexperimentalstudies2-6haveshown
thatconicalintersectionscanprovidethemechanismfor
extremelyfastchemicalprocesses,e.g.photo-dissocia-
tion,photo-isomerization,andinternalconversiontothe
electronicgroundstate.Time-dependentquantumwave-
packetcalculationshaveestablishedthatradiationless
transitionsviaconicalintersectionsbetweenelectronic
statescantakeplaceonatimescaleof10fsorless.Itis
nowwidelyacceptedthatconicalintersectionsareomni-
presentinpolyatomicmolecules,andarefundamentally
important for understanding reaction mechanisms in
photochemistryandphotobiology.7-10
3 .2 .1 .2 Prior State-of-the-Art
Femtosecond time-resolved methods have been
appliedtochemicalreactionsrangingincomplexityfrom
bond-breakingindiatomicmoleculestodynamicsinlarg-
erorganicandbiologicalmolecules, andhave led to
breakthroughsinourunderstandingoffundamentalpro-
cesses.11,12Asachemicalreactioninitiatedbyapump
pulseevolvestowardproducts,oneexpectsthatboththe
electronicandnuclearcomponentsunderobservation
fuelproductionweneedanexperimentalcapabilitythat
willprobethecentralmechanismofenergyconversionin
molecules.Thecombinationofhighaveragepowerand
ultrafast pulses provided by NGLS X-ray lasers will
enableustoprobeindetailthecoupledelectronicand
nuclearmotionthatareincreasinglyacknowledgedto
determinethemolecularpathwaysinlight-drivenreac-
tions.This new class of experiments will allow us to
developnewprinciplesthatwillguidethedesignofmol-
eculesthatwillbeessentialcomponentsofrenewable
energytechnologiesrangingfromsolarfuelproduction
toutilizationandstorage.
How Does the Combination of Nuclear Dynamics and Electron Dynamics Cause Charge Migration in Large Molecules?
Chargemigrationinmoleculesisnotjustthemove-
mentofelectrons.1Thenucleimustalsomoveinorderto
directandlocalizechargeatanewlocation.Thisisakey
stepintheoperationoflightharvestingmolecularsys-
tems,whethernaturalorartificial.Theroleofenergetic
barriersandthedegreeoftheirreversibilityiscentralto
theprocessofpassingchargefromonestructuretothe
next.Electronicsuperpositionstatescaninitiatecharge
flowevenonatime-scaleofafewfemtoseconds,which
mustbeinvestigatedtounderstandtheroleofthesubtle
nuclearmotionsthatguidesystemsfromonestateto
anotherthroughimportantconicalintersectionsofpotential
t = 0 t = 4 fs
t = 8 fs
Figure8Simulation of charge migration in a molecule with nuclei fixed. A localized electron hole is created, migrates to the opposite end of the molecule but returns on the femtosecond time scale in the absence of simultaneous nuclear motion. (Courtesy L. Cederbaum32)
21
3 . SCIENCE DRIVERSFUNDAMENTAL ENERGY AND CHARGE DYNAMICS
system, very little progress has been made from the
experimentalside,mainlyduetothelimitationsofexist-
ingtoolsandphotonsources.NGLSX-raylaserswith
MHzrepetitionrateandshortpulseswillchangethat.The
excitationwillbeperformedwithaUVphotonandthe
probewillbeperformedwithadelayed,ultrashortX-ray
pulsetunedjustabovethecarbonK-edge.
Hereonecanuseareactionmicroscope(COLTRIMS)
typeofdetector toperform“rare-event” coincidence
measurements and provide complete momentum-
resolvedmeasurementsateachtime-stepbyresolving
themomentaofthephotoelectron,Augerelectron,and
two positively charged ionic fragments, as shown in
willchange,andinthecaseofnon-adiabaticcouplings
thesetwoareentangled.Variousexperimentalpump-
probetechniqueshavebeenusedinthepast,buteach
givesaccesstospecificaspectsofthecomplexnon-adia-
baticreactiondynamicswhileremainingmoreorless
blindtoothers.Amongtheexperimentalmethodsused
nowaretransientabsorption,time-resolvedionproduc-
tion, time-resolved photoelectron spectroscopy, and
morerecently,time-resolvedmolecularframephotoelec-
tronangulardistribution.13-20Thislattermethodallowsa
directmonitoringoftheevolvingexcitedstateelectronic
configurationsduringthechemicalreaction.
3 .2 .1 .3 An Entirely New Capability at NGLS
Inordertoprobesimultaneouslytheelectronicaswell
asthenuclearpartofthecoupledsystem,onehasto
measurealloftheaboveelementsincoincidencebyper-
formingakinematicallycompleteexperimentat each
time step.Akinematicallycompleteexperimentisonein
whichthemomentaofallthecomponentsofasystem
beingpumpedaremeasuredsimultaneously(typically
viaprobe-pulseionization,followedbyTOFspectroscopy
ofthechargedfragments).Kinematicallycompleteexper-
imentsatthirdgenerationlightsourcesaresuccessfully
andwidelyusedtostudystationaryground-statemole-
cules using reaction microscopes.21-24The NGLS will
enabletheextensionoftheseexperimentstoevolving
excitedstatedynamics.Itwilladdthecriticaldimension
oftimeonthefemtosecondandsubfemtosecondscales.
3 .2 .1 .4 . An Example Experiment
Asanexample,theπ→π*transitioninethyleneserves
asaprototypicalsystem.Ethylenehasattractedanenor-
mousamountofattentionfrombothexperimentalists
and theorists for its highly non-Born-Oppenheimer
behavior.25-28Varioustheoreticalmethodspredictthat
afterπ→π*excitation,themoleculeexperiencesanultra-
fastdecaybacktothegroundstatethroughtwoconical
intersections,oneoccurring
ata twisted-pyramidalized
structureshowninFigure9
andtheothernearanethyli-
deneconfiguration(HCCH3)
whereoneofthehydrogens
has migrated across the
double bond. Despite the
apparent simplicityof this
0
2
4
6
PyramidalizationTwist
S1
S0
Ener
gy /
eV
8
10
12
Figure9 Illustration of a conical intersection that provides control for directing chemical reaction pathways. (Courtesy Todd Martinez)
H
Ethylene
Fermtosecond delayUV pump
Photoelectron Auger electron
IonIon
X-ray probe
C
H
C
H H
Ethylidene
Figure10Illustration of coupled electronic and nuclear motion in ethylene. Isomerization dynamics are initiated with a UV excitation pulse (left), and the molecular structure is probed a variable fem-tosecond time delays using an X-ray pulse to ionize the molecule.
UV-visible-THz pump, X-ray probe
Two-color, X-ray probe (sub-fs)
Time-resolved Reaction Microscope
22
3 . SCIENCE DRIVERSFUNDAMENTAL ENERGY AND CHARGE DYNAMICS
willdetect incoincidencetwophotoelectronsemitted
fromthetwodifferentmetalcenters.Thephotoelectron
energyfromeachcenterisexquisitelysensitivetothe
valenceelectrondensityaroundthecenter.Anillustration
measurementofthecoincidentphotoelectronenergiesis
showninFigure12.The2Dcoincidencemapswillpro-
vide,ateachtimestep,afingerprintofthestateofthe
valencechargedensityateachmetalcenter,thusprobing
thechargeflowasthechargeoscillatesbackandforth
betweenthesites.Thisisaverypowerfulcapabilitythat
will take the traditional time-resolved photoelectron
spectroscopy(TRPES) techniquetomulti-dimensional
spectroscopy,inthiscasetwo-dimensionaltime-resolved
photoelectronspectroscopy2D-TRPES.Tofurtherinsure
thatthetwophotoelectronsareemittedfromasingle
molecule,theionicfragmentsaredetectedincoincidence
withtheelectronsviaatime-of-flighttechnique.
Figure10.Thedetectionoftheheavierfragmentswillpro-
videamolecularframeofreferenceanddeterminethe
energythatischanneledintonuclearmotion.Thecom-
pletetransientenergymapofthefourparticlesprovides
critical informationonthestronglycoupledelectronic
andnucleardynamicsatandaroundtheconicalintersec-
tion.ThephotoelectronangulardistributionsandAuger
electronangulardistributionsinthemolecularframeare
exquisitelysensitiveto theelectronicstatesandtheir
symmetriesaswellasthepositionofthenucleiatthe
timeoftheprobe.29-32Thiscoincidentdetectionofallthe
fragmentsasafunctionofdelaytimefromtheUVexcita-
tion,combinedwiththeoreticalwork,willgiveaunique
insightthatwillallowustounderstandthecoupledelec-
tronicandnucleardynamics,andelectroncorrelationin
thevicinityofconicalintersections.
3.2.2 Direct Probe of Charge Flow in Molecules: Multi-Color X-ray Pump-Probe Experiments
3 .2 .2 .1 A New Capability at NGLS
MultipleX-raypulsescanserveasprobesofcharge
migrationincomplexmolecules.Asimpleexampleinvolves
the electron dynamics in dimeric metalloporphyrins.
Suchmetalloporphyrinsaremostlyineithertheend-on
(linear)ortheside-on(cofacial)configuration.Whilecofa-
cialdimersrepresentthesimplestmodelsystemforbio-
logicallightharvestingcomplexes,end-ondimersareof
interestduetotheirpotentialapplicationasmolecular
wires. It has recently been postulated that quantum
coherencemightplayanimportantroleinthetransferof
excitationenergyinbiologicallightharvestingcomplexes
withhighquantumefficiencies.33Byemployingmetallo-
porphyrin dimers with dissimilar metal centers, e.g.,
Fe(II)-Ni(II),itwillbepossibletotracktheelectronmotion
alongtheaxisofthedimerbyprobingthecoreleveltran-
sitionsassociatedwitheachmetalcenter.Theelemental
specificityofX-rayprobingmakesitpossibletodeter-
mineonwhichsubunittheelectrondensityislocalizedas
afunctionoftime.
NGLSwillprovideamuchmorepowerfultoolbeyond
existingcapabilitiesbyenablingsimultaneousprobing
withtwoX-raysattwodifferentsites.EachX-rayistuned
toafeweVabovethecorrespondingedgeofoneofthe
metals,asshowninFigure11.Insuchanexperimentwe
UV pump at t = 0
eFe
eNi
Fe
N N
N N
Ni
N N
N N
X-ray probe at t = T/2eNi eNi
eFe eFe
X-ray probe at t = T
Figure11 Dimeric metalloporphyrin complex. Ultrafast optical excitation (e.g. of the charge-transfer band) initiates inter- and intra-molecular charge dynamics that can be probed with element specificity via time-resolved photoelectron spectroscopy.
Figure12 Illustration of 2D coincidence maps of photoelectron spectra. Charge oscillations between Ni and Fe centers will lead to characteristic shifts of the respective photoelectron spectra that will be out of phase by a half oscillation period.
23
3 . SCIENCE DRIVERSFUNDAMENTAL ENERGY AND CHARGE DYNAMICS
evolve,forexampleduringchargetransferprocesses,or
duringreactionsthatchangethemoleculargeometry.
Core-holecorrelationspectroscopyprobesthecorrela-
tionsbetweencore-excitedvalencestatesattwodistinct
atomic sites in a molecule.34 Multidimensional X-ray
RamanspectroscopyexploitsonestimulatedX-rayRaman
processtocreateavalenceexcitation(localizedatone
atomicsite),andasecondstimulatedRamanprocessto
probethemigrationofthatexcitationtodifferentatomic
sites(withoutanycoreexcitations).35Multidimensional
X-rayspectroscopywillprovidecriticalinsightintocorre-
latedelectronsystemsandmolecularcomplexeswith
strongcouplingbetweenelectronicandnucleardynamics.
Beamlines for Investigating Charge and Energy Flow in Molecules
CoincidenceexperimentsandReactionMicroscope
experimentsofchargeandenergyflowinmoleculeswill
relyonvisible/VUV-pumpandsoftX-ray-probeathigh
repetitionratestocapturerarecoincidenceevents.Some
experimentswillrelyontheseededNGLSbeamlineas
describedinSection5(Table2).Themostdemanding
two-colorcore-hole-correlationmulti-dimensionalspec-
troscopyexperimentswillrelyontwo-colorsub-femto-
secondcapabilitiesofbeamline2.
3.2.3 Imaging Energy Flow in Large Molecules Using Multi-Dimensional X-Ray Spectroscopy
A new kind of X-ray spectroscopy that can be devel-
oped only at NGLS: MultidimensionalX-rayspectroscopy
(asdescribedinsection2.3.3)incorporatestime-ordered
sequencesofcoherentX-raypulsestogenerateasignal
thatisafunctionofmultipletimedelaysand/orphoton
energies.Theresulting2Dsignalmapsfollowcoherent
charge flow and energy relaxation between specific
atomic sites. Figure 13
shows a schematic four-
wavemixinggeometry(as
istypicallyemployedinthe
visibleregime).Inthecase
of the dimeric porphyrin
complexes as described
above, with X-ray pulses
tunedtotheNandNi(orN
andFe)absorptionedges,
off-diagonalfeaturesinthe
core-levelcorrelationspec-
troscopymapsshowthedegreeofcorrelationbetween
valence charges associated with N and with Ni (Fe).
Furthermore, suchmaps revealhow thesecorrelations
N-1s Ni-2p(Fe-2p)
Valence coupling
Time
Sample
t1 t2 t3
k1
k1
k2
k2
k3
k3
k4
k4
kS
Figure13 Schematic multi-dimension-al spectroscopy in dimeric porphyrin complexes. X-ray pulse sequences tuned to the N-1s and Ni-2p (or alter-natively Fe-2p) probe correlations between N-2p and Ni(Fe)-3d levels. Alternatively, X-ray pulses tuned to the Fe and Ni 2p-3d transitions may probe d-d transitions, correlations, and charge flow between the Ni and Fe sites.
Attosecond spectroscopy
Stimulated X-ray Raman (CXRS) – wave mixing
Core-hole correlation – wave mixing
see Section 4.3
24
3 . SCIENCE DRIVERSFUNDAMENTAL ENERGY AND CHARGE DYNAMICS
6. Bixon, M. and J. Jortner, Intramolecular radiationless transitions. J.
Chem. Phys., 1968. 48: p. 715-726.
7. Satzger, H., et al., Primary processes underlying the photostability of iso-
lated DNA basis: Adenine. PNAS, 2006. 103: p. 10196.
8. D. Polli, et al., Conical intersection dynamics of the primary photoisomer-
ization event in vision. Nature, 2010. 467: p. 440.
9. C.E. Crespo-Hernandez, et al., Ultrafast excited-state dynamics in nucle-
ic acids. Chem. Rev., 2004. 104: p. 1977-2019.
10. Schultz, T., et al., Efficient deactivation of a model base pair via excited-
state hydrogen transfer. Science, 2004. 306: p. 1765-1768.
11. Zewail, A., Femtochemistry: past, present , and future. Pure and Applied
Chemistry, 2000.72: p. 2219.
12. Zewail, A., Atomic-scale dynamics of the chemical bond. J. Phys. Chem.
A, 2000. 104: p. 5660.
References:
1. Lünnemann, S., A.I. Kuleff, and L.S. Cederbaum, Ultrafast charge migra-
tion in 2-phenylethyl-N,N-dimethylamine Chemical Physics Letters, 2008.
450: p. 232-235.
2. Yarkony, D.R., Diabolical conical intersections. Rev. Mod. Phys., 1996. 68:
p. 985-1013.
3. Baer, M. and B.G. D., eds. The Role of Degenerate States in Chemistry.
Advances in Chemical Physics. Vol. 124. 2002, J. Wiley & Sons: Hoboken
Publishing Company: Hackensack, NJ.
4. Domcke, W., D.R. Yarkony, and H. Koppel, eds. Conical Intersections:
Electronic Structure, Dynamics, and Spectroscopy. Advanced Series in
Physical Chemistry. Vol. 15. 2004, World Scientific Publishing Company:
Hackensack, NJ.
5. Levine, B.G. and T.J. Martinez, Isomerization through conical intersec-
tions. Annu. Rev. Phys. Chem. , 2007. 58: p. 613-634.
Time-resolved Reaction Microscope Experiments
Thefollowingestimatesillustratethescientificneedforahigh-repetition-rateultrafastXraylaserforfundamental
researchonchargeandenergyflowinmolecularcomplexesusingreactionmicroscopetechniques:
Required integrated flux on the sample: ~1019 photons for 100 time points*
ph/pulse (usable) Rep . rate [Hz]Time to do experiment
Time resolution
StorageRing 106[3] 107[1] 10 days 100ps
PulsedFEL 1010[2] 102 100 days ~fs
NGLS 109[2] 106 3 hrs ~fs
[1] Rate limit:~107Hz,determinedbyTOFenergyanalyzer.
Coincidencemeasurementsrequire<<1event/pulse,andtypically10-5oftheeventssatisfythecoincidencecriteria.
Thesampleconcentrationandflux/pulsearebalancedtosatisfythecriteriaof<<1event/pulse
[2] Temporal requirement:Fewfspulseduration,low-chargemode
[3] Bandwidth limit: 0.1%BWand~10xlossesfrommonochromatoroptics
Nominal Storage Ring Source:
Flux ~5x1015ph/s/0.1%BW@1keV(withoutmonochromatorlosses)
Rep.rate 5x108Hz
Pulseduration 100ps
Nominal Storage Ring Source with Bunch Tilting:
Flux ~6x1012ph/s/0.1%BW@1keV(~106ph/pulse/0.1%BW)(withoutmonochromatorlosses)
Rep.rate 6x106Hz
Pulseduration ~1ps
*Forcomparison,atypical(non-timeresolved)COLTRIMSexperimentataStorageRingrequiresdaysofsignalaccumulationat~1MHzrepetitionrate,withafluxof~1012ph/seconthesample,or~1017photonsintotal(foreffectivelyonetimepoint).
25
3 . SCIENCE DRIVERSFUNDAMENTAL ENERGY AND CHARGE DYNAMICS
13. Stolow, A., A.E. Bragg, and D.M. Neumark, Femtosecond time-resolved
photoelectron spectroscopy. Chem. Rev., 2004. 104: p. 1719.
14. O. Gessner, et al., Femtosecond multidimensional imaging of a molecular
dissociation. Science, 2006. 311: p. 219-222.
15. Liu, S.Y., et al., Time-resolved photoelectron imaging using femtosecond
laser and VUV free-electron laser. Phys. Rev. A, 2010. 81: p. 031403(R).
16. Suzuki, T., Femtosecond time-resolved photoelectron imaging. Ann. Rev.
Phys. Chem., 2006. 57: p. 555.
17. Stolow, A. and J.G. Underwood, Time-resolved photoelectron spectros-
copy of non-adiabatic dynamics in polyatomic molecules. Advances in
Chemical Physics, 2008. 139: p. 497.
18. Reid, K.L., Photoelectron angular distributions. Annu. Rev. Phys. Chem.,
2003. 54: p. 397.
19. Bisgaard, C.Z., O.J. Clarkin, and G.R. Wu, Time-resolved molecular frame
dynamics of Fixed-in-space CS2 molecules. Science, 2009. 323: p. 1464.
20. Wollenhaupt, M., Engel.V., and T. Baumert, Femtosecond laser photo-
electron spectroscopy on atoms and small molecules: prototype studies
in quantum control. Annu. Rev. Phys. Chem., 2005. 56: p. 25.
21. Doerner, R., et al., Cold target recoil ion momentum spectroscopy: a
‘momentum microscope’ to view atomic collision dynamics. Phys. Rep.,
2000. 330: p. 95.
22. Ullrich, J., et al., Recoil-ion and electron momentum spectroscopy: reac-
tion-microscope. Rep. Prog. Phys., 2003. 66: p. 1463.
23. T. Jahnke, et al., Multicoincidence studies of photo and Auger electrons
from fixed-in-space molecules using the COLTRIMS technique. J.
Electron Spectr. and related Phenomena, 2004. 141: p. 229.
24. M. S. Schöffler, et al., Ultrafast probing of core hole localization in N2.
Science, 2008. 320: p. 920.
25. Kosma, K., et al., Ultrafast dynamics and coherent oscillations in ethylene
and ethylene-D4 excited at 162 nm. J. Phys. Chem. A 2008. 112: p. 7514.
26. Stert, V., et al., Femtosecond time-resolved dynamics of the electronically
excited ethylene molecule. Chem. Phys. Lett. , 2004. 388: p. 144.
27. Ben-Nun, M., J. Quenneville, and T.J. Martinez, Ab-initio Multiple
Spawning: Photochemistry from first principles quantum Molecular
Dynamics. J. Phys. Chem. A, 2000. 104: p. 5161.
28. Tilborg, J.v., et al., Femtosecond isomerization dynamics in the ethylene
cation measured in an EUV-pump NIR-probe configuration. J. Phys. B:
Atom. Mol. Opt. Phys. , 2009. 42: p. 081002.
29. T. Osipov, et al., Carbon K-shell photoionization of fixed-in-space C2H4.
Phys. Rev. A, 2010. 81: p. 033429.
30. Landers, A., et al., Photoelectron diffraction mapping: molecules illumi-
nated from within. Phys. Rev. Lett., 2001. 87: p. 013002.
31. A. Rudenko, et al., Exploring few-photon, few-electron reactions at
FLASH: from ion yield and momentum measurements to time-resolved
and kinematically complete experiments. J. Phys. B: Atom. Mol. Opt.
Phys., 2010. 43: p. 194004.
32. Krasniqi, F., et al., Imaging molecules from within: Ultrafast angstrom
scale structure determination of molecules via photoelectron holography
using free-electron lasers. Phys. Rev. A, 2010.81: p. 033411.
33. Liddel, P.A., et al., Photoinduced electron transfer in a symmetrical dipor-
phyrin-fullerene triad. Phys. Chem. Chem. Phys., 2004. 6: p. 5509.
34. Schweigert, I.V. and S. Mukamel, Coherent Ultrafast Core-Hole
Correlation Spectroscopy: X-Ray Analogues of Multidimensional NMR.
Physical Review Letters, 2007. 99(16): p. 163001.
35. Tanaka, S. and S. Mukamel, Coherent X-ray Raman spectroscopy: A non-
linear local probe for electronic excitations. Phys Rev. Lett., 2002. 89(4): p.
043001.
26
3 . SCIENCE DRIVERSADVANCED COMBUSTION SCIENCE
3.3 AdvancedCombustionScience
Combustionisnowandwillremainformanydecades
themostimportantmeansofenergyutilizationonearth.
Theenormousbenefitsofmoderncombustiontechnolo-
gies(e.g.reliableelectricity,rapidtransportation,heating
andcoolingetc.),areaccompaniedbynegativeconse-
quences,suchasthehealtheffectsofcombustionparticu-
lates,photochemicalsmog,andanthropogenicclimate
change.Itisincreasinglyimportanttoutilizecombustion
withgreaterefficiencyandfewerharmfulimpactsonthe
planetanditsinhabitants.Forexample,inthetransporta-
tionsector,newenergysourcessuchasbiomass-derived
fuelsofferanopportunitytooptimizethefuelstreamfor
newhighlyefficientengines,andtodevelopnovelfuels
that will help reduce greenhouse gas emissions and
enhancenationalenergysecurity.Climatechangecon-
cernscreateanurgentneedforthesesolutions,reflected
inthegoalfor80%greenhousegasemissionreductions
intheU.S.by2050.Becauseoftheinherentadvantagesof
liquidhydrocarbons,e.g.intransportabilityandenergy
density,itislikelythattheywillcontinuetobeusedas
energycarriersviafutureenergytechnologiessuchas
solarfuels.Accordingly,thereisanincreasingneedfor
predictivemodelsofenginecombustionthatareaccurate
fromthescaleofmoleculesandelectronsthroughthe
macroscopicscaleofenginecylinders.
ThegroundbreakingcapabilitiesofNGLSwillbeinstru-
mentaltocreatethesciencebaseforpredictivecombus-
tionmodels.Withthepromiseofnewdetailedexperiments,
andtheastonishingadvancesincomputation,aconcerted
effortwillbeabletodelivertrulyrigorousscience-based
modelsofcombustion,withgenuinepredictivepowereven
forunexploredfuelsandcombustionstrategies.Validating
suchmodelswillentailground-breakingexperimentalprobes
of combustionchemistryandphysics,manyofwhich
couldberealizedwiththehighrepetitionrate,highaver-
agepower,andcoherentradiationpromisedbyNGLS.
The extraction of useful energy from combustion — the liberation of heat by combining a fuel and an oxidizer in a rapid chemical reaction — provides the foundation for the transportation age. The internal combustion engine has dramatically altered the landscape for the movement of goods and people over the past century, and will continue to dominate transportation for decades. Air travel will depend on combustion for the foreseeable future. While the efficiency of combustion is continually improving, developing a fundamental basis for predict-ing performance in novel engine designs, with the concurrent emergence of dramatically different energy carri-ers such as biomass based fuels, will engage research for many decades. Validation of new high-level computational models of combustion and engine design will require ground-breaking experimental techniques to probe — at high pressure and temperature — the physical and chemical processes of combustion, from the injec-tion of liquid fuel sprays, through turbulent reacting flows, to pollutant formation.
NGLS will provide an arsenal of X-ray tools to image and quantify the dynamics of rapid combustion processes under realistic engine conditions. This includes the visualization of concentration gradients and turbulent flows, with chemical specificity, for the first time. The high repetition rate of NGLS will provide time-resolved tomo-graphic reconstructions of fuel spray breakup and soot particle formation in a truly real-time (high frame rate) movie of the physics governing the ignition and completion of the combustion cycle. With the available high pulse energy, X-ray coherent anti-Stokes Raman spectroscopy will reveal chemical speciation of the heteroge-neous mixture during the combustion cycle.
NGLS X-ray lasers will provide the advanced experimental tools for validation of predictive models that will per-mit high performance, high efficiency, and minimum environmental impacts to be designed directly into engines, a goal that is urgently needed to address national and global challenges in energy security and climate change.
27
3 . SCIENCE DRIVERSADVANCED COMBUSTION SCIENCE
discoveriesusinglaserdiagnostics.Itisclearthatmost
emergingenginetechnologieswillusedirectinjectionof
liquidfuelintothecombustionchamber.Itisimperative
tounderstandtheentirespray,howitbreaksup,mixes,
vaporizesandburns,inordertodevelopfullypredictive
modelsforenginecombustion.Althoughtheprocessof
fuel/airmixturepreparationviaanatomizingspraycon-
trolscombustionefficiencyandemissionsformationin
everytypeofdirect-injectedengine,itispoorlyunder-
stood,evenattheleveloffundamentalfluidmechanics.
Furthermore,manypotentiallyimportantcouplingsofthe
sprayfluidmechanicsandchemicalenvironmenthave
simplynotbeenexplored.Inordertomakefurtherprog-
ress,significantleapsarerequired.
Asasimpleexample,itisspeculatedthatundernor-
maldieselengineoperatingconditions,asmallfuel-rich
flameisstabilizedjustdownstreamoftheliquidjet,lead-
ingtosootformationatthetipoftheflame.2However,in
thisregion,thefuel/airratioandtheexistenceofaflame
haveneverbeenmeasured.Furthermore,undervery
highlevelsofexhaustgasrecirculation(EGR,usedto
lowerNOproduction)thisregionofthesprayappearsto
becompletelychanged,especiallyasitevolvesdown-
stream.3Again,thefuel/airratioisunknown,asisthe
chemical effect of EGR entrainment, or the differing
effectsof fuel-boundor freeoxygen.Exploring these
issuesdemandsinformationabouttheinteriorofsprays,
withchemicaldetailneverbeforeavailable.Finally,the
performanceofinjection,asformanyaspectsofengine
Challenges and Opportunities in Combustion Science
Becausecombustionreliesonacomplexinterrelation-
shipofchemistryandturbulentfluidmechanics,itspro-
cessesexhibitinhomogeneitiesandcorrelationsacrossa
widerangeoflengthandtimescales.Themostpowerful
classofexperimentalmethodsforturbulentcombustion
isdynamic imaging,thatis,resolvingthespatialandtem-
poraldistributionsinacombustionprocess.Furthermore,
becausethecouplingofchemicalandfluid-dynamical
phenomenaliesattheheartofcombustion’scomplexity,
chemically-specific imagingiskeyforcombustioninves-
tigations.NGLSX-raylaserswillimagethefundamental
chemistryandphysicsthatgoverntheentireprocessof
combustion,fromfuelspraystogas-phasecombustion
to particulate formation and evolution.The following
presentsthreecriticalareasincombustionscience(fuel
sprays,turbulentreactingflows,andparticulateformation
andoxidation)wherethesebreakthroughcapabilitiescan
addressproblemsthatremaindifficultorimpossibleto
solvewithothertechniques.Finally,wediscusshowthe
characteristicsofNGLSwillprovideunprecedentednew
experimentalcapabilitiestoaddressthesechallenges.
Chemistry and Physics of Fuel Sprays
Combustionenginesareundergoingradicalchanges,
broughtonlargelybynewscientificandtechnological
A B C D E
Figure14 A diesel spray in a chamber with a surrogate wall (lhs) to investigate wall interactions (lhs) and compare to a free jet (rhs), images cap-tures with a high speed camera: a) early in the injection, b) late in the injection, c) OH chemi-luminescence, d) Temperature via soot black body fits, e) relative soot levels [via emission (κ) determinations]. (From Eismark et al.1)
28
3 . SCIENCE DRIVERSADVANCED COMBUSTION SCIENCE
Ramanscatteringare low.
Here,thehighpower,coher-
ence, and narrow band-
widthofNGLSX-raylasers
willenablecoherentstimu-
lated Raman spectrosco-
pies7 that canbringmany
orders of magnitude
increaseinefficiencyoverspontaneousRaman.X-ray
coherentanti-StokesRamanspectroscopy(XCARS),for
example,alongaline-imaginggeometry8asshownin
Figure15,isapossiblewaytospatiallyresolvethechang-
esinchemicalbondingacrossthereactingspray,evenfor
highly scattering environments.The changes in the
molecularK-edgespectraaremappedontothefrequency
offsetoftheXCARSsignal.Thetemporalresolutionwill
bedeterminedbytheamountofaveragingrequired(as
dictatedbytheavailableaveragepower),butcoherent
X-rayRamanspectroscopyopensauniquewindowinto
the interiorchemistryofa fuelspray.Becauseall the
X-raywavelengthscanbechosentotransmitthroughthe
spray,andbecausetheXCARSsignalarisesonlyfromthe
areawhere thebeamscross, chemical information is
obtainedatchosenlocationswithinevenadensespray.
3.3.2 Cinematic Imaging of Reacting Flows
Thestochasticnatureofturbulentreactingflowsisa
criticalfactorincombustordesign,andunderstanding
turbulence-chemistryinteractionsiskeytodeveloping
robustandefficientcombustionstrategies.Despitesig-
nificant advances in 1D and 2D imaging of turbulent
flames,manyimportantobservablesarestillnotaccessi-
blewithcurrentimagingtechniques.Byitsnature,the
spatio-temporalstructureofturbulenceisnotrepeatable,
andthereforecomparisonsbetweennumericalsimula-
combustion,isoftengovernedbystochasticprocesses
andstatisticallyunlikelyadverseevents,suchaswallor
pistonwetting,makingtemporalandspatialresolution
important.
TheexperimentdepictedinFigure14illustratesthe
differencesbetweenchemicalenvironmentswhencom-
paringafreejettoawallinteraction.Inthecombusting
spraythechemicalbondingenvironmentofcarbon,for
example, moves: from liquid fuel to vaporized fuel;
throughthecombustionprocess;tocarbonmonoxide,
carbondioxide,ortoparticulatecarbon(soot).Correlation
ofthesechemicaltransformationswiththephysicsofthe
spray will be a tremendous breakthrough that will
becomepossiblewithNGLS.
Probingthemasstransport,differentialevaporation,
andoxidativechemistryoffuelflowsatthehighpres-
sures (~30 atm) and temperatures (~1000K) that are
directly relevant tohigh-efficiency/low-pollutionnext
generation engine designs is extremely challenging.
Lightscatteringinadensespraycanconfoundoptical
methods,requiringstrategiessuchasultrafastballistic
imaging4,5andX-rayabsorption6toprobethecritical
sprayformationregion,wheretheliquidcoreinitially
breaksup.Infact,evenwiththecurrentstateoftheart,
spatialandtemporalimagingofthefluidstructuresisat
bestincomplete.Imagingtheinteriorofthespraywith
chemicalspecificityisessentiallyimpossibleatpresent.
TherevolutionarycapabilitiesofNGLSpresentavery
promisingroutetomeetthesechallenges.Thepotential
fortime-resolvedchemically-specifictomographyoftur-
bulentgas-phasereactingflowsisdiscussedbelow(and
inSection4.2).Atthedensitiesofanevaporatinghigh-
momentumfueljet,transmissionimagingnearthecar-
bonoroxygenK-edgesissimplyimpossiblebecauseof
thesmallpenetrationdepths.Nevertheless,forfunda-
mental fluiddynamics,single-pulsedirectabsorption
tomographyinthe2-3keVregion(NGLSharmonics)will
permit unprecedented resolution of transient three-
dimensionalliquidstructureswithseparationofdroplet
andliquidcorefeatures.
ThetrulytransformationalaspectsofNGLS,however,
areintheabilitytochemicallyresolvespatio-temporal
structures.Thecouplingofthechemicalnatureofmulti-
phasereactingflowswiththefluidmechanicsislargely
unexplored.Inprinciple,X-rayRamanscatteringcanpro-
videsimilarchemicalinformationtonear-edgeabsorp-
tionspectroscopy,butcrosssectionsforspontaneous
Fuel spray(reactive flow)
Figure15 X-ray CARS diagram for chemically specific line imaging.
XCARS or Stimulated X-ray Raman (CXRS) – wave mixing
see Section 4.3
29
3 . SCIENCE DRIVERSADVANCED COMBUSTION SCIENCE
andwillrequirecinematic imagingexperiments(high
framerateimaging)tomakeacontinuous“movie”of
scalarandvectorflameproperties.Amongthepressing
needsforexperimentalmeasurementsare4Dmeasure-
mentsofspecies,temperature,mixturefraction(fueldis-
tribution), and scalar dissipation (rate of molecular
mixing) spanning the full range of turbulence length
scales.Thesemeasurementswillprovidenewinsights
into important phenomena, such as localized flame
extinctionandignition.HighrepetitionrateX-raylasers
offernewandrevolutionaryopportunitiestomakesuch
measurements.
Thefollowingsectionsdescribeafewofthepossible
experimental imagingprobesthatwillprovidecritical
insightintoturbulence-chemistryinteractionsandfacili-
tatethedevelopmentandvalidationofnext-generation
turbulentcombustionmodels.Accuratemodelsofthe
coupling between fluid dynamics and chemistry are
neededtoadvancethepredictivecapabilitiesofhigh-
fidelity,large-eddysimulations(LES)anddirectnumeri-
calsimulations(DNS).
Theimagingtoolsforspatio-temporalinterrogationof
turbulentflamesincludesingle-pulseX-rayfluorescence
imagingandsingle-pulsetomographyusingeitherfluo-
rescenceordirectabsorption.Thenarrowbandwidth
radiation(<100meV)ofNGLSwillbecriticalinproviding
chemicalspeciation,andphasecoherentimagingdemod-
ulation(cameraswithpixel-by-pixellock-indetection)will
increase sensitivity. Furthermore, the combinationof
highrepetitionrateandhighpulseenergyiscriticaland
notavailablefromsynchrotronsoranyenvisionedtable-
toplasersource.Finally,X-rayimagingwillbecombined
with(synchronized)conventionallaserdiagnosticsthat
indicateregionsoflocalflameextinctionorvelocityfield
measurements.
Time-resolvedX-rayfluorescenceimagingwillusethe
NGLSbeams(formattedintoalasersheet)forexcitation
atthelow-energysideofthecarbonK-edgenear284eV.10
Red-shifted fluorescence (265–283 eV) arising from
valenceelectronsfillingthe1scoreholewillbeimaged
perpendiculartothelasersheet.11Thefluorescencewill
notbesubstantiallyabsorbedbytheflameduetoitsred-
shift.Absorptioncrosssectionsof~10-18cm2perCatom,
fluorescencequantumyieldsof~3x10-3,and90%quan-
tumefficiencyimagingdetectorswillprovidesufficient
signaltonoiseforsingle-pulseimaging.Cinematic2D
imagesacquiredevery10µs(utilizingthe100kHzNGLS
tionsandexperimentsarecurrentlyperformedonasta-
tistical basis using conditionally averaged quantities
(mixturefraction,localstrainrate,dissipationrate,spatial
scales,etc.).Therelevantcorrelationsbetweenspecies
concentrations and flow conditions are incompletely
characterizedduetoalackoftemporalresolutionandthe
absenceofinformationonthethirdspatialdimension.
Theultimategoalofexperimentalreactingflowsisto
measurecomplete4Dmovies(space+time)ofthefluid
dynamics while simultaneously resolving molecular
identitiesandconcentrations.Achievingthisgoalwill
permittheuseofexperimentally-derivedinitialboundary
conditionsfornumericalflamesimulations.Thisapproach
willallowadirectcomparisonofthemodelwithexperi-
ment,andwillbearevolutionaryleapforwardinunder-
standingthecouplingofchemistryandfluiddynamics
thatisattheheartofcombustioninrealdevices.
AlthoughVUVradiationfromsynchrotronshasmade
major advances in our understanding of combustion
chemistry(e.g.,thediscoveryofenols,anewclassof
combustionintermediate9),theproposedexperiments
onturbulence-chemistryinteractionsrequirethecapabili-
tiesofahigh-repetition-rateX-raylaser.Direct4Dcom-
parisonwithcombustionmodelsarenotyetpossible,
TunableX-rayFEL
Laser-sheetformingoptics
High-repetition rate camera
Figure16 Planar X-ray fluorescence imaging of a turbulent flame.
30
3 . SCIENCE DRIVERSADVANCED COMBUSTION SCIENCE
otherspecies).Bycontrast,
in X-ray fluorescence, the
fluorescencelifetimeisgov-
erned by the dominant
Auger electron emission
lifetime(~10fs).16Thislife-
timeisthreetofourorders
of magnitude shorter than
thetimebetweenmolecular
collisionsinanatmospheric
pressure flame; therefore
theX-rayimagewillbeunaf-
fectedbycollisionalquench-
ing, greatly simplifying
quantitativeinterpretation.
Powerfulmulti-dimensionalvariantsonthistechnique
(asdescribedinmoredetailinSection4.3)willprovide
unprecedenteddetailforinvestigatingturbulentreacting
flows.Four-dimensional(space+time)imagingispossi-
blebyscanningthelasersheettomultipledifferentposi-
tionswithinthedepthoffieldofasingleimagingdetector
(seeSection4.2).Inthiswayafullspatialandtemporal
mapcanbeacquiredwithsufficienttimeresolutiontofol-
lowtheevolutionofflamestructures(usingNGLSrepeti-
tionratesupto1MHz).Chemicalspeciationcouldbe
achievedbytuningtheexcitationtonearedgefeatures
characteristicofimportantfunctionalgroups,providing
criticalinformationontherelativeconcentrationsofe.g.
aromatic,aliphatic,carbonyl,oretherfunctionalities.The
excitationlightmayalsobetunedtotheredsideofthe
oxygenedge(543eV)orthenitrogenedge(410eV)to
probe the distributions of species containing these
atoms.
Ansecondapproachto4Dchemicallyresolvedimag-
ingissinglepulsetomography—obtainingprojectionsof
theflametoreconstructitsspatialstructure,withsuffi-
cientframeratetofollowitstemporalstructure.Weenvi-
siontwotomographicapproachesappliedatthecarbon
K-edge,basedonfluorescenceandabsorption,eachwith
itsownstrengthsandchallenges.
Fluorescenceexcitationtomographyisavariantofpla-
nar fluorescence imaging described above. However,
insteadofformingathinX-raylasersheet,thebeamis
expandedtooverlapasignificantvolumeoftheflame.
Fluorescence is imaged at multiple viewing angles
(~20–100 views) using an array of imaging detectors.
Becausefluorescenceexcitationisalinearprocess,the
repetitionrate)willbefastenoughtocorrelatestructures
fromoneframetothenext.Spectrallyintegratedfluores-
cence(265–283eV)willenableimagingofthetotalcar-
bon atom distribution, which when coupled with
temperaturemeasurements(fromRayleighscattering)
willprovidespatio-temporalmapsofmixturefraction.
Mixturefraction,ξ,isacentralquantityinthetheory
andmodelingofturbulentnon-premixedandpartially
premixedcombustion.Thestateofmixingbetweenthe
fuelandoxidizerstreamsisquantifiedbymixturefrac-
tion,andtherateofmolecularmixingisgivenbythesca-
lar dissipation rate, χ = 2D(∇ξ∙∇ξ), where D is the
mixture-averagedmassdiffusivity.Multi-dimensional
mixture fractionmeasurementsarerequired todeter-
minethescalardissipationrate.Opticallaser-basedmea-
surementsofmixturefractioninflamesareachallenge
becauseofspatialvariationsintemperatureandchemical
composition. Single-point and line measurements of
mixturefractionarefeasibleusingsimultaneousRaman/
Rayleigh/CO-LIFtomeasureallthemajorspeciesconcen-
trationsandtemperature.12However,thisapproachis
not practical for multi-dimensional measurements of
mixturefraction.
State-of-the-artmethodsfor2Dmixturefractionimag-
ing13-15aresingle-pulsemeasurementsthatdonotpro-
videinsightintothetemporalevolutionofthe3Dflame
structure.This structuregovernsphenomenasuchas
localizedextinctionandre-ignition. Inaddition, these
techniquesrelyonmeasurementsofmultiplereactive
speciestoconstructaconservedscalar(aquantitythatis
neitherconsumednorproduced,e.g.mixturefraction).
X-rayfluorescenceimagingofcarbonwillenablespa-
tiallyandtemporallyresolvedmeasurementsofallcar-
bon-containing species in the flame. Because X-ray
fluorescenceislargelyanatomicprocess,thismeasure-
mentwilltracktheredistributionofcarbonatomsthat
originatedfromthefuel,independentofchemicalreac-
tionsthatredistributethemintheflame.Theconcentra-
tion of carbon atoms is a true conserved scalar that
cannotbedirectlymeasuredwithoptical techniques.
Spatiallyresolvedcarbonconcentrationswillbecoupled
withRayleighscatteringtodetermineacarbon-based
mixturefraction.
Quantitativevisible/UVfluorescenceimagingiscom-
plicatedbyspatiallydependentfluorescencecollisional
quenchingrates(i.e.,convertingsignaltoconcentration
requiresconcentrationmapsandquenchingratesforall
Time-resolved X-ray fluorescence
Time-resolved XAS
Real-time cinematic imaging
Time-resolved tomography
see Section 4.2
31
3 . SCIENCE DRIVERSADVANCED COMBUSTION SCIENCE
sitizedsolarcells,19andasacatalystsupportindirect
methanolandotherfuelcells.20Asamajorcontributorto
anthropogenicaerosols, soot isdetrimental toatmo-
spheric visibility,21-24 global climate25,26 and human
health.Moreover,after-treatmentcontrolofparticulates
addssubstantial costanddecreases theefficiencyof
vehicles,makingreductionofin-cylindersootproduction
ahighlyimportanttechnicalareaforenginemanufacturers.
Thetheoryofsootformationhasbeenreviewedperi-
odically22,23 and many contemporary problems have
beendiscussedinarecentworkshop.27Yetbecauseofthe
intricatenatureofsootformation,nottomentionthelarge
number of tightly coupled elementary processes
involved,themechanismandkineticsofsootproduction
remainspoorlyunderstood.Althoughthereisincreasing
consensusonchemicalpathwaysfortheformationofthe
firstandsecondaromaticrings,currenttechniquesare
notwellsuitedtostudythedetailed,time-resolvedchem-
istryof latergrowthoroxidationphases.This lackof
quantitativeorevenqualitativeunderstandingposes
severechallengestofutureenginedesignandoptimiza-
signallevelswillbethesameasinplanarimaging.The
extraexpenseofmultiplecameraswillenablefullspatial
and temporal reconstruction of the flame structure.
Chemical specificity would be achieved as described
aboveforplanarfluorescenceimaging.
Analternativeapproachtofluorescenceisspatially
andchemicallyresolvedabsorptiontomographyatthe
carbonK-edge.Thisapproachpromisesbettersignal-to-
noisethanfluorescence(duetothelowquantumyieldof
fluorescence),butrequiresasophisticateddesigntocre-
ateanangulararrayofexcitationbeams.The3Dspatial
structureof the flamecanbe tomographically recon-
structedfromthesetofprojectedabsorptionimagesat
variousangles.Thissingle-pulsetomographywilldeliver
thedesired4Dcinematicimagingatthe100kHzrepeti-
tionrateofNGLS.Chemicalfunctionalgroupspeciation
couldbeachievedby tuning theexciting radiation to
appropriatenear-edgefeatures,asdescribedabove.This
single-pulsetomographicapproachwouldbearevolu-
tionarycapabilitynotonlyforreactingflows,butalsofor
imagingmanyirreproducibleobjects,andreliesonthe
characteristicsofNGLSradiation(simultaneouslyhigh
repetitionrateandpulseenergy).
AkeyadvantageofalltheseX-raybasedtechniques,
compared to similar approaches in the UV-IR-optical
regime,isthatsignalsfromthesecoreelectronspectros-
copieswillnotbeweakenedbydiminishingBoltzmann
quantumlevelfractionsathighertemperatures,norby
unfavorable Franck-Condon factors.The new multi-
dimensionalimagingcapabilitieswillbeinstrumentalin
makingprogressinunderstandingignitionandextinction
phenomena.Understandingthedynamicsofthesetran-
sientprocesses is increasingly importantbecausethe
peakefficiencyinmanyadvancedcombustionsystemsis
obtained near their stability limits, where the risk of
extinctionandexcesspollutantformationisalsogreater.
3.3.3 Uncovering the Chemistry and Kinetics of Particle Formation and Oxidation
Today’sscientificinterestincombustionparticlefor-
mationchemistryhasbeendirectedtowardsbothmiti-
gatingairpollutionproblemsthatinvariablyresultfrom
theuseoffossilfuels,andtowardsoptimizingproperties
oftechnologicallyusefulnanoparticlesproducedbycom-
bustion.17Recentapplicationsincludetheuseofsootor
carbonnanoparticlesasacathodecatalyst,18indye-sen-
Fuel + Oxidizer
CO, H2, CO2, H20, C2H2
Figure17 A conceptual model of soot formation. (Adapted from Reference 30)
32
3 . SCIENCE DRIVERSADVANCED COMBUSTION SCIENCE
thechemical composition, structureandmorphology
evolvesasafunctionofitsreactionhistory.Forexample,
recentstudiesshowthatthenascentsootparticlescan
haveanaromatic-core/aliphaticshellstructureandthe
ratioofaliphatic-to-aromaticconstituentscanvarywidely
asafunctionofthelocalflameconditionandparticlehis-
tory,30,31 as seen in Figure 18. Currently, we lack the
appropriatetoolsforinsituprobingoftheparticlechemi-
calcomposition,morphology,andreactivity.Thesecom-
plexitieshaveledtothecurrentsituationthatneitherthe
reactants nor the products are defined in the kinetic
descriptionofthesootingprocess.Withoutrevolutionary
newexperimentalcapabilitiesthatenableafundamental
approachanalogous to thoseemployed ingas-phase
reactionkinetics,themodelofsootformationwillremain
phenomenologicalratherthanrigorousandpredictive.
NGLSX-raylaserswillenablenewimagingtechniques
todirectlyinterrogatethetime-dependentelectronicsur-
facestructure,chemicalcomposition,sizeandmorphology
duringtheevolutionfrompolycyclicaromatichydrocar-
bons(PAHs)toparticulate,includingthecriticalandpoor-
lycharacterizedparticlenucleation,massandsizegrowth
andoxidationprocessesofnascentparticles.Particlesat
differentstagesofgrowthcanbeinterrogatedinsituin
flamesorreduced-dimensionalreactors,orheldassingle
freely-suspendedparticles.Byemployingthehighphoton
fluxcontainedwithinasinglepulsetoprobetheevolu-
tionofproducts,theheterogeneousreactionkineticsmay
beexaminedatafundamentallevel.Time-resolvedfluc-
tuationSAXS(small-angleX-rayscattering)andrelated
approaches(describedinsection3.5and4.1)willbepow-
erfulnewtoolsforunderstandingparticulateformation
andgrowthduringcombustion.
tion.Sootformspersistentlyinmanytypesofcombus-
tionengines,andthisproblemcanbesolvedonlywhena
quantitativeanddetailedknowledgeaboutparticlenucle-
ation, mass/size growth, aggregation and oxidation
becomesavailable.
Forexample,ourcurrentlevelofkineticsmodelingfor
sootoxidation28remainsrootedinasemi-empiricalglob-
alkineticsapproachusuallycalledthe“Nagle/Strickland-
Constable”expression.29 It isbecomingmorewidely
appreciatedthatsootoxidationmaybeevenmoreimpor-
tantthanformation,andyetwehaveonlyaroughglobal
expressionatourdisposal.Aswithsootformation,the
sootoxidationproblemishighlycomplex.Sootagesover
time,andatthestagewhereoxidationbecomesimpor-
tantinanengine,fueliscondensingonthesurfaceaspar-
ticlesaggregate.Tobreakthisproblemintoelementary
steps one must first characterize the complexity and
natureoftheproblem.
The overall process of
soot formation, from pre-
cursor formation, particle
inception,andfinallytosoot
mass/size growth (see,
Figure17)occurs typically
overafewmilliseconds.At
theendofthereaction,soot
particles typically contain
106Catoms.Therearetwo
fundamentalchallengesthatlimitourunderstandingof
particleformationkinetics.First,theaveragerateofaddi-
tionofCatomstotheprecursormoleculeis~1atom/ns.
Followingthisrapidprocessrequiresexceptionaltempo-
ralandspatialresolution.Second,duringparticlegrowth,
C3; H = 0.7cm
20 nm( i ) ( ii )
z (n
m)
x (nm)
a
h
420
Distance from Burner Surface, H (cm)
8
6
4
2
0 0.4 0.5 ? 0.6 0.7 0.8 0.9 10
( iii )[Alfphatic C-H][Aromatic C-H]
Equivalent Diameter, Dv?AFM (nm)
Aspe
ct ra
tio, h
/a
0.4
0.3
0.2
0.1
0.00 5 10 15 20 25 30
0.51.0
??
??
?
?
?
m/z0 200 400 600 800 1000
78 78 78 78 78
78 78 78 78
74 74?
?
Figure18 Panel (i) TEM micrograph, panel (ii) AFM images and aspect ratio, and panel (iii) micro-FTIR characterization of aliphatic-to-aro-matic C–H ratios, and thermal-desorption chemical ionization mass spectrometry of nascent soot collected from a 16.3% C2H4–23.7% O2–Ar flame33.
Particulate formation:
Fluctuation X-ray Scattering
Giga-shot diffractive imaging
see Sections 3.5 and 4.1
33
3 . SCIENCE DRIVERSADVANCED COMBUSTION SCIENCE
suchasoxidesofsilicon,titanium,andiron.Beyondpro-
vidingatestforpredictivecombustionmodelsaimedat
minimizingoreliminatingsootproductioninpractical
combustiondevices,theyofferaplatformforoptimizing
combustionsynthesisofnovelmaterials.
3.3.4 NGLS Impact — Combustion Science
ThefullycoherentX-rayradiationfromNGLS,deliv-
eredwithhighaveragepower,highrepetitionrate,mod-
eratepeakpower,anduser-selectabletime/bandwidth
characteristicsarethekeyfeaturesofthislightsource
thatdifferentiateitfromcurrentlightsources,andwill
enableexplorationofnewfrontiersincombustionsci-
ence.Theneedforreal-timecinematicimagingofnon-
repeatablephenomena,suchasdescribedhere,cannot
bemetwitheithercurrentX-rayFEL’s(<100Hzrepetition
rate),norwithsynchrotrons(manyMHzrepetitionrate,
butlowpulseenergy).Specifically,NGLSwillforthefirst
timeenableawiderangeofnon-linearcoherentspec-
troscopiesinthesoftandhardX-rayregimeswhilesimul-
taneouslyenablingtime-resolved cinematicimaging.
First,thecoherentnatureofFELradiationwillallowthe
implementationofmanynon-linearspectroscopictech-
niques(e.g.,softX-raycoherentanti-StokesRamanspec-
troscopy,XCARS,andsoftX-rayholography)previously
developedintheopticalregime.Thesecoherenttech-
niqueswillprovideauniquecombinationofspatialimaging
withchemicalspeciesidentificationthatwillbeunaffected
bytheBoltzmannandFranck-Condonlimitationsofoptical
spectroscopyprobes.Theabilitytotradetemporalpulse
widthforspectralpulsewidthwillbeespeciallypowerful
withnon-linearspectroscopies.Forexample,a0.2fspulse
hasafullbandwidthof~9eV,allowingcoherentwave-
packetexcitationofalargeportionofthenear-edgeX-ray
absorptionsthatdistinguishbondingenvironmentsof
carbon,nitrogen,andoxygen.Conversely,longerpulses
(withnarrowerbandwidth)canbeusedtoselectivelyexcite
resonances of chemical subsets within a chemically
diversesample.Thisbandwidthflexibilitymaybecombined
intheXCARSexperiment,coherentlydrivingmanynear-
edgeexcitationswithbroadbandx-rayradiationinthepump
andStokespulses,andspectrallyresolvingeachofthese
chemicalsubsetsinthedispersedsignalbyutilizinganar-
rowbandprobe.Theuniversalityofcore-levelspectroscopy
providesimportantadvantagesovermolecule-specific
Previousexperimentalevidencesuggestssootnucle-
ationoccursthroughtheclusteringofPAHs.Figure19
depicts a possible reaction sequence as predicted in
molecular dynamics (MD) calculations byVioli and
coworkers.32Criticalquestionsincludetheroleofaro-
maticπradicalsintheclusteringprocess,33andtheroles
ofenergytransferandvibrationalrelaxationinstabilizing
thecluster.34-37Core-levelNEXAFSspectroscopyand
harderX-rayRamananaloguesofferthepossibilityto
simultaneouslyprobethechemicalbondingenvironment
andthespatialstructureofsootgrowth.Inparticularthe
radicaland/orbiradicalsitesthatarepostulatedtobethe
keyreactivesitesformolecularweightgrowthoroxida-
tionofnascentsoot33havespectralfingerprintsinthe
carbonK-edgeregion.
Kineticexperimentsonsingleparticlesmaybeenvi-
sionedusingafreelysuspendedparticleextracteddirectly
from a flame and immobilized by optical tweezers.
NanoscaleX-raycinematicimagingoftheparticlecan
providetime-resolveddataonitsstructure,morphology,
composition,and reactionkinetics.Tobridge thegap
betweenfundamentalkineticstudiesonnascentsootand
sootformationinflames,simultaneoustemporallyand
spatially resolved X-ray CARS and Mie scattering in
steadyorunsteadyflamescouldprobethedynamicsof
particleformation.
Understandingtheexceptionallyrichchemistryofcar-
bonhasyieldedseveralNobelprizes,mostrecently(2010)
toGeimandNovoselovforgraphene,aswellasthatto
Smalley,Curl,andKrotoin1996forfullerenes.Theinves-
tigationofmolecularweightgrowthinflamesfollowsin
thetraditionofthischemistry,butthescopeofcombus-
tionparticulategrowthiswiderthancarbon.Studiessuch
asthoseenvisionedabovewillprovideabetterunder-
standingofparticlegrowthandoxidationmechanisms
andkineticsfortechnologicallyimportantnanomaterials
Figure19PAH clustering/particle nucleation as predicted by molecular dynamics (A. Violi, University of Michigan).
34
3 . SCIENCE DRIVERSADVANCED COMBUSTION SCIENCE
chromatorsonbeamlines1or3),whilenon-linearX-ray
mixingexperimentswill require thehighestpossible
peakpowerandshortestpulsesavailableonbeamline2.
Combustionparticulate formationexperimentswill
relyon“diffractanddestroy”methodsusingthe3rdand
5thharmonicswiththehighestfluxperpulseontheseed-
edNGLSbeamline1at100kHzrepetitionrates,andon
theunseededSASEbeamline3,atMHzrepetitionrates
(ashigh-speeddetectorsallow)asdescribedinSections
5and6.6.Choiceofwavelengthwillbedeterminedby
balancingthescatteringefficiencyandtherequiredreso-
lutionforparticularsamples.
References:
1. J. Eismark, et al., Role of formation and transortation of hydroxyl radicals
for enhanced late soot oxidation in a low emissions heavy-duty diesel
engine, in THIESEL 2010 Conference-on Thermo- and Fluid Dynamic
Processes in Diesel Engines. 2010: Valencia, Spain.
2. J.E. Dec, A Conceptual Model of DI Diesel Combustion Based on Laser-
Sheet Imaging, in SAE Technical Paper 970873. 1997. p. DOI:
10.4271/970873.
3. Musculus, M. and L. Pickett, Chapter 33: In-cylinder spray, mixing, com-
bustion, and pollutant-formation processes in conventional and low-
temperature-combustion diesel engines, in Advanced Direct Injection
Combustion Engine Technologies and Development: Volume : Diesel
Engines, H. Zhao, Editor. 2009, Woodhead Publishing Ltd.: Cambridge, UK.
4. Schmidt, J.B., et al., Ultrafast time-gated ballistic-photon imaging and
shadowgraphy in optically dense rocket sprays. Appl Optics, 2009. 48: p.
B137-B44.
5. Linne, M.A., et al., Ballistic imaging of liquid breakup processes in dense
sprays. Proc. Combust. Inst., 2009.32: p. 2147-61.
6. Wang Y, et al., Ultrafast X-ray study of dense-liquid-jet flow dynamics
using structure-tracking velocimetry. Nature Phys, 2008. 4: p. 305-309.
7. Tanaka, S. and S. Mukamel, Coherent x-ray raman spectroscopy: A non-
linear local probe for electronic excitations. Phys Rev Lett 2002. 89.
8. Kliewer, C., Quantitative one-dimensional imaging using picosecond
dual-broadband pure-rotational CARS. submitted to Applied Optics.
https://share.sandia.gov/crf/crfnews.php?id=307.
9. Taatjes, C.A., et al., Enols are common intermediates in hydrocarbon oxi-
dation. Science, 2005. 308: p. 1887-1889.
10. R. Manne and J. Chem, Physics 1970. 52.
11. Ayoolan, B.O., et al., Spatially resolved heat release rate measurements
in turbulent premixed flames. Combust Flame, 2006. 144: p. 1-16.
12. R. S. Barlow and J.H. Frank, Effects of turbulence on species mass frac-
tions in methane/air jet flames. Proc. Combust. Inst, 1998. 27: p. 1087-1095.
13. J. H. Frank, et al., Mixture fraction imaging in turbulent nonpremixed
hydrocarbon flames. Proc. Combust. Inst, 1994. 25: p. 1159-1166.
opticalspectroscopicmethodsinprobingglobalcombus-
tionphenomena,asdiscussedintheprevioussections.
Second,thehighrepetitionrateofNGLSwillallowcin-
ematicimagingontimescales(~10μsto1μs)thatare
fundamentallyimportantincombustionenvironments.
These time-scalesare fastenoughtoallowframe-by-
framecorrelationoffluiddynamicsandchemistry,which
isnotpossibleusingexistinglightsourcesortable-top
sources.Third,themoderatepeakpowerswillbesuffi-
cientlyintensetodrivenon-linearprocesses,butnotso
intenseastodestroythetargets.Fourth,the3rdand5th
harmonicsoftheFELwillprovidehigh-powernarrow-
bandwidthX-raysatshorterwavelengths.Thegreater
penetrationdepthsofthesephotonswillenablenon-lin-
ear X-ray spectroscopy38 to probe combustion with
chemicalandspatialresolutioneveninsidehighlyscat-
teringorabsorbingenvironmentssuchasdensesprays.
Finally,thehighaveragepowerandbrightnessofNGLS
willallowtheinterrogationofsamplesatmuchhigher
(i.e.,directlyrelevant)pressuresthanwasheretoforepos-
sibleatsynchrotronfacilities.Siliconnitridewindowsand
differentialpumpingwillbeusedtointerfacetheFELto
thehightemperaturesandpressuresofthesamples.
Newexperimentsincombustionsciencemadepossi-
blebyNGLSpromisetobeamajorleapforwardinthe
developmentofpredictivecombustionmodels.These
models, when validated against high fidelity experi-
ments,willbeinvaluableindesigningbothnewfuelsand
combustorsoveramuchlargerparameterspacethanis
possiblewithcurrentengineeringapproaches.Science-
basedpredictivecombustionmodelingwillbenefitsoci-
etythroughpositiveimpactsonenergy,humanhealth,
environmentalhealth,nationalsecurity,andeconomic
competitiveness.
Beamlines for Advanced Combustion Science
Combustionimagingandchemicalspeciationexperi-
mentswillrelyontheun-seededNGLSbeamline3,pro-
vidingthehighestaveragefluxandrepetitionrate,andon
theseededbeamline1providingthehighestresolution
(<50meVwithoutamonochromator).SoftX-raychemical
speciationwillbedoneinthepre-edgeregionofcarbon
(280eV),withsomeexperimentsexploitinghigher-ener-
gyphotonsatthe3rdharmonic(upto3.6keV)inorderto
balanceabsorptionandpenetrationdepth.Someexperi-
mentswillrequirehigherenergyresolution(withmono-
35
3 . SCIENCE DRIVERSADVANCED COMBUSTION SCIENCE
28. Vishwanathan, G. and R.D. Reitz, Development of a Practical Soot
Modeling Approach and Its Application to Low-Temperature Diesel
Combustion. Combust. Sci. Tech., 2010. 182(8): p. 1050 - 1082.
29. Nagle, J. and R.F. Strickland-Constable, Oxidation of carbon between
1000-2000°C, in Proceedings of the 5th Carbon Conference. 1962:
Pergamon, Oxford. p. 154-164.
30. Cain, J.P., et al., Micro-FTIR study of soot chemical composition - evi-
dence of aliphatic hydrocarbons on nascent soot surfaces. Phys. Chem.
Chem. Phys., 2010. 12: p. 5206-5218.
31. Cain, J.P., et al., Evidence of aliphatics in nascent soot particles formed
in premixed ethylene flames. Prog. Energy Combust. Sci., 2010. 33: p. doi:
10.1016/j.proci.2010.06.164.
32. Chung, S.-H. and A. Violi, Peri-condensed aromatics with aliphatic
chains as key intermediates for the nucleation of aromatic hydrocar-
bons. Proceedings of the Combustion Institute, 2010. 33: p. in press.
33. Wang, H., Formation of nascent soot and other condensed-phase mate-
rials in flames. Proceedings of the Combustion Institute, 2010: p. in press
(doi:10.1016/j.proci.2010.09.009).
34. Violi, A., A.F. Sarofim, and G.A. Voth, Kinetic Monte Carlo molecular
dynamics approach to model soot inception. Combustion Science and
Technology, 2004. 176: p. 991-1005.
35. Violi A and Venkatnathan A., Combustion-generated nanoparticles pro-
duced in a benzene flame: A multiscale approach. Journal of Chemical
Physics, 2006. 125.
36. Schuetz, C.A. and M. Frenklach, Nucleation of soot: Molecular dynamics
simulations of pyrene dimerization. Proceedings of the Combustion
Institute, 2002. 29: p. 2307-2314.
37. Wong, D., et al., Molecular dynamics simulations of PAH dimerization, in
Combustion Generated Fine Carbonaceous Particles, H. Bockhorn, et al.,
Editors. 2009, KIT Scientific Publishing: Karlsruhe. p. 247-57.
38. Harbola, U. and S. Mukamel, Coherent stimulated x-ray raman spectros-
copy: Attosecond extension of resonant inelastic x-ray raman scatter-
ing. Physical Review B, 2009. 79.
14. M. B. Long, et al., A technique for mixture fraction imaging in turbulent
nonpremixed flames. Ber. Bunsenges. Phys. Chem., 1993. 97: p. 1555-
1559.
15. J. Fielding, et al., Polarized/depolarized rayleigh scattering for determin-
ing fuel concentrations in flames. Proc. Combust. Inst., 2003. 29: p. 2703-
2709.
16. A. J. Seen and F. P. Larkins, Ab initio studies of molecular x-ray-emission
processes - Ethanol. Phys.B-At. Mol. Opt. Phys., 1992. 25: p. 4811-4822.
17. Strobel, R. and S.E. Pratsinis, Flame aerosol synthesis of smart nano-
structured materials. J. Mater. Chem., 2007. 17: p. 4743-56.
18. Kay A. and G. M., Low cost photovoltaic modules based on dye sensi-
tized nanocrystalline titanium dioxide and carbon powder. Solar Energy
Materials and Solar Cells, 1996. 44: p. 99-117.
19. Oregan B. and Gratzel M., A low-cost, high-efficiency solar-cell based
on dye-sensitized colloidal TiO2 films. Nature, 1991. 353: p. 737-40.
20. Bashyam R. and Zelenay P., A class of non-precious metal composite
catalysts for fuel cells. Nature, 2006. 443: p. 63-66.
21. Horvath, H., Atmospheric Light-Absorption — A Review. Atmospheric
Environment Part A-General Topics, 1993. 27: p. 293-317.
22. Kennedy, I.M., Models of soot formation and oxidation. Prog. Energy and
Combust. Sci., 1997. 23: p. 95-132.
23. Richter, H. and J.B. Howard, Formation of polycyclic aromatic hydrocar-
bons and their growth to soot — a review of chemical reaction path-
ways. Prog. Energy Combust. Sci., 2000. 26: p. 565-608.
24. McEnally, C.S., et al., Studies of aromatic hydrocarbon formation mecha-
nisms in flames: Progress towards closing the fuel gap. Prog. Energy
Combust. Sci., 2006. 32: p. 247-94.
25. Hansen, J. and L. Nazarenko, Soot climate forcing via snow and ice albe-
dos. PNAS, 2004. 101: p. 423-8.
26. Service, R.F., Climate change — Study fingers soot as a major player in
global warming. Science, 2008. 319: p. 1745.
27. Bockhorn, H., et al., eds. Combustion Generated Fine Carbonaceous
Particles. 2009, KIT Scientific Publishing: Karlsruhe.
36
OVERVIEW OF REVOLUTIONARY X-RAY SCIENCE TOOLS AT NGLS
X-raysathighrepetitionrate,withtimescalesofattosec-
ondstohundredsoffemtoseconds,comparabletoelec-
tronicandnuclearmotion inmolecules,andthehigh
brightnessofNGLSbeamsallowingspectro-nanoscopy
downtosub10nmspatialresolutionopenspreviously
unthinkablepossibilitiesfordiscoveringhowspecificele-
mentaryreactionstepsproceedonheterogeneouscata-
lysts.Suchanunderstandingwillfacilitatethediscovery
ofnovelreactionpathwaystomakemoreselective(green
chemistry)andmoreefficient(energysaving)catalysts.
3.4.1 Heterogeneous Catalysts in Action — Microscopy and Dynamics
It iswellknownthatcatalystsareneitherstaticnor
homogeneousentitiesduringoperation.Theenergetic
processesofphononandelectronexchangesoccurringat
their surfaces, and the formation of chemical bonds
betweenthereactantmoleculesandthecatalystcanleadto
restructuringofthecatalyst.Knowingtheatomicandelec-
3.4 Catalysis
Catalyticreactionsareofvitalimportanceinvirtually
allareasofenergygeneration.Examplesareindustrial
processingoffossilfuels;reductionofharmfulemissions
frompowerplantsandcars;large-scaleproductionof
chemicals;retrievalofstoredenergyaselectricalenergy
orheat;andalternativeenergyuseandconversion(fuel
cells, artificialphotosynthesis).Theworld’s increased
needforenergyinthecomingdecadescanbesustained
only ifnew,moreefficientandcleanerprocesses for
energygenerationandtransformationarefound.This
requiresthedesignofnewclassesofcatalysts,basedon
inexpensiveandabundantmaterials,withtheversatility
toprocessnewsourcesofbiofuelsandcatalyzeexisting
reactionsmoreefficientlyandecologically.Twokeyper-
formance goals are increased output (i.e. yield) and
decreasedwasteproduction(i.e.selectivity).NGLSwill
enable unique spectro-nanoscopy experiments with
unprecedentedspatialandtemporalresolutionthatwill
provide transformational knowledge to help achieve
thesetwogoals.Theabilitytodeliverultrafastpulsesof
Catalysis is critical to nearly all energy production and utilization cycles. The process whereby a chemical activa-tion barrier is lowered to permit a normally unfavorable chemical reaction to occur on a rapid time scale, catalysis has revolutionized humankind. The Haber-Bosch catalytic process in the early 1900s addressed the fixation of nitrogen, leading to the production of fertilizer, and resulting in the award of two Nobel prizes. The automobile catalytic converter is a remarkable success story, producing an effectively clean engine exhaust, using complex catalysts as the driving force to eliminate carbon monoxide and nitrogen oxides.
NGLS will provide the capability to follow the changes in structure of catalytic sites during the processes of catalytic conversion. Just as gas phase dynamics studies with lasers have pioneered a wealth of understanding about transition states, reaction pathways, and energetics, NGLS will unfold these stories using X-Ray spec-troscopies for the first time on dynamic catalytic sites. NGLS provides the ability, through coherent imaging, to visualize the individual locations and movements of a complex set of metal atoms, set in precise configuations by a supramolecular framework. In situ electrochemical and photochemical processes will be analyzed by pump-probe X-ray absorption spectroscopy, and time-resolved ambient-pressure photoemission spectroscopy, made possible by the high pulse repetition rates, short pulses, and high fluxes of NGLS.
Today, novel catalytic processes are paramount to successful water splitting reactions using sunlight and, indeed, for the internal processing in every living cell. A comprehensive understanding of artificial enzyme catalysis will provide visionary tools for the production and utilization of energy in the future.
37
3 . SCIENCE DRIVERSCATALYSIS
To elucidate the differences between synchrotron
investigations and those of NGLS, consider a recent
example.AttheALSresearchershavedemonstratedthat
acatalystchangesitsstructureinresponsetochangesin
reactantcompositionduetothechemistryoccurringatits
surface.IntheexampleshowninFigure20,thecore-shell
structure of bimetallic nanoparticle Rh-Pd catalysts
changesdramaticallyinresponsetotheadsorptionand
associatedreactionsatthesurface.Underoxidationcon-
ditions,involvingforexampleNO,CO,andO2,Rhiscon-
centrated in the shell in oxidized form, while Pd is
concentratedinthecoreregion.Thedistributionreverses
uponreducingconditions.Whileunique,thisinforma-
tion,obtainedbytakingadvantageofthedifferentmean
freepathsofphotoelectronsoftheRhandPdduringin
situXPS,providesonlytime-averagedinformationover
anaveragedspatialdistribution.Thecombinationoftem-
poralresolution,spatialresolution,andchemicalspecific-
ity,availablefromNGLSX-raylasers,willrevealhowand
whythesechangesoccurbyprovidingdetailedinforma-
tiononatomicpositionsandbondinggeometriesofeach
atominthecatalystandadsorbates.Thisnewknowledge
willenablethedevelopmentofcatalyststhatcanadapt
theirstructuretooptimizereactivityandselectivity.
Inanotherexample,bimetalliccatalystsconsistingof
PtnanoparticlespromotedwithSn,Ga,orInexhibitvery
highactivityandselectivityforthedehydrogenationof
lightalkanes(C2toC4)intoH2andalkenes.Bothproducts
ofthisreactionarevaluable,sinceH2isalwaysrequired
forheteroatom(S,N,O)removalfromenergyfeedstocks,
andlightalkenescanbeconvertedtoproductsthatcan
beblendedintogasolineanddiesel.RecentstaticXAS
studieshaveshownthatthepromotingelementsmigrate
tronicstructureofspatially-resolvedcatalyticsurfaces
andinterfaces,duringthereactionanditstimeevolution,
isfundamentalforpredictivecatalystdesign.Unfortunately
todayitisnotpossibletofollowthisstructuralevolutionon
relevant timescales: fs topseccharacteristicofatomic
motion,ornstomscharacteristicofdiffusionandmateri-
alsevolution.Determininghowthisrestructuring,occur-
ring over picoseconds or longer, is connected to the
thermalandelectronicprocessesofthereactionsthatit
precedes,isfundamentalforunderstandinghowthecata-
lystoperates.Althoughsomeattemptstoobtainsuch
informationhavebeenperformedwithultrafasttrans-
missionelectronmicroscopyandelectrondiffraction,
NGLSwillprovideuniqueandtrulyrevolutionarywaysto
visualize dynamic catalytic processes. In particular,
advancedcoherentimagingtechniquessuchascoherent
diffractiveimagingandptychography(discussedinSections
4.1and4.2),willexploitthehighcoherenceandintensity
ofNGLSX-raylaserstoprovidechemically-specificimaging
atthenanoscale.Evenifthehighintensityislikelytodestroy
thecatalystsandtheadsorbedmolecules,theaccumulation
ofrepeatedscatteringpatternsofnewandnearlyidentical
particlescanbedeconvolvedtorestore,throughmodern
imageprocessingmethods,therealspacenanostructure.
Fromtheexperimentalstandpoint,itisimportantto
developinstrumentationthatmakespossiblethestudyof
catalystsintheirworkingenvironment,i.e.,underhigh
pressureofreactantsandathightemperature.Thisisnot
trivialforelectronspectroscopyexperiments;thephoto-
electron mean-free path in gas environments is very
short.Forexamplea500eVelectronwilltravelapproxi-
mately3mmthrough1mbarofO2beforeacollisionwith
agasphasemoleculecausesittolosetheinformationit
carries(kineticenergy,direction).ScientistsattheALS
pioneeredtheuseofdifferentially-pumpedspectrome-
terstoaccomplishthis.1,2Thetechnique,calledambient
pressurephotoelectronspectroscopy(APPES)3,4isnow
beingimplementedinmanysynchrotronfacilitiesaround
theworld.APPEShasledtomanyimportantdiscoveries
incatalysis,5-8waterandenvironmentalscience,9,10fuel
cells,11,12andotherenergyrelatedscience.Togetherwith
X-rayabsorptionspectroscopy(XAS),evenliquidenvi-
ronmentsandelectrochemistryexperimentsarenow
possibletostudyatsynchrotronsources.1However,syn-
chrotron-basedstudiesarenotcapableofobtainingthe
time-evolvingstructuresthatwillberequiredinfuture
generationsofcatalyticscience.
Atom
ic fr
actio
n
Rh0.5Pd0.5
Rhodium
Palladium
Reactants
Shell
Rh Kαl
18 19
PdPd
Rh
Rh ex-situ TEM
20 21 22 23 24 2
Pd Kαl Core-Shell
Core
NO
1.00.90.80.70.60.50.40.30.20.1
0 NONO+CO NO+CO O2
Figure20In-situ XPS (Pgas = mbar) shows core-shell atom exchange in Rh-Pd catalyst nanoparticles induced by oxidation and reduction reactions of NO, CO, H2, and O2. (From Reference 13)
38
3 . SCIENCE DRIVERSCATALYSIS
environment,non-covalentinteractionscanplayacritical
roleinstabilizingtransitionstates.Suchenvironments
offer thepossibilityof (1)precise controlbya three-
dimensionalframeworkinwhichsubstratesinteractwith
oneanotherandwiththehost;and(2)precisecontrolof
theratesatwhichthereactingmoleculesenterandleave
thecatalyticcenter.
Natureexploitsthissupramolecularprincipletodesign
biologicalcatalyticsystemssuchasenzymesandribo-
zymes.Heresupramolecularframeworksarecriticalfea-
turesoftheactivesitesinwhichbiologicalreactionsoccur
withexquisiteselectivityandhighlyenhancedreaction
rates,comparedtothoseexhibitedbyanalogousprocess-
esthattakeplaceinthegasphase,onsurfaces,orinsolu-
tion.However,evenbiologicalchemistshavehistorically
focusedprimarilyonthefeaturesoftheactivesites,and
areonlynowbeginningtounderstandtherolethatthe
supramolecularframeworkplaysincontrollingcatalysis.
Asoneexample,recentinsightssuggestthat“conforma-
tionalgating”playsasignificantroleinenzymecatalysis
(seeforexample,Reference14).Todate,thehomogenous
catalysisfieldhasfocusedlittleattentionondesigningabi-
ologicalsystemsthattakeadvantageofsupramolecular
controltoproducecatalyststhatdemonstratethelevelof
chemoselectivityandrateenhancementtypicallyattribut-
edtoenzymes.However,recentworkbyRaymondand
Bergmanhavedemonstratedsupramolecularcatalysts
(Figure21)thatapproachenzyme-likerateaccelerationsof
overamillionfold.15Amongmanypotentialapplications,
onemayexploitsuchassembliestoencapsulatemetal
complexescapableofcatalyzinghighlyselectivecarbon-
hydrogenbondactivationreactions.
NGLSX-raylaserswillbeessentialtothedevelop-
ment of this new field of supramolecule catalysis by
enablingcompletecharacterizationofthesupramolecu-
larassembliesinwaysthatarenotachievablewithpres-
ent sources. In particular, time-resolved X-ray
spectroscopy techniques
(XANES and EXAFS) will
probe the local bonding
geometry, coordination,
andbond-distancesofcata-
lystswhiletheyareoperat-
ing in solution, and with
temporal resolutions that
canfollowthefundamental
chemistry and turn-over
inandoutofthePtnanopar-
ticles during the overall
reaction, and during cata-
lystreprocessingtoremove
accumulated carbon.The
NGLSwillprovideaunique
opportunity to observe
theseprocessesasafunc-
tionofbothtimeandspace
via time-resolve ambient-
pressure photoemission,
XAS,andXES.Ofparticular
interest are the real-time
dynamicsandchemistryof
carbon deposition on the
nanoparticle surface and
the migration of these
depositsontothesupport-
ingmaterial.Understandinghowpromotingelements
affecttheinitiationandgrowthofcarbonaceousdeposits
andhowtolimittheseprocesseswillprovideessential
informationforthedesignofsuperiordehydrogenation
catalysts.
3.4.2 Homogeneous Catalysis
Bothheterogeneousandhomogenouscatalysishave
focusedhistoricallyon reactions controlledby short-
rangeinteractions,usuallybetweenasmallnumberof
moderate-sizedmolecules.Whilethisapproachinhomo-
geneouscatalysishasledtothedevelopmentandunder-
standing of many interesting and useful processes,
especiallyintheareaofasymmetriccarbon-carbon,car-
bon-hydrogenandcarbon-heteroatombondformation,
animportantfuturedirectionincatalysisliesinthedevel-
opmentofnewreactionscontrolledbylonger-range,or
“supramolecular,”interactions.
Homogeneous “supramolecular” catalysis differs
fromheterogeneoussurface-catalyzedreactionsinthat
thelatterhastraditionallyfocusedon“externalspace,”in
whichreactingmoleculesbindtoextendedsurfaces,and
thosemoleculesexchangerapidlybetweenthebound
stateandanotherphasesuchasaliquidsolutionorgas
phase.Incontrast,supramolecularchemistrytargetsthe
studyofan“internalmolecularspace”,wherereactive
substratesenteraconfinedenvironmentsurroundedon
allsidesbythesupramolecularframework.Insuchan
UV-visible-THz pump, X-ray probe
Time-resolved XAS, XES, XANES
Time-resolved ambient-pressure XPS (APPES)
Spectro-nanoscopy (<10 nm)
Coherent diffractive imaging
Ptychography
see Section 4.2
Fluctuation X-ray scattering
Giga-shot diffractive imaging
see Section 4.1
39
3 . SCIENCE DRIVERSCATALYSIS
(Figure22),but only on long time scales(typically100’sof
msorlonger).Significantadvanceswillbemadewith
NGLS,whereitwillbepossibletoprobethedynamicsof
oxidationprocessesonultrafasttimescalesandtopene-
trateindepthandachievespatialparametersfarbeyond
thelimitedexperimentspossibleatpresentX-raysources.
Figure22showsaschematicsetupandfirstresultsofa
recentinsituelectrochemicalstudyoftheformationof
copperoxides (CuO, cupricoxideandCu2O, cuprous
oxide)asafunctionoftheelectrodepotential.1Similar
studieswillbeinvaluabletoolsinstudiesofrenewable
energysciences,suchasLi+-basedbatteries.
Spectroscopicstudiesofsolidfuelscellshavealsobeen
performedinsituforthefirsttimeattheALSusingambient
pressureXPS(APPES).TheexampleinFigure22showsthe
simultaneousmeasurementofsurfacepotentialandcerium
oxidationstateacrosstheelectrodegapbetweenPtand
CeOinthepresenceofwatervaporandH2.Pump-probe
spectroscopicexperiments,withsub10nmspatialresolu-
tion,wheretheelectrontransfertoH2OtoformO-andHare
followedasafunctionoftime,willbeseminalindeter-
miningelementaryreactionsteps.Pumpexcitationcanbe
accomplishedviaUVpulseirradiationoftheanodeand
cathode,ordirectexcitationofthemolecules.Onecanalso
envisiontheuseofultrafastswitchingdevicestogether
withthehighpulseenergyX-rayprobesofNGLStofirst
injectelectronsintotheelectrodematerials,thenfollow
withprobeanalysis(XPS,XAS)ofthechargestructureof
themoleculeorsurfacespecies,includingtheconduction
bandintheelectrolyte.Furthermore,thehighbrightness
ofNGLSwillallowthestudyofcharge-transferprocesses
withtheuseofin-situRIXS,andwillrevealthedynamics
ofiontransportusingstimulatedphotondesorption.
3.4.4 Pump-Probe Catalysis Studies at NGLS
Pump-probetechniquesatNGLSwillmakeitpossible
toinvestigatethemostfundamentalstepsincatalysis
thatspantherangefromattosecondstopicoseconds.
Experimentsthatwillbecomepossibleincludethecom-
binationofspectroscopyandmicroscopy,withsub-10nm
spatialresolution(spectro-nanoscopy),followingtheini-
tialexcitationof:
•electronicstatesviacharge-transferexcitationsand
photo-ionizationprocessesonfemtosecondandsub-
femtosecondtimescales(photochemistry,electro-
chemistry,electronictransitionsinredoxcycles)
ratesinrealtime—fromfstoms.Inaddition,NGLSX-ray
laserswillenablequalitativelynewapproachesforimag-
ingspontaneousdynamicsofheterogeneousensembles
ofmacromoleculesinsolution,asdescribedinSection
4.1.Thisnewapproachisbasedoncollectingbillionsof
coherentdiffractiveimagestoresolvetheconformational
dynamicsofsupramolecularcatalystsandprovidenew
insighttoprocessessuchasconformationalgating.The
experimentalapproachcloselyfollowsthatdescribedin
Section4.1forimagingheterogeneousensemblesofpro-
teinconformations.Thenewunderstandingofsupramo-
lecularcatalysisfromNGLSexperimentswillenablethe
developmentoffundamentaloperatingprinciplesthat
willdramaticallyacceleratethepresentiterativeprocess
oftargetedcatalysisdesign.
3.4.3 Fuel Cells, Electrochemical Reactions, Batteries
Understandingandcontrollingelectrochemicalreac-
tionsisparticularlyimportantinmanytechnologiesand
processes,rangingfrombatteriestopotassiumchannels
incellmembranes.SoftX-rayspectroscopyhasbeen
extensivelyemployedforex-situinvestigationsofelec-
trochemicallyactivematerials,butitsuseasanin-situ
probehas laggedbehindhardX-rayexperiments for
technicalreasons.TheALSandothershaverecentlydem-
onstratedthefeasibilityandpowerofusingsoftX-ray
XPS,XAS,XES,andRIXStodeterminethestructuraland
chemical changes of electrochemical systems during
operation. Scientist have explored evaporated metal
electrodes during cyclic voltammetric experiments
= Galll. 1
NH O
O O
O O
OHN
12-
Figure21 The water-soluble, self-assembled, tetrahedral assem-bly shown to catalyze the Nazarov cyclization of 1,3-pentadienols with extremely high levels of efficiency. Left: blue lines represent bisbidentate ligands. Right: space-filling model.
40
3 . SCIENCE DRIVERSCATALYSIS
3 .4 .4 .1 Use of THz Pump Pulses to Excite Nuclear
Motion (Heat, Vibrations)
Nuclearmotions,inthetimescaleoffstops,canbe
excitedbyTHzpulses.Thiswilltriggerthesubsequent
reorganizationof theelectronicwavefunctions in the
HOMOandLUMOlevelsofthemoleculeofinterest.The
abilitytodeterminethiselectronicstructurewillallowfor
anexplorationoftransitionstatesandreactionpathways
inprocessestriggeredbyheating.Unlikeheating,how-
ever,whichdistributesenergytoanumberofvibration
modesaccordingtotheBoltzmandistribution,THzexcita-
tioncanexcitespecificvibrationalmodes.Followingare
examplesofreactionsthatcanbenefitfromthistypeof
experiment.
COisafundamentalingredientintheFischer-Tropsch
reactiontomakegasolinefuelsfromcoal.Itiscurrently
undergoingastrongrevival,withcountrieslikeNorway
investingheavilyintheprocess.Themostfavoredmecha-
nisminvolvesCOdissociationtoCandO,whichcanbe
mediatedbyhydrogen.16ExcitingtheC-Ostretchmode
• specificmolecularvibrationalmodesviacoherent
mid-infrared andTHz excitation on 0.1-1 ps time
scales
• transientthermally-drivenreactionsviaTHzexcita-
tiononpstimescalesandlonger
NGLSexperimentsatthehighesttemporalresolution
willrelyonrepetitiveprocesses,wherethesystemcanbe
preparedrepeatedlyforeachpulse.Forexample,reac-
tionsonnanoparticlescouldbeperformedusingagas-
phase jet of particles (or a liquid droplet injector as
described in Section 4.1.3) crossing the NGLS X-ray
beam.Reactantsarepre-absorbedoneachnanoparticle,
forexamplebycrossingajetofparticleswiththereactant
moleculesofinterest.Excitedstatedynamicsandtransi-
tion-statespeciesarerevealedviaXPS,XASorRIXSfol-
lowinginitialexcitationbyaTHz,UV,orX-rayphoton
pulse,orpulsede-beam.
0.0Electrochemically active region
O2–
H2O H2
YSZAu
Au
580 nme–Pt
–1.0 –0.5 0Distance (mm)
0.5
PtYSZ
X-rays
CeO2-x
CeO2-x
0
2
4
6
8
0.5
1.0
Surf
ace
pote
ntia
l (V)
CE
WEPEEK
Support tubeSi3N4
Fluorescenceout
X-raysin
–1.0x10–5
–0.5 –0.4 –0.3Potential (V)
Photon energy (eV)In
tens
ity (a
. u.)
Cu2+Cu+
–0.2 –0.1 0.0 0.1
920 930 940 950 960 970
–5.0x10–6
0
5.0x10–6
RE
–0.9 V—Reduction300 nm Cu in 2 mM NaHCO3
Cu2+ Cu/Cu+
0 V—Oxidation300 nm Cu in 2 mM NaHCO3
300 nm Cu in thin film in air
Cu foil
Cu L2,3-edge XAS
In situ electrochemical XAS
Figure22 Left: schematic electrochemical cell assembly for in situ XAS-ray absorption spectroscopy studies, cyclic voltammogram of a Cu thin film working electrode in NaHCO3 solution, XAS of Cu L2,3-edge after reduction1. Right: Ambeint pressure X-ray photoemission spec-troscopy (APPES) measurements of surface potential and oxidatation state of solid-oxide fuel cell electrodes in operation.
41
3 . SCIENCE DRIVERSCATALYSIS
compositions,andnewreactionconditions.AftertheTHz
pulse,X-raypulseswillthenbeusedtoprobetheelec-
tronicstructureoftheexcitedmolecule,viaXPS,XESand
XAStofollow,withattosecondsorfemtosecondssteps,
andtheirevolutionoverseveralpicoseconds.
3 .4 .4 .2 Use of UV Pulses and/or Electrochemical Charge
Transfer (Pump) to Excite Electronic Levels
Hereweenvisionexcitingmolecularorbitalsandelec-
troniclevelsdirectly,ratherthanatomicvibrations.Inthis
experimentwecaneitherinitiallypopulateemptyLUMO
orbitalsandconductionbands,orcreateholesinHOMOs
orvalencebands.Asequenceoffemtosecondandatto-
secondtimedelayedprobeX-raypulseswillfollowthe
wavefunctionevolutionviaXPS,XAS,XESorRIXSover
timeperiodsextendingtopicoseconds.Inthefollowing
wedescribeexamplesofprocessesandreactionsinvolv-
ingelectrontransfertomolecularorbitals.
Inthephotoelectrochemicalsplittingofwatertohydro-
genandoxygen,photonsfirstinteractwithoneormore
lightabsorbingelementstocreateelectron-holepairs.The
holesdiffusetooneendoftheabsorberwheretheyinteract
withacatalystthatpromotestheoxidationofwatertoO2
andliberatestwoprotons.Theprotonsthendiffusethrough
apolymericmembraneandreactwiththephoton-gener-
atedelectronsatasecondcatalysttoformH2.Thedynamics
ofelectron-holetransport,andofelectronandholeinter-
actionwiththecatalystsateachendofthelightabsorber
arepoorlyunderstoodbecauseofthedifficultyinmea-
suringthesephenomena.Similarly,thedynamicswith
whichchargedspeciesinteractwithmoleculesandanions
orcationspresentnearthecatalystsurfacearelargely
unknown.NGLSX-raylaserswillbeusedtoprobethese
processesviapump-probeexperimentswherefewfem-
tosecondtoseveralhundredfemtosecondlightpulses
(fromsynchronizedvisiblelasers)areusedtogenerate
electron-holepairs.Thedynamicsoftheinteractionofthese
chargedspecieswithcatalystsandmoleculescanthenbe
followedbyXASandXPS.Time-resolvedXES,RIXS,and
RamanspectroscopyusingUVstimulationshouldalso
enable the observation of molecular transformations
occurringinthepresenceofaliquidelectrolyte.These
studieswillcontributeinformationcriticaltounderstanding
thelifetimesofchargedspeciesproducedatthesurfaceof
electrocatalysts,whichmaybemetals,metaloxides,ormetal
complexes.Theinfluenceofchangesinthecomposition
andstructureofelectrocatalystswillalsobeinvestigated
mightbefollowedbydissociation.Otherchannelsthat
canbeselectivelyexcited includeCO-metalsubstrate
frustratedmodes(translation,rotation,bending).NGLS
experimentswillmakeitpossibletofollowthewavefunc-
tionofthemolecularorbitalsinresponsetotheexcitation
ofsuchvibrations,andthushelpdiscoverwhichparticu-
larmodeleadstospecificintermediatesorproducts.For
example, theexcitationofCObendingmodes (when
adsorbedonacatalyst)mightchangethegeometryofthe
moleculesothatelectronsfromthecatalystsubstrate
mightjumpintoanti-bondingorbitalsofthemoleculesor
conversely,electronsfromtheHOMObondingorbitals
mightmovetothecatalyst.Commoncatalystsinclude1st
rowtransitionmetals(Fe,Ni,Co)andalloysofthosewith
Cu,Pt,etc.
Methanoloxidationisamuchstudiedchemicalreaction
toproducepartiallyoxidizedproductslikeformaldehyde,
whileavoidingtheundesirablebutthermodynamically
favoredtotaloxidationtoCO2andwater.CatalystslikePt,
Au,Ag,Cu,andtheiralloysareusedtoaccomplishthisin
anefficientandhighlyselectivemode.17,18Theexcitation
ofC-Hbondsinboundmethoxidesbyheat(viaselected
THzpulsesthatcanselectivelyexcitedifferentvibration
modesdirectlyinthemoleculeorinthecatalystmetal
atoms),willleadtotherearrangementoftheH,CandO
atomstoformintermediates.Capturingtheevolutionof
theorbitalwavefunctionsfollowingtheexcitationwill
provideuniqueinformationonthemechanismofthevar-
iouspossiblereactionpathways,whichwillthenmakeit
possibletostudyandselectdifferentcatalystmaterialsor
CH3OH + O2
Methanol oxidataion on metal alloy clusters
(1) CH3OH + ½O2 → CH2O+ H2O
(2) CH3OH + 3/2O2 → CO2+ 2H2O
hv
hv
e-
CO2
296
C 1s XPS
CH2O CH
3OH
292 288 284Binding energy (eV)
Figure23 Experimental schematic of pump-probe studies of metha-nol oxidation on metal alloy clusters.
42
3 . SCIENCE DRIVERSCATALYSIS
References:
1. Jiang, P., et al., In Situ Soft X-ray Absorption Spectroscopy Investigation
of Electrochemical Corrosion of Copper in Aqueous NaHCO3 Solution. E.
Chem. Com., 2010. 12: p. 820.
2. Ogletree, D.F., et al., A differentially pumped electrostatic lens system for
photoemission studies in the millibar range. Rev. Sci. Instr., 2002. 73: p. 3872.
3. Salmeron, M. and R. Schlögl, Ambient pressure photoelectron spectros-
copy: A new tool for surface science and nanotechnology. Surf. Sci.
Rep., 2008. 63: p. 169.
4. Bluhm, H., et al., In situ x-ray photoelectron spectroscopy studies of gas/
solid interfaces at near-ambient conditions. MRS Bull., 2007. 32: p. 1022.
5. Ketteler, G., et al., In situ Spectroscopic Study of the Oxidation and
Reduction of Pd (111). J. Am. Chem. Soc., 2005. 127: p. 18269.
6. Tao, F. and e. al., Reaction Driven Restructuring of Rh-Pd and Pt-Pd Core
Shell Nanoparticles. Science, 2008. 322: p. 932.
7. Tao, F. and e. al., Break-Up of Stepped Platinum Catalyst Surfaces by
High CO Coverage. Science, 2010. 327: p. 850.
8. Teschner, D. and e. al., The Roles of Subsurface Carbon and Hydrogen in
Palladium-Catalyzed Alkyne Hydrogenation. Science, 2008. 320: p. 86.
9. Ghosal, S. and e. al., Electron Spectroscopy of Aqueous Solution
Interfaces Reveals Surface Enhancement of Halides. Science, 2005. 307:
p. 563.
10. Andersson, K. and e. al., Autocatalytic water dissociation on Cu(110) at
near ambient conditions. J. Am. Chem. Soc., 2008. 130: p. 2793.
11. Zhang, C.J., et al., Measuring fundamental properties in operating solid
oxide electrochemical cells by using in situ X-ray photoelectron spec-
troscopy. Nature Materials, 2010. 9: p. 944.
12. Gabaly, F.E., M.E. Grass, and e. al., Measuring individual overpotentials in
an operating solid-oxide electrochemical cell. Phys. Chem. Chem. Phys.,
2010. 12: p. 12138.
13. Tao, F., et al., Surface Structure and Chemistry of Bimetallic
Nanoparticles under Reaction Conditions. J. Am. Chem. Soc., 2010. 132:
p. 8697-8703.
14. Danyal, K., et al., Conformational Gating of Electron Transfer from the
NItrogenase Fe Protein to MoFe Protein. J. Am. Chem. Soc., 2010. 132: p.
6894-6895.
15. Hastings, C.J., et al., Enzymelike Catalysis of the Nazarov Cyclization by
Supramolecular Encapsulation. Journal of the American Chemical
Society, 2010. 132(20): p. 6938-6940.
16. Ojeda, M., et al., CO Activation Pathways and the Mechanism of the
Fischer Tropsch Synthesis. Journal of Catalysis, 2010.272: p. 287.
17. Lichtenberger, J., D. Lee, and E. Iglesia, Catalytic Oxidation of Methanol
on Pd Metal and Oxide Clusters at Near Ambient Temperature. Phys.
Chem. Chem. Phys., 2007. 9: p. 4902.
18. Liu, H. and E. Iglesia, Selective Oxidation of Methanol and Ethanol on
Supported Ruthenium Oxide Clusters at Low Temperatures. J. of Phys.
Chem. B, 2005. 109: p. 2155.
inthismannerprovidinginvaluableinformationforguiding
theidentificationofcatalystpropertiesforsuchhigheffi-
ciencyprocessesasthesplittingofwatertogenerateH2or
thereductionofCO2toproducemethanolandotherfuels.
AnotherexampleisthesplittingofO2moleculesthat
precedesmostoxidationprocesses.Oneimportantstep
hereistheformationofchargedO2-species.Howdoes
O2-formfromO2andhowdoesitevolvetoOatoms?
Howdoestheprocessdependonthesubstrate(e.g.oxide
filmsinoxidationreactions,metalsinthecatalyticpro-
cessofCOandNOoxidation)?CO2andH2Oaresomeof
manystrategicallyimportantmoleculesoflowmolecular
weight.Theyareofgreatinterestinprocesseslikephoto-
synthesis,CO2conversiontousefulchemicals,andwater
photo-splitting.UVlasersorshortsoftX-raypulsescan
beused topopulatespecificLUMOwithelectronsor
HOMOswithholes,followedbyprobingwithXPS,XAS
orRIXS.
Beamlines for Catalysis Research
Time-resolved spectroscopy experiments (XANES,
EXAFS,XES,APPES)oncatalyticsystemswillrelypri-
marily on the seeded NGLS beamlines 1 and 2 as
describedinSection5(Table2).Theseexperimentswill
useone-color(andinsomecasestwo-color)X-rayprobes
tofollowthelocalchemicalenvironment,bonding,and
coordinationattransition-metalL-edges(andK-edgesof
C,O,Netc.)inthesoftX-rayrange.EXAFSprobesoflocal
structuraldynamicswillrelyonhardX-raysatthe3rdand
5thharmonics toprobetransition-metalK-edges.Soft
X-rayRIXSexperimentswillrelyonthehighenergyreso-
lution(andhighaverageflux)ofNGLSbeamline1 in
long-pulseseededoperation.
Diffractiveimagingexperimentswillrelyon“diffract
anddestroy”methodsusingthe3rdand5thharmonics
withthehighestflux/pulseontheseededNGLSbeamline
1at100kHzrepetitionrates,andontheun-seededSASE
beamline3,atMHzrepetitionrates(ashigh-speeddetectors
allow)asdescribedinSections5and6.6.Choiceofwave-
lengthwillbedeterminedbybalancingthescatteringeffi-
ciencyandtherequiredresolutionforparticularsamples.
Akeycomponentoftheseexperimentswillbeahigh-
speedparticleinjectorsynchronizedtotheCWSCRFlinac
(seeSection4.1).
43
3 . SCIENCE DRIVERSNANOSCALE MATERIALS NUCLEATION
Characterizingandcontrollingmatterfarfromequilib-
rium,andachievingsynthesisofmaterialswithtailored
propertiesthroughcontrolatthemolecularlevelaretwo
oftheoutstandinggrandchallengesinmaterialsscience.
Understanding the processes through which crystals
nucleatefromsolutioniscentraltobothchallenges.The
importanceofnucleationphenomenaiswidespread:they
areharnessedtoformallthebiomineralsrequiredbyour
bodies;theyunderpinenvironmentalandatmospheric
phenomena;andtheyenablethesynthesisoftechnologi-
calmaterials includingnanoparticlesandcatalysts.1,2
However,experimental,theoreticalandsimulationmeth-
odsforrevealingthemolecular-scaleprocessesthatcon-
trolnucleationallfacemajorchallenges.Nucleationisan
inherently stochastic process, requiring interactions
amongmanyatomsormolecules,andtheearlieststable
nucleiarehighlytransientobjects,withalargethermody-
namicdrivingforcefor furthergrowth.Consequently,
singlenucleationeventsarerareandhighlytransient.
Theyarevirtuallyimpossibletocaptureeitherinexperi-
mentorrealisticatomisticsimulations.Thislimitsourabil-
ity to control nucleation processes in technological
settings,andinhibitsourabilitytoevenlearntheprinci-
plesfordirectedcrystalformationfromthebestknown
examples—biomineralizingproteins.3
NGLSofferstheopportunitytoperformanewkindof
scientificexperimentthatisdesignedforstudyingrare
eventssuchasnucleationinhomogeneoussolution.The
highrepetitionratewillallowaverylargenumberofindi-
vidualreactorvolumestobeanalyzedatspecifiedtime-
points followingsamplepreparation.X-rayscattering
from individualnanoscalecrystallites,enabledby the
highpulseintensityandfastdetectorreadout,willprovide
atomisticand/ormorphologicalinformationattheearliest
stages of crystal formation and growth. Detection of
amorphousprecipitateswillbeaccomplishedbyaverag-
ing the X-ray exposures creating powder patterns.
Moreover,eachX-raypulsecanbeprecededbyoptical
Understanding the processes by which crystals nucleate from solution is a fundamental challenge in materials science with far reaching relevance. Nucleation phenomena affecting our everyday life are harnessed to form all the biominerals required by our bodies, underpin environmental and atmospheric phenomena, and enable the synthesis of technological materials including nanoparticles and catalysts. Recent discoveries have revealed that nucleation pathways are far more complex than envisioned in classical theories, which also fail to provide an energetic basis for observed nucleation rates. Development of a new predictive theory requires an understand-ing of these pathways. However, nucleation events and the molecular processes that control phase, composi-tion, morphology, and final materials properties are exceedingly difficult to capture experimentally or through simulation.
NGLS will be a new tool for nucleation studies uniquely able to capture the dynamic aspects of spontaneous and directed crystal formation and growth. It will allow a new kind of scientific experiment to be performed that is ideal for studying rare events such as nucleation. Because NGLS will provide intense X-ray pulses at a high rep-etition rate, single-shot diffraction images can be acquired from large numbers of supersaturated droplets, allowing snapshots of the earliest crystal nuclei to be discriminated. Furthermore, this approach will offer unprecedented insight into the dynamic process of biologically directed mineralization, revealing the protein-inorganic bonding interactions that direct the nucleation of amorphous precursor phases and the crystallization of oriented nanocrystals.
3.5 NanoscaleMaterialsNucleation
44
3 . SCIENCE DRIVERSNANOSCALE MATERIALS NUCLEATION
formationofapre-nucleationassociationofionsprecedesa
(ratelimiting)crystallizationstep.Forcrystalformationin
aqueoussolutionsinparticular,thispathwayislikelya
consequence of the complex interactions between
hydratedionsandsolventmoleculesthatmustoccurfor
anorderedcrystaltoform.Bycombiningsingleandaver-
agedsnap-shotdiffractionpatternsacquiredfromsolu-
tionsasafunctionoftime,NGLSwillenabletwo-step
nucleationpathwaystobeidentifiedandquantified.
3.5.2 Statistical Description of Crystal Nuclei Size Distributions during Nucleation and Growth
Followingtheturbulentmixingoftworeactionsolu-
tions,asupersaturatedsolutionwillbetranslatedacross
anNGLSbeameitherasdiscreteliquiddroplets8orasa
streamthroughawindowlessmicrofluidiccell.9Liquid
dropletscanbepreparedwithsubmicrondiameters(see
Section4.1.3),loweringthebackgroundwaterscattering,
whileflowcellsareanticipatedtopermitanalysisof>10µm
fluidchannelsatsub-microsecondtimescalesfollowing
mixing.Conditionswillbevariedsothat(withinPoisson
statisticslimits)eitherzeroorasinglecrystalnucleusis
presentineachanalyzedsolutionvolume.
AsillustratedinFigure25,wewillacquiresingle-pulse
coherentX-raydiffractionpatternsat3.6keVand100kHz.
EachX-raypatternwillbeindividuallyanalyzedtoidentify
andindexBraggreflectionsfromcrystallitesnucleatedin
solution.10EachBraggreflectionfromasmallcrystallite
ismodulatedbytheshapefactorofthecrystallite,provid-
ingapartialdescriptionofthecrystallitemorphology.11
interrogationpulsesdesignedtoprovideadditionalinfor-
mationonaqueouscomplexespresentinsolutionprior
tocrystalnucleusformation.Theseapproacheswillallow
unprecedentedquantitativetestsofmodelsofcrystalfor-
mationthroughhomogeneousnucleationofinorganic,
organicandproteincrystals.
Inaddition,NGLSoffersnewapproachesforacquiring
molecular-scalestructuralinformationfrommacromole-
culesthatcanbeharnessedtorevealthechemicalinter-
actions at protein-mineral interfaces. Biomineral
formationfrequentlytakesplaceuponahighlyorganized
proteinmatrixthatconfersorientationtotheforming
inorganicphase.Whetherintheformoforganizedfibrils,
polymericsheets,orhighly-orderedlattices,itisbelieved
thattheproteinmatrixdefinesthemolecularcontacts
thatleadtocontrolovercrystallographicorientationand
biastheenergylandscapetowardssite-specificnucle-
ation.However,alackofstructuralprobeswithtimereso-
lutioncommensuratewiththecharacteristictimescaleof
nucleationprocesseshasseverelylimitedinvestigations
ofstructuraldynamicsduringnucleationatanorganic
matrix.
3.5.1 Homogeneous Nucleation
Theestablishedframeworkforunderstandingnucle-
ationisClassical Nucleation Theory (CNT),whichcalcu-
latesthethermodynamicstabilityofformingcrystallites
asasumofcontributionsfrombulkandinterfacialfree
energyterms.4,5Themajorpredictionofthismodelis
thatearlycrystallitesareunstablebelowacriticalsize,
andkineticmodelsofnucleationhavesoughttodeter-
minehowratesofdiffusion,dissolutionandgrowthlead
to thesuccessfulappearanceofstable (andgrowing)
nuclei in saturatedsolutions.1,6While the conceptual
basisforthemodelisappropriateformanysystemsits
shortcomings were appreciated from the beginning.
Becausecentralthermodynamicsquantitiessuchasinter-
facialfreeenergyrelyonmacroscopicapproximations,
andkineticparameterssuchasratesofadsorptionordis-
solutionaregenerallyinaccessibletomeasurement,pre-
dictionsofcrystalnucleationrateshaveerredbyorders
ofmagnitudeeveninthemostidealcases.
Furthermore,ithasbecomeclearrecentlythatthere
arealternativepathwaysforcrystalformationbeyondthe
stepwiseadditionofmonomersfromsolution.1,7Figure
24illustratestheTwo-StepNucleationModel,inwhichthe
Classical Nucleation Model
Growth
(a)
(b) (c)
Two-step Nucleation Model
Figure24Scheme of Classical and Two-Step nucleation path-ways. (a) Formation of crystallites through aggregation of mono-mers; (b) Formation of amorphous prenucleation cluster; (c) crys-tallization. (After Erdimir et al.1)
45
3 . SCIENCE DRIVERSNANOSCALE MATERIALS NUCLEATION
thatyielddescriptionsofallparticlesnucleatedinasys-
tem.6,16Todate,quantitativetestsoftheoreticalpredic-
tionshavebeenbasedoncrystallitenumber17oraverage
crystallitesize.18
Forcrystalsystemsthatformfromsolutionviathe
Two-Steppathway,theamorphousprecursorswillnot
yielddetectableBraggpeaks.However,foragiventime-
pointallsingle-pulsepatternsthatyieldnoBraggpeak
willbeaveragedtoproduceapowderdiffractionpattern
inwhichdiffusescatteringringswillbedetectableabove
background.Although individual-particleanalysiswill
notbepossibleforamorphousprecursors,theapproach
willrevealthekineticsofprecursorformationpriortothe
nucleationofindividualcrystallitesthatwillbeidentified
asdescribedabove.Both thermodynamicandkinetic
contributions to the pathway can be addressed. For
example,becausestudiesofthestabilityofnanophase
materialshaverevealedthestrongcontributionofsur-
facefreeenergyoncrystallitestability19weanticipate
thatthenucleationstepishighlysize-dependentandwe
willobserveaminimumsizeatwhichcrystalsfirstappear.
Moreover, numerous materials can be crystallized in
more than one polymorph, and may undergo phase
transformationsduringgrowth.Suchprocesses,andthe
impactsofconditionsandadditivesonpolymorphselec-
tion,canbestudiedandcomplementedbycurrentand
anticipated methods for molecular simulations using
transitionpathsampling.20-22
Ifthesamplecontainsidenticalobjects,theaccumulation
ofX-raydiffractionpatternsatmanyorientationsallows
directreconstructionofthearbitrarymorphologyofthe
object(thelow-resolutionelectrondensitydistribution).It
wasrecentlyshown,however,thatforcrystallitesthatvary
indimensionbutwhichconsistofidenticalunitcells,the
collectionofBraggspotprofilescanbeanalyzedtoreveal
thedistributionofcrystallitedimensions.12Thisapproach
willprovidesignificantlymoreinformationoncrystallite
dimensionsthancanbeobtainedbyconventionalX-ray
methodssuchastheScherrerequationforpowderX-ray
diffraction,13orbyusingsmall-angleX-rayscattering.
TheX-rayscatteringstudiescanbecomplementedby
priorspectroscopicinterrogationusingopticallaserpuls-
es.Forexample,second-ordernonlinearopticalimaging
ofchiralcrystals(SONICC)hasbeendemonstratedforsub-
wavelengthdetectionofproteincrystalsinturbidcrystal-
lizationsolutions.14Fornon-centrosymmetriccrystals
thatexhibitbulksecondharmonicgeneration(SHG)this
approachwillprovideaneasilydiscriminatedsignalindi-
catingthepresenceofacrystalliteintheanalysisvolume.15
NGLSexperimentswillprovideprobabilitydistribu-
tionfunctionsforcrystallitesizeasafunctionofsolution
chemistryandtime,followingtheformationofasuper-
saturatedsolution.Thiswillbethefirstdirectexperimen-
talobservationofthedistributionofcrystallitespresent
in solution during crystal nucleation and will allow
unprecedentedtestsoftheoriesofnucleationandgrowth
t
Droplet ofsupersaturatedsolution
X-ray scattering
Nanocrystal
visX-ray
[3, 1, 0]
[3, -1, 0]
[3, 0, 0]
[4, 0, 0]
[4, -1, 0]
Optical scattering
Figure25Scheme of NGLS experiment in which size and morphological information on single crystallites are obtained by single-pulse coherent X-ray diffraction. Crystallite size and shape modulates the Bragg peak profiles as illustrated by simulated scattering pattern from 17x17x30 unit cell Photosystem I nanocrystal.11 Depending on experimental requirements, optional visible or infra-red analysis pulses can be designed that precede the X-ray diffraction analysis pulse.
46
3 . SCIENCE DRIVERSNANOSCALE MATERIALS NUCLEATION
complexpathinvolvingclustersofyetadifferentstoichi-
ometryfollowedbyACP,followedbyoctacalciumphos-
phate(OCP)andfinallyHAP.25
Despite thetinydimensionsandhighly-anisotropic
propertiesoftheHAPnanocrystals,thehierarchicalorga-
nizationofthecollagenmonomersintohelices,helices
intofibrils,fibrilsintobundles,andbundlesintomacro-
scopicboneresultsinamaterialwithnearlyisotropic
mechanicalpropertiesandremarkablefracturetough-
ness.Unfortunately,themolecularcontactsthatdirect
thelocationandorientationofthecrystalnucleusandthe
pathwayofphaseandcompositionalevolutionthatleads
tothefinalcrystallineproductremainamystery.
3 .5 .3 .1 Protein-Directed Hydroxyapatite Nucleation at
Collagen Crystal Surfaces
Wewillacquiremultiplesingle-pulsecoherentX-ray
diffractionpatternsofcollagenfibrilsinliquiddropsthat
areeithersupersaturatedrelativetohydroxyapatite(HA)
formationorwhicharephosphate-free.Byfirstaveraging
singleX-rayexposuresofnon-mineralizedcollagenwe
willobtainalow-resolution
electrondensitymapofthe
fibrilsundertheexperimen-
talconditionsagainstwhich
the known amino acid
sequence can be aligned.
Thiswillprovideanatomis-
tic depiction of the nucle-
ationsitesinthefibrilsthat
directHAnucleation.
3.5.3 Biologically-Directed Heterogeneous Nucleation
Livingsystemsprovideexquisiteexamplesofmateri-
alssynthesiswithtailoredpropertiesviamolecular-level
control. Biomineralization, in particular, is a process
throughwhich livingorganismsproducematerials to
solvefunctionalrequirementsbyexertingmolecular-level
controloverinorganiccrystalgrowth.Hereorganicmatri-
cesactastemplatestodirectthenucleationstage.Non-
equilibriumphasesarestabilizedbytheintroductionof
soluble macromolecules that modulate atomic-scale
growthkinetics.Thisenableslivingorganismstoproduce
awidevarietyofcrystallinenanostructureswithfunc-
tionsasdiverseaslightharvesting,magneticsensing,
andmechanicalsupport.3,23
Among the myriad biomineral systems found in
nature,mineralizationofcollagenousproteinsbycalcium
phosphatesisoneofthemostimportant.Itcomprisesthe
skeletalanddentalstructureofhigherorganismsand
presentsanexquisiteexampleofanorganizedprotein
matrix,highlydirectednucleation,andanevolvingmin-
eralnucleus.23,24Attheshortestlengthscale,thecolla-
genmonomerformtriplehelicesthatstackparalleltoone
anotherwithastaggeredgeometry thatcreatesaso-
called“hole zone” (Figure 26). Nanocrystal plates of
hydroxyapatite(HAP,themoststablecalciumphosphate
phase)afew10sofnmacrossand<5nminthickness
form within these hole zones.The formation of HAP
appearstobeprecededbydepositionofamorphouscal-
ciumphosphate(ACP)nanoparticlesofadistinctstoichi-
ometry.Moreover,invitroexperimentssuggestanvery
a b c dHole zone
D ~ 67nm
D ~67nm
250 nm
Figure26Depictions of the Type I Collagen crystals that direct the nucleation of hydroxyapatite at “hole zone” regions between C- and N-terminus protein regions. (a) Atomic force microscopy (AFM) image of aligned collagen fibrils exhibiting the periodic bands (Tao, DeYoreo unpublished). (b) AFM image of single fibril.26 (c) Low-resolution electron density map of collagen helix obtained from single crys-tal diffraction.27 (d) Reconstructed collagen structure revealing amino acid sequence.
Fluctuation X-ray scattering
Giga-shot diffractive imaging
see Section 4.1
47
3 . SCIENCE DRIVERSNANOSCALE MATERIALS NUCLEATION
SCRFlinac(seeSection4.1.3),provideforsamplereplace-
mentonapulse-by-pulsebasis.
References:
1. Erdemir, D., A.Y. Lee, and A.S. Myserson, Nucleations of Crystals from
Solution: Classical and Two-Step Models. Accounts of Chemical
Research, 2009. 42(5): p. 621-629.
2. Laaksonen, A., V. Talanquer, and D.W. Oxtoby, Nucleation —
Measurements, theory and atmospheric applications. Annual Review of
Physical Chemistry, 1995. 46: p. 489-524.
3. Dove, P.M., J.J. DeYoreo, and S. Weiner, Biomineralization. Reviews in
Mineralogy and Geochemistry, ed. J.J. Rosso. Vol. 54. 2003, Virginia:
Mineralogical Society of America.
4. Oxtoby, D.W., Homogeneous nuceation — theory and experiment.
Journal of Physics-Condensed Matter, 1992.4(38): p. 7627-7650.
5. Gunton, J.D., Homogeneous Nucleation. Journal of Statistical Physics,
1999. 95(5/6): p. 903-923.
6. Kelton, K.F., A.L. Greer, and C.V. Thompson, Transient nucleation in con-
densed systems. Journal of Chemical Physics, 1983. 79(12): p. 6261-6276.
7. Gebauer, D., A. Völkel, and H. Cölfen, Stable prenucleation calcium car-
bonate clusters. Science, 2008. 322: p. 1819-1822.
8. Weierstall, U., et al., Droplet streams for serial crystallography of pro-
teins. Experiments in Fluids, 2008. 44(5): p. 675-689.
9. Vig, A.L., et al., Windowless microfluidic platform based on capillary
burst valves for high intensity x-ray measurements. Review of Scientific
Instruments, 2009. 80: p. 115114.
10. Leslie, A.G.W., The integration of macromolecular diffraction data. Acta
Crystallographica Section D, 2006. 62: p. 48-57.
11. Kirian, R.A., et al., Structure factor analysis of femtosecond microdiffrac-
tion patterns from protein nanocrystals. Acta Crystallographica Section
A, 2010.inpress.
12. Spence, J.H.C., et al., Ab-initio phasing of femtosecond diffraction from
many nanocrystals. submitted to Physical Review Letters, 2010.
13. Kazanci, M., et al., Complementary information on in vitro conversion of
amorphous (precursor) calcium phosphate to hydroxyapatite from
Raman microspectroscopy and wide-angle X-ray scattering. Calcified
Tissue International, 2006. 79: p. 354-359.
14. Wampler, R.D., et al., Selective Detection of Protein Crystals by Second
Harmonic Microscopy. Journal of the American Chemical Society, 2008.
130(43): p. 14076-14077.
15. Kissick, D.J., et al., Nonlinear Optical Imaging of Integral Membrane
Protein Crystals in Lipidic Mesophases. Analytical Chemistry, 2010. 82(2):
p. 491-497.
16. Noguera, C., et al., Nucleation, growth and ageing scenarios in closed
systems I: A unified mathematical framework for precipitation, conden-
sation and crystallization. Journal of Crystal Growth, 2006. 297: p. 180-186.
Under conditions for which HA nucleation is just
beginning,wewillacquireandaveragemultipleX-ray
exposurestorefineamodeloftheHA—collageninter-
face.Becausethecollagensubstrateimpartsaconsistent
crystallographicorientationto theHAcrystallites it is
expectedthattherewillbeaninterfacialstructurethatis
conservedatallnucleationsites.Fibriltwistingmaylimit
the resolutionofelectrondensitymapsderived from
scatteringdata,sothiswillbelimitedbypreparingthe
shortestcollagenfibers.However,becausethesequence
andstructureof theorganicsubstrate isknown,data
refinementcanincorporatemodelsoftorsionaldisorder.
Thisapproachwillenabletheaccumulationofinorganic
ionsinthegapregionstobevisualizedasabuild-upof
excesselectrondensityandthusidentifywhetherthefor-
mationofadisorderedprenucleationclusterprecedes
crystallization.Themorphologyofbothamorphousand
crystallineHAstructureswillbefollowedwithtime,offering
unprecedentedinsightsintothedynamicprocessofbio-
logically-directed mineralization.The protein-crystal
bondingwillbeestablishedeitherdirectlyfromelectron
densitymaps,orbyaligningcollagen-HAmodelstolower-
resolutiondata.
Thereisnoprecedentforsuchaneffort.Z-contrast
electrontomographyTEMhasrevealedbonemorphology
atfew-nmresolution,butwithnoreconstructionofpro-
teinstructureorchemistry.28Theproposedsingle-pulse
X-raymethod,avoidingtheenormousradiationdamage
associatedwithelectrondiffraction,hasthebestpotential
forobservinghowprotein-mineral interactionsguide
dynamicmineralizingprocessesofenormousmedical
significance.
Beamlines for Nanomaterials Nucleation Research
Materialsnucleationexperimentswillrelyon“diffract
anddestroy”methodsusingthe3rdand5thharmonics
withthehighestflux/pulseontheseededNGLSbeamline
1at100kHzrepetitionrates,andontheun-seededSASE
beamline3,atMHzrepetitionrates(ashigh-speeddetec-
torsallow)asdescribedinSection5and6.6.Choiceof
wavelengthwillbedeterminedbybalancingthescatter-
ingefficiencyandtherequiredresolutionforparticular
samples.Akeycomponentoftheseexperimentswillbea
high-speedliquiddropletinjectorsynchronizedtotheCW
48
3 . SCIENCE DRIVERSNANOSCALE MATERIALS NUCLEATION
23. Mann, S., Biomineralization: Principles and concepts in bioinorganic
materials chemistry. 2001, New York: Oxford University Press.
24. Weiner, S., W. Traub, and H.D. Wagner, Lamellar bone: Structure-
function relations. Journal of Structural Biology, 1999. 126(3): p. 241-255.
25. Habraken, W.J.E.M., et al., The role and composition of calcium phos-
phate prenucleation clusters. submitted, 2010.
26. Cisneros, D.A., et al., Creating ultrathin nanoscopic collagen matrices for
biological and biotechnological applications. Small, 2007. 3(6): p. 956-963.
27. Orgel, J., et al., The in situ supermolecular structure of type I collagen.
Structure, 2001. 9(11): p. 1061-1069.
28. Grandfield, K., et al., Visualizing biointerfaces in three dimensions: elec-
tron tomography of the bone-hydroxyapatite interface. Journal of the
Royal Society Interface, 2010. 7(51): p. 1497-1501.
17. Kelton, K.F. and A.L. Greer, Test of classical nucleation theory in a con-
densed system. Physical Review B, 1988. 38: p. 10089-10092.
18. Viswanatha, R., et al., Growth mechanism of nanocrystals in solution:
ZnO, a case study. Physical Review Letters, 2007. 98(25).
19. McHale, J.M., et al., Surface energies and thermodynamic phase stability in
nanocrystalline aluminas. Science, 1997. 277(5327): p. 788-791.
20. Auer, S. and D. Frenkel, Quantitative prediction of crystal-nucleation
rates for spherical colloids: A computational approach. Annual Review
of Physical Chemistry, 2004. 55: p. 333-361.
21. Desgranges, C. and J. Delhommelle, Molecular simulation of the
Nucleation and Growth of Gold Nanoparticles. Journal of Physical
Chemistry C, 2009. 113: p. 3607-3611.
22. ten Wolde, P.R. and D. Frenkel, Computer simulation study of gas-liquid
nucleation in a Lennard-Jones system. Journal of Chemical Physics,
1998. 109(22): p. 9901-9918.
49
3 . SCIENCE DRIVERSDYNAMIC NANOSCALE HETEROGENEITY IN MATERIALS
Avarietyofnanostructuredsystemsexhibitdynamical
heterogeneitydriveneitherthermallyorbyadiabaticvari-
ationofexternalfields,includingmolecularswitches,1,2
polymer melts,3 colloidal suspensions,4 magnetic
domains,5-14 single-center fluorophores,15,16 biopoly-
mers,17-20andchargeandorbitaldomainsincomplex
oxides.21-24Animportantchallengeinstudyingsuchsys-
temsistheneedtoprobeaverybroadrangeoftemporal
andspatialscales.Incomplexmaterials,spontaneous
fluctuationsofelectronandspinorderingwillstartto
dominateatnanometerlengthscales,andmayprovide
aninherentlowersizelimitfordevices.Relevantmodes
inproteinfoldingandfunction,forexample,spanfrom
molecular-scalevibrationsatTHzfrequenciestomacro-
molecular-scalelibrationsonthescaleof1Hz.Dynamical
heterogeneityisoftenassociatedwithfastorultrafast
intermittentnanoscaleevents thatspawnstatistically
self-similarspatialand/ortemporalstructures.Powerlaw
dependenciesandtheabsenceofcharacteristiclength
andtimescalesareofcentralimportanceinunderstand-
ingtheemergingmacroscopicproperties,thoughthis
connectionisrarelyunderstoodindetail.
Dynamical nanoscale heterogeneity impacts a multitude of important processes, ranging from protein librations that are crucial to biological function, to superparamagnetic fluctuations that limit the density of information that can be stored on a hard drive. An important aspect of such processes is the way they connect thermally-driven ultrafast events on the nanoscale with kinetic phenomena on the macroscale. This connection has been intensely studied for many decades and in many different contexts, since it governs the emergence of complex material properties from simple microscopic interactions.
NGLS will revolutionize the ability to probe emergent phenomena through its sensitivity to very broad ranges of time and length scales, in combination with its incisive X-ray contrast mechanisms. Highly coherent X-ray pulses will enable ‘probe-probe’ correlation spectroscopy measurements of spontaneous ultrafast processes. For example, thermally driven spin flips or polaron motion in a transition metal oxide will be probed on the relevant nanometer length scale. Closely related time-series correlation spectroscopy measurements will probe longer length and time scales in these systems, where fast events cross over to domain wall or microphase boundary motion. The richness of modern nanoscience, as manifested uniformly in physical, chemical and biological materials, begs for the very diverse array of X-ray tools provided by NGLS.
Length (nm)
Wavevector (Å-1)
106 104 102 100
100
100
105
1010
1015
10-5
10-10
10-15
10-6 10-4 10-2 100
Ener
gy (e
V)
Freq
uenc
y (H
z)
XPCS NGLS
VisibleRaman
Scattering
VisibleBrillouin
Scattering
VisiblePhoton
CorrelationSpectroscopy
XXPPPCCCSSS3rd Gennneerratioon
Soouurrcceces
Inelasticcc SScatt.:X-ray
IInnnnnInI elae sticc nn eeutronn
scattt.
NNeutroonsssppiin echho
Figure27Time and length scales accessible by various experi-mental techniques. X-ray photon correlation spectroscopy operates in a key area not accessible by other techniques. NGLS will open up new frontiers in correlation spectroscopy by allowing to probe materials over a broad range of length and time scales.
3.6 DynamicalNanoscaleHeterogeneityinMaterials
50
3 . SCIENCE DRIVERSDYNAMIC NANOSCALE HETEROGENEITY IN MATERIALS
estsofbasicsciencewiththeneedsofemergingclean,
efficient,andaffordabletechnologies.
Understandinghowtopredictandcontrolnanoscale
dynamicalheterogeneitywillrequirerevolutionarynew
toolslikeX-raylasers.Simplystated,thehighcoherence
ofanX-raylaserwillallowtheseincoherentlydrivenpro-
cessestobeprobedinunprecedenteddetailbyproject-
ingtheirinherentcomplexityintofar-fieldspeckle-diffraction
patterns,asshownschematicallyinFigure29.Withultra-
fastpulsesathighrepetitionrates,asequenceofsuch
patternscanbecollectedandanalyzedtodeterminethe
underlyingfluctuationsoveranunprecedentedrangeof
spaceandtimescales(Figure27).Ifthesystemisstaticat
thelengthscaleprobed,specklepatternscollectedatdif-
ferent timeswillbe identical,andperfectlycorrelated
witheachother.Ifthesystemisnotstatic,thespecklepat-
ternswilldecorrelate.Measuringthatdecorrelationreveals
thedynamicsthroughtheintermediatescatteringfunction
S(q,t),whichistheFouriertransformofthedynamical
structurefactorS(q,ω).Thisapproachisadirectdescen-
dentofconventionaldynamiclaserlightscattering,28-30
though with high enough spatial resolution to probe
nanoscalephenomena,andwiththepowerfulcharge,
magnetic,andorbitalcontrastmechanismsendemicto
X-raytechniques.Muchprogressinthedevelopmentof
thistechnique,(typicallyreferredtoasX-rayphotoncor-
relationspectroscopy,XPCS),hasbeenachievedoverthe
pastdecadebyspatiallyfiltering(attremendouslossof
flux)thepartiallycoherentradiationfrom3rdgeneration
synchrotronfacilities.31-38Thecoherentfluxofspontane-
ousundulatorradiationislowandthissignificantlylimits
Nanoscaledynamicalheterogeneitywillposesignifi-
cantchallengesindevelopingthecomplexmaterialsfor
next-generationnano-devices.This isalreadyamajor
issueinmagneticrecordingtechnologies,wherethermally
drivensuperparamagnetic fluctuationsdetermine the
ultimatestoragedensitythatcanbeachieved.25-27Future
deviceswillhavetoaddressissuessuchasfluctuations
thatmaylimittheperformanceofmagneticread-heads
basedoncolossalmagnetoresistanceinaspin-andorbital-
orderedmanganitematerial(depictedschematicallyin
Figure28).Conversely,understandinghowtocontrol
nanoscaleheterogeneityisanimportantcomplementto
optimizingdrivennanoscaledynamicsusing,forexam-
ple,pump-probetechniquesdescribedelsewhereinthis
proposal:thesearefastbecomingtheYinandtheYangof
nanotechnology.The X-ray correlation spectroscopy
experimentsdescribedbelow,enabledbythecababilities
ofNGLSX-raylasers,willilluminatenanoscaleordering
andfluctuationphenomenon,therebymeldingtheinter-
Magneticnanobits
Readdevice
Spin-orbitorderedmaterial
Active layer:complexmaterial
Figure28A future generation read-device. It uses complex mate-rials as active layer. Exploitation and engineering of lattice, elec-tronic and spin coupling makes the device ultra small and ultra fast.
Incoming X-rays
Illuminated area
CCD camera
qx, qzFrame 1 2 3 4 N
Magnetic sample yx
z
8
8
d
Pinhole
Figure29A series of ultrafast scattering snapshots are taken. For a static system, the speckle pattern on the detector remains the same, and individ-ual snapshots are correlated. For a dynamical system, the speckle pattern will change, and measuring the time constant for the decorrelation gives information about the intermediate scattering function S(q,t), which is the Fourier transform of the dynamical structure factor S(q,ω).
51
3 . SCIENCE DRIVERSDYNAMIC NANOSCALE HETEROGENEITY IN MATERIALS
3.6.2 Specific Examples
Example 1: Correlation Spectroscopy Experiments in
Complex Oxides
Anoutstandingfundamentalquestionincomplexcor-
relatedmaterialsishowthevariouslowenergydegrees
offreedomarecoupled.Forexample,abilayermanga-
nitesimultaneouslyshowssomecombinationofspin,
charge,lattice,andorbitalorder.Whatarethetemporal
andspatialbehaviorsofthecollectivemodesforthese
competingorders?Howaretheseorderparameterscou-
pled?Theseordersgenerallyoccuratdifferentwavevec-
tors. Determining the thermally driven equilibrium
dynamicsatthesevectorswillilluminatethenatureofthe
fluctuatingdegreesoffreedomandthecouplingbetween
them.Inthecaseofsimplecontinuoustransitions,the
dynamicsofthevariousorderparameterscanbedeter-
minedandextrapolatedtothecriticalpointtoprobefun-
damental exponents and self-similar behaviors.The
experiment canbe repeatedatdifferentwavelengths
(e.g.,MnL-edgeandOK-edge)toprobeelementspecific
fluctuationinformation,assuggestedinFigure30.
the spatial and temporal dynamic range that can be
achieved,asshowninFigure27.Thehighcoherentpower
of4thgenerationsourcesalongwithdevelopmentoffast
2Ddetectorswilldramaticallyexpandthedynamicrange
ofXPCSinbothspaceandtime.
3.6.1 Science Case: Nanoscale Fluctuation And Dynamical Heterogeneity
Critical Gap
Theequilibriumkineticanddynamicalphenomenaof
acomplexsystemontimescalesrangingfrommillisec-
ondstopicoseconds,andonlengthscalesfromanano-
metertoamicronremainlargelyunexplored.Studiesof
dynamicalheterogeneityofthermallyandadiabatically
drivenspontaneousfluctuationsarefundamentallydif-
ferentfromtriggeredprocessesstudiedinultrafastpump-
probemeasurements.X-ray correlationspectroscopy
probesthese‘inherent’orspontaneousdynamicsontheall-
importantnanometerandlongerlengthscalewheremac-
roscopicpropertiesbegintoemerge.Nearacriticalpoint,
forexample,thermallydrivenequilibriumfluctuations
becomesignificantandleadtoself-similarbehaviorsin
spaceandtimethatproduceremarkablepropertieslike
criticalopalescence.Nanoscaleheterogeneity(notalways
nearcriticalpoints)ismanifestincomplexsystemsranging
fromproteinfunctiontodomainstructuresincomplex
magneticandsuperconductingoxides.Inthesesystems,
intermittentbehaviors(someofwhichexhibitself-simi-
larity)oftenspanbothnanometerandmicrometerlength
scalesandultrafastdynamicalandslowkinetictimescales.
Akeyfeatureofnanoscienceingeneral,istheduality
betweenreal-andmomentum-spaceproperties.NGLS
X-raylaserswillallowustocombineholographywith
correlationspectroscopytoprobethisdualityinarevolu-
tionaryway.Thiswillleadforexampletomoviesofthe
formationandevolutionofasingledomainwallnearan
orderingtransitionwithnano-topicosecondtimeresolu-
tionorofthermalfluctuationsthataretheanalogofcriti-
calopalescence.Butsuchmoviescouldalsobeanalyzed
statisticallytodeterminethespace-timecorrelationfunc-
tionG(r,t),whichistheFouriertransformofS(q,ω),sothat
nanostructuresandraredynamicaleventscanbeprobed
onthesamefootingasstatisticalpropertiesthatarecon-
nectedtousefulmacroscopicproperties.Thismeldingof
real-spacewithmomentum-spacesensitivitieswillbea
keyfeatureofmanycoherence-basedexperimentsatNGLS.
X-ray beam
CCD at OO peak
CCD at AF peak
Complexoxide device
Figure30Illustration of an experimental set up for dynamical pho-ton correlation spectroscopy at the antiferromagnetic (AF) and orbital-order (OO) Bragg peaks. The above figure shows a part of the speckle pattern obtained for a Pr0.5Ca0.5MnO3 OO peak and a bilayer AF peak. The important point is that the peaks have differ-ent ordering temperature and spatial correlation length. It is con-ceivable that the electronic order peak will have a different fluctu-ation time scale than the magnetic order peak. Probing such inher-ent electronic and magnetic fluctuation will provide insight into the spin-lattice coupling mechanism that forms the basis of correlated effects in complex oxide systems.
52
3 . SCIENCE DRIVERSDYNAMIC NANOSCALE HETEROGENEITY IN MATERIALS
interactionoffluxlines,pinningandde-pinningeffectsas
wellasspeedandcharacteristicsof flux linemotion.
Understandingthevortexdynamicsthroughcorrelation
spectroscopywillhelpin‘pinscapeengineering’ofvorti-
ces,whichisanessentialandintegralparttomanufac-
turenextgenerationsuperconductingwires.Combining
real- andmomentum-spaceapproachesprovides tre-
mendousadvantages,inparticular,imaginginrealtime
thepinninganddepinningofvariousvortexlineswhile
alsomeasuringthecorrelationfunctionsthatdetermine
macroscopicproperties.
Example 3: Ultrasoft Modes in Complex Materials
Thereisenormousinterestincharacterizingthelow-
energymodesofbroadclassesofcomplexmaterials,
includingtwo-leveltunnelingcentersthatareubiquitous
inmostglasses,spin-andcharge-densitywavedynam-
icsinlayeredmaterials,and‘orbitalwaves’incomplex
oxides.Inmanyexperimentstodate,thedynamicsof
Example 2: Vortex Dynamics in a Superconductor
Sincethediscoveryofhightemperaturesuperconduc-
tivity,thepromiseofzeroresistancedevicesforelectric
powerapplicationsuchasgenerators,motors,andtrans-
missionlinesoperatingnear liquidnitrogentempera-
tures,hasfueledintense,worldwideresearchefforts.39
Powerapplicationssharethecommonrequirementthat
thesuperconductorhastobeabletocarrylargecurrent
densitiesinthepresenceofstrongmagneticfieldscon-
sistingoftheself-fieldofthetransportcurrentandexter-
nal fields present in motors and generators. In the
presenceofanappliedmagneticfield,typeIIsupercon-
ductorsarepermeatedbyquantizedvorticesofmagnetic
fluxasshowninFigure31(top).Themagneticinduction
inthesurroundingsuperconductingmaterialiszero.
Whena supercurrent flows, there isdissipationof
energyunlessthesevorticesare‘pinned’insomeway,as
tobeinhibitedfrommovingundertheinfluenceofthe
Lorentzforce.Onlyifthefluxlinescanbeimmobilized
willthesuperconductorsustainthehighcurrentdensities
necessaryforpracticalapplications.Thisso-calledflux
pinningarisesfromthepresenceoflocalizeddefectsor
crystallineimperfectionsthatreducetheenergyofaflux
linesuchthatittendstoremainpinnedatthislocation.
Hence,optimizingthesuperconductorforpowerapplica-
tionsinvolvesmakingthesuperconductingmaterialless
perfectbyinducingsuitabledefectsforpinningmagnetic
flux lines.This requires identifying theelectronicand
magneticpropertiesaswellasstructuralcharacteristics
ofthepinningcenters.Further,thenormalconducting
fluxlinecorehasaradiuscorrespondingtothecoher-
encelengthofthesuperconductor,ξ,whilethedimen-
sionsofthesupercurrentvorticesisdeterminedbythe
Londonpenetratingdepth,λ.ξandλarematerialdepen-
dentandthe1to100nmrange,theideallengthscalefor
softX-raytechniques.
Thedynamicsofthefluxlinelatticeinthepresenceof
anexternal fieldand transport current, aswell as its
responsetoexternalexcitations,needstobestudiedto
determinetheinteractionbetweenfluxlinesandflux-pin-
ningdefects.XMCDspectroscopyhasbeendemonstrat-
edtobeaneffectivemeanstoidentifythevorticies(as
illustratedinFigure31,bottom).Thus,XPCSstudiesat
theCuL-edge(exploitingthestrongXMCDdifferential
scatteringandabsorptioncross-sections)willbeapower-
fulprobeofthevortexstatedynamicsonthenanoscale.
Thesestudieswillprovidenovelinformationaboutthe
XMCD
(arb
. uni
ts)
XA(a
rb. u
nits
) 1.0
0.5
0.0
0.00
-0.05
930 940Photon energy (eV)
950
YBaCuO
T = 20KH = ±9T
Figure31Top: Illustration of fluxoids (red) surrounded by current vortices (green) in a type II superconductor exposed to a magnetic field (blue) (figure courtesy J. Hoffman, Harvard). By manipulating the applied magnetic field, magnetic vortices form in a Type II superconductor. When a current is applied, the Lorentz force drives dynamic behavior of the magnetic vortices. This movement dissipates energy and produces resistance thereby limiting the maximum current that can flow through the superconducting wire. Bottom: X-ray magnetic circular dichroism (XMCD) signal at the Cu L3,2 edges of YBaCuO measured in external fields of 9 T and at T = 20 K. (Figure courtesy E. Arenholz)
53
3 . SCIENCE DRIVERSDYNAMIC NANOSCALE HETEROGENEITY IN MATERIALS
volumeσ.Thus,foraminimumsignal-to-noiseratio,the
shortestaccessibletimeinterval(measurablecorrelation
time)ΔtscalesasB-2.RecentstudiesattheALS,forexam-
ple,showthatdynamicsofnanoscaleorbitaldomainsin
acomplexoxidecanbeprobedonatimescaleof~1sec-
ond.42ThehigherbrightnessoftheNSLS-IIwillimprove
thetimeresolutioninsuchexperimentstotensofmilli-
seconds.Accessingthemicrosecondandnanosecond
timescalesthatarerelevantforfunctionaldeviceswill
requireafully-coherent,high-repetitionratesourcepro-
vidingafactorof103orgreaterimprovementinaverage
brightness. Reaching the fundamental time scales of
vibrationaldynamics,charge-transfer,andchargecorre-
lationwillrequireanultrafastsoftXraylaserincombina-
tionwithtwo-pulse(splitanddelay)probingasdescribed
belowtoestablishthecorrelationtimeofthespecklepat-
tern.Highrepetitionrateisindispensibletoensurethat
thefluxperpulseremainswithintolerablelimitswhile
maintainingthehighaverageflux.
3 .6 .3 .1 Time Series Technique: 10 ns — Many Second
Time Scale Dynamics
The‘sequentialmode’followstheprotocoldescribed
above:atemporalautocorrelationfunctiong2(q,t)ispro-
duced froma timedsequenceof specklepatterns,as
showninFigure29.Thepresentlyaccessibletemporal
rangeinthisapproachislimitedbytherepetitionrateof
presentX-rayFELs.IntheSASEmode,NGLSwilloperate
initiallyat1MHz(withanupgradepathto100MHz),dra-
maticallyexpandingtherangeofexperimentsthatcanbe
performed.The high NGLS repetition rate will make
experimentsonsystemswithverylowscatteringcontrast
possibleinboththesequentialandthesplitanddelay
modesdiscussedbelow.ThehighNGLSrepetitionrate
will also open for study the important time regime
between ~100 ns and ~30 ms.This will not be easily
probedatotherfacilitiesbecausedelaylineslongerthan
~100nsbecomeunmanageable inthesplitanddelay
approach.Also,theminimumtimescaleaccessibleinthe
sequentialXPCSapproachisdeterminedbythesource
repetitionrate.
3 .6 .3 .2 Split-Pulse Delay Line Technique:
Sub-Picosecond to Nanosecond Dynamics
Theultrafast‘split-pulsemode40,41forXPCSrelieson
superimposedpairsofspecklepatternscollectedwith
time-delayedX-raypulses.Thedecayinspecklefringe
thesemodesareprobedintheenergydomainusingquasi-
elasticscatteringofneutronsorphotons:theycansome-
timesalsobeobservedinpump-probeexperiments.The
fulltransverseandlongitudinalcoherenceandthehigh
repetitionrateofNGLSX-raylaserswillrevolutionize
suchstudiessincewewillbeabletoprobethermally-
driven(spontaneous)modesinthetimedomainwithcor-
respondingly ultrahigh energy resolution. Using the
split-pulsetechniquedescribedbelow,wewillbeableto
studynanoscalefluctuationsonafstonstimescale,cor-
responding to an the energy regime spanning 1 eV
through1μeV,andlimitedonlybythetimedelayavail-
able.Suchstudieswilllieattheforefrontofquasielastic
X-rayscatteringandwillcomplementrelatedneutron
scatteringstudieswiththeabilitytoprobesmallsamples
withthemultitudeofX-rayspectralcontrastmechanisms.
Suchexperimentswillbeenabledbythepropertiesof
NGLSandwillalsorelyonsignificantadvancesinour
abilitytomanipulatesoftX-raybeams.Inadditiontothe
needforasplitanddelaylinediscussedbelow,thefull
longitudinalcoherencewillallowheterodynedetection
experimentsthatwillconnecttime-domainXPCSexperi-
mentstoemergingveryhighresolutioninelasticX-ray
scattering techniques.Heterodynedetection isnearly
alwaysincorporatedindynamiclaserlightscatteringin
theopticalregimesinceitallowsdirectmeasurementof
the field-fieldcorrelation function,g1(q,t). In turn, for
under-damped,propagatingmodeslikethosediscussed
above,g1(q,t)providesameasure,throughFouriertrans-
formation,ofω(q)aswellasthemodedamping.
3.6.3 New Technical Capabilities
Techniquesnowunderdevelopmentatfirst-genera-
tionX-rayFEL’swillprovide the foundation formajor
advancesinXPCSresearchinthenextfewyears.40,41
Thesetechniques,combinedwiththeveryhighrepetition
rateofNGLSX-raylaserswillopenforstudyentirelynew
timeregimesofdynamicnanoscaleheterogeneitythat
arenotaccessiblewithpresentsynchrotronorlowrep-
rateX-rayFELsources.
Atpresent3rdgenerationlightsources,SXPCScapa-
bilitiesareseverelylimitedduetothelowcoherentflux
available,andthesubtletyoftheunderlyingspectralcon-
trastmechanisms.Thesignal-to-noiseratioofsuchexper-
iments scalesas t1/2Bσwith the sampling time t, the
sourcebrightnessB,andthescatteringcross-sectionper
54
3 . SCIENCE DRIVERSDYNAMIC NANOSCALE HETEROGENEITY IN MATERIALS
Beamlines for Investigating Dynamic Nanoscale Heterogeneity in Materials
Studiesofdynamicnanoscaleheterogeneityusing
SXPCSwillrelyonthehighaveragefluxprovidedbythe
un-seededNGLSbeamline3,providingpulseswith~0.5µm
naturalcoherence length,at1MHzrepetitionrate,as
describedinSection5(Table2).Experimentsrequiring
longercoherencelength(narrowbandwidth)willexploit
theseededbeamline1operatingat100kHz(oralterna-
tively employ a monochromator on beamline 3).
Importantrequiredcapabilitiesaretunabilityacrossthe
transition-metalL-edgesinthesoftX-rayrangeandpolar-
izationcontrol.
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3. Russell, E.V. and N.E. Israeloff, Direct observation of molecular coopera-
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9. Perkovic, O., K. Dahmen, and J.P. Sethna, Avalanches, Barkhausen
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11. Spasojevic, D., et al., Barkhausen noise: elementary signals, power laws,
and scaling relations. Phys. Rev. E, 1996. 54: p. 2531.
12. Urbach, J.S., R.C. Madison, and J.T. Markert, Interface depinning, self-
organized criticality, and the Barkhausen effect. Phys. Rev. Lett., 1995.
75: p. 276-279.
13. Vazquez, O. and O. Sotolongo-Costa, Dynamics of a domain wall in soft-
magnetic materials: Barkhausen effect and relation with sandpile mod-
els. Phys. Rev. Lett., 2000. 84: p. 1316-1319.
visibiltyisdrivenbythermallymediateddecorrelation
occuringinthetimeintervalbetweenthetwopulsesand
canberelatedtog2(q,t).Thisapproachwillprobeultra-
fast,thermallydrivendynamicsforthefirsttime,withatime
resolutionlimitedonlybytheX-raydelayline.Anexample
ofsuchanexperimentatNGLSisthedynamicformationof
magneticdomainsfromaparamagneticstate.Asthesys-
tempassesthroughtheNeeltemperature,thespinsstartto
orderandfluctuateintheformof‘spindroplets’.Thespin
fluctuationisveryfast,butasthetemperatureisfurther
loweredthedomainsstarttoforminordertominimize
theenergy.Thesplitpulsemodewillallowthetemporaland
spatialdependenceofthesespindropletstobemeasured.
3.6.4 Sample Damage and Modification at 4th Generation Light Sources:
Sampledamage/modificationduetotheintenseFEL
pulsesaresignificantissuesthathavebeenextensively
considered, and are discussed briefly inAppendix 1.
These issuesareparticularly relevant in thesplitand
delayapproach,wherethefirstX-raypulseinapaircan-
notdisruptthephenomenabeingmeasuredbythesec-
ondpulse.Thecapabilitytousemoderatepeak-power
pulses,whilemaintaininghighaveragepowerviahigh-
repetition ratewill beessential.The smaller inelastic
extinctionlengthwillmaketheproblemmoreserious
sincethepulseenergywillbedepositedinasmallervol-
ume.However,thisdecreasedextinctionlengthispartially
compensatedforbythelowerenergyperphoton.Itis
becomingincreasingclearthatonegetsthebestcontrast
indiffractiveimagingexperimentsbyusingawavelength
thatisclosertothedesiredresolutionandnotarbitrarily
shortwavelengthstoavoidradiationdamage.Theimpor-
tanceof limitingsampledamageordisruptionof the
statesbeingmeasuredmeansthatthenumberofpho-
tons/pulsethatcanbeeffectivelyusedwillbecomparable
atallFELfacilities.ThehigherNGLSrepetitionrate—up
toamilliontimeshigherthantheLCLSinNGLSSASE
mode—willmakeexperimentsonsystemswithverylow
scatteringcontrastpossible,inboththesequentialand
thesplitanddelaymodes.
55
3 . SCIENCE DRIVERSDYNAMIC NANOSCALE HETEROGENEITY IN MATERIALS
28. Berne, B.J. and R. Pecora, Dynamic Light Scattering. 1976, New York:
Wiley.
29. Chu, B., Dynamic Laser Light Scattering. 1991, San Diego: Academic
Press.
30. Schmitz, K.S., Dynamic Light Scattering of Macromolecules. 1990, San
Diego: 1990.
31. Sutton, M., A review of x-ray intensity fluctuation spectroscopy. C.R.
Physique, 2007. 9: p. 657.
32. Lengeler, Coherence in X-ray physics. Naturwissenschaften, 2001. 88(6):
p. 249-260.
33. Livet, P., Diffraction with a coherent X-ray beam: dynamics and imaging.
Acta Cryst. A, 2007. A63: p. 87.
34. Sutton, M., Coherent X-ray diffraction, in Third Generation Hard X-Ray
Synchrotron Radiation Sources: Source Properties, Optics and
Experimental Techniques, D. Mills, Editor. 2002, John Wiley and Sons:
New York.
35. Grübel, G. and F. Zontone, Correlation spectroscopy with coherent
X-rays. J. Alloys and Compounds, 2004. 362: p. 3.
36. Mochrie, S.G. Equilibrium Dynamics of Complex Fluids studied via X-ray
Photon Correlation Spectroscopy at 8-ID at the APS. 2005; Available
from: http://8id.xor.aps.anl.gov/UserInfo/Analysis/slslecturenotes.pdf.
37. Madsen, A. X-ray Photon Correlation Spectroscopyray Photon
Correlation Spectroscopy. 2008; Available from: http://www.esrf.eu/
events/conferences/Tutorials/Slideslecture8.
38. Grübel, G., A. Madsen, and A. Robert, X-ray Photon Correlation
Spectroscopy, in Soft-Matter Characterization B. Borsali and R. Pecora,
Editors. 2008, Springer: Berlin.
39. Larbalestier, D., Gurevich, A., Feldmann, D.M., and Polyanskii, A., Nature,
2001. 414: p.368; Foltyn, S.R., Civale, L., MacManus-Driscoll, J.L., Jia, B.
Maiorov, Q.X., Wang, H., and Maley, M., Nature Mater., 2007. 6: p.631.
40. Gutt, C., et al., Measuring temporal speckle correlations at ultrafast x-ray
sources. Opt. Express, 2009. 17: p. 55.
41. Grübel, G., et al., XPCS at the European X-ray free electron laser facility.
Nucl. Instrum. Methods B, 2007. 262: p. 357.
42. Turner, J.J. and et al., Orbital domain dynamics in a doped manganite.
New Journal of Physics, 2008. 10(5): p. 053023.
14. Zapperi, S., et al., Dynamics of a ferromagnetic domain wall: Avalanches,
depinning transition, and the Barkhausen effect. Phys. Rev. B, 1998. 58: p.
6353.
15. Verberk, R., A.M. van Oijen, and M. Orrit, Simple model for the power-law
blinking of single semiconductor nanocrystals. Physical Review B, 2002.
66(23): p. 233202.
16. Issac, A., C. von Borczyskowski, and F. Cichos, Correlation between pho-
toluminescence intermittency of CdSe quantum dots and self-trapped
states in dielectric media. Physical Review B, 2005. 71(16): p. 161302.
17. Lu, H.P., L. Xun, and X.S. Xie, Single-Molecule Enzymatic Dynamics.
Science, 1998. 282: p. 1877.
18. Deniz, A.S., S. Mukhopadhyay, and et al., Lemke, Single-molecule bio-
physics: at the interface of biology, physics and chemistry. J. Roy. Soc.
Interface, 2007.6(18): p. 15-45.
19. Prakash, M.K. and R.A. Marcus, An interpretation of fluctuations in
enzyme catalysis rate, spectral diffusion, and radiative component of
lifetimes in terms of electric field fluctuations. PNAS, 2010. 104: p. 15982.
20. Chen, S.-J., RNA Folding: Conformational Statistics, Folding Kinetics, and
Ion Electrostatics. Annual Review of Biophysics, 2008. 37(1): p. 197-214.
21. Alers, G.B., A.P. Ramirez, and S. Jin, 1/f resistance noise in the large
magnetoresistance manganites. Appl. Phys. Lett., 1996. 68: p. 3644.
22. Hardner, H.T., et al., Non-Gaussian noise in a colossal magnetoresistive
film. J. Appl. Phys., 1997. 81: p. 272.
23. Podzorov, V., et al., Mesoscopic, non-equilibrium fluctuations in inhomo-
geneous electronic states in manganites. Europhys. Lett., 2001. 55: p.
411-7.
24. Raquet, B., et al., Noise Probe of the Dynamic Phase Separation in
La2/3Ca1/3MnO3. Phys. Rev. Lett., 2000. 84: p. 4485.
25. Weller, D. and A. Moser, Thermal effect limits in ultrahigh-density mag-
netic recording. Magnetics, IEEE Transactions on, 1999. 35(6): p. 4423-
4439.
26. O’Grady, K. and H. Laidler, The limits to magnetic recording — media
considerations. Journal of Magnetism and Magnetic Materials, 1999.
200(1-3): p. 616-633.
27. McDaniel, T.W., Ultimate limits to thermally assisted magnetic recording.
J. Phys: Cond. Matter, 2005. 17: p. R315.
56
3 . SCIENCE DRIVERSQUANTUM MATERIALS
remarkablepropertiesofthesematerials.Thisisanambi-
tious goal, with tremendous potential impact across
diversetechnologyareas:fromefficientenergytransport,
storage,andconversion;tolow-power/high-speedinfor-
mationprocessingandcommunication;tohigh-density
informationstorage;tomaterialsandnano-structures
withengineeredthermal,mechanical,andelectricalprop-
ertieswithmyriadapplications.
Collective Modes
Thescientificchallengeistounderstandhowexotic
andpowerfulpropertiesofquantummaterials“emerge”
fromthecollectiveorcoordinatedbehaviorofthecon-
stituentcomponents.Thesearepropertiesthatarenot
predictablebyconsideringtheindividualparticles(e.g.
3.7 QuantumMaterials
“Quantummaterials”refersbroadlytosystemsthat
arenotadequatelydescribedbysimplesingle-electron
band-models.Suchmodelsandrelatedtheoriesprovided
foundationalknowledgeforthesemiconductorrevolu-
tionofthe20thcentury.Quantummaterialsarepromising
materialsforthe21stcentury,forwhichwesorelylackan
equivalent knowledge foundation.These materials
includeunconventionalsuperconductors,multiferroics,
topologicalinsulators,colossalmagnetoresistancecom-
pounds,andnano-structureswheresurface/interface
effectsandquantumconfinementgiverisetonewphysics,
newproperties,andimportantnewfunctionalities.
NGLSwillenablequalitativelynewapproaches for
understandingquantummaterials.Thisnewknowledge
willbeessential inordertodeveloptheprinciplesfor
directedmaterialsdesignandsynthesistoexploitthe
“Quantum materials” are materials in which electrons — through quantum entanglement — behave collectively in ways we are unable to predict from the reductive models and experimental approaches that guided the development of 20th century semiconductor technologies. Materials in which electrons are naturally quantum-entangled, such as high-Tc superconductors and colossal magnetoresistive manganites, have been at the heart of some of the greatest surprises in 20th century material science. New quantum material systems exhibiting unique emer-gent properties are being discovered every year. Ideas inspired by these materials compose a large part of the innovative landscape at the frontier of modern electronics, including quantum information technologies, super-conducting electrical grids, and nano-device engineering. However, better understanding and control of the materials themselves is essential to develop their potential for these applications.
NGLS X-ray lasers will provide qualitatively new experimental capabilities to observe the energetically fragile many-electron dynamics of quantum materials. The high repetition rate and high peak brightness proposed for NGLS will enable new nonlinear photoemission techniques that directly probe electron correlations. Photon hungry spectroscopies such as Resonant Inelastic X-ray Scattering (RIXS) will finally achieve the requisite energy and momentum resolution to characterize correlated states for effective comparison with theoretical predictions. Ultrafast time resolution will enable the observation of correlated states as they develop, and as they respond to specific excitations of the material. Importantly, the availability of high repetition rate makes it practical to inves-tigate these fragile states with moderate pulse energies (while maintaining high average power) in order to avoid disrupting the states being measured.
Direct probes of charge correlations and their dynamics have been long-recognized as a critical capability gap of modern materials science. Bridging this gap requires the capabilities of NGLS X-ray lasers, and will propel the application of quantum materials in technology areas ranging from efficient energy transport, to low-power/high-speed information processing, to high-density information storage.
57
3 . SCIENCE DRIVERSQUANTUM MATERIALS
NGLSwillenableentirelynewapproachesfordirectly
probing collective or emergent behavior in quantum
materials, approaches that are not available through
existingtechniquesandfacilities. Inthefollowing,we
presentselectedexamplesof futureexperiments that
illustratethescientificimpactofNGLSX-raylaserson
ourunderstandingofquantummaterials.
Amongthemostexcitingmaterialsstudiedtodayare
ultrathinatomicfilmssuchasgraphene,3layeredcom-
poundsincludingthehigh-Tcsuperconductors4,5(cuprates
andpnictides),andcolossalmagnetoresistivematerials
(layered manganites).Theoretical and experimental
researchoverthelastthreeyearshasledtotheidentifica-
tionofnew“topologicalinsulator”statesofmatterinwhich
electronswithinonenanometerofacrystalsurfacehave
uniqueandrobustpropertiesthatarehighlyvaluedfor
devices,suchasnovelsuperconductingandmagnetic
states.6-8Allofthesematerialsachievefunctionalproperties
frominteractingelectronsthatmovepredominantlyalong
layersintheircrystalstructure.Furthermore,inallcases
existingmeasurementscanprovideonlylimitedinforma-
tionaboutthekeyelectronicprocesses.Highaveragebright-
ness,ultrafastpulses,andhighlyphasecoherentX-rays
fromNGLSwillmakeitpossibletotakeX-raytechniquesinto
newregimesoftime-,energy-,space-,momentum-,and
spin-resolution,providingcriticalinformationtounderstand
bothartificiallyandnaturallynano-structuredquantum
materials(Figure33)atthescienceandtechnologyfrontier.
electrons,atomsetc.)operatinginisolation.Thepara-
digmforunderstandinganinteractingelectronsystemin
termsofthechargedcollectivemodesdatesbacktothe
1950’streatmentbyPinesandNozieresoftheinteracting
electrongas.Theydescribedthelow-energyfermionsas
Landauqausiparticles,andidentifiedtheelementarycol-
lectiveexcitationasthewell-knownplasmon.Theformer
areobserved,forexample,asapeakintheone-particle
spectralfunction,A(k,ω),andthelatterasapeakinthe
two-particle,dynamicstructurefactor,S(q,ω).
Remarkably,whilethestudyoffermionicquasiparticles
inmodernquantummaterialsisnowwelladvanced,weare
still lackinganeffectivemeanstostudythecollective
modes. Angle-resolved photoemission spectroscopy
(ARPES)measuresdirectlyA(k,ω), and in thepast15
yearshasemergedasthesinglemostpowerfulprobeof
quantummaterials.However,S(q,ω)the essential observ-
ableofaninteractingelectronsystem,hashardlybeen
measuredattherelevantscaleinanyquantummaterial.
Theproblemisthat,becausethegroundstatesofquantum
materialsarisefromasubtlebalanceamongcompeting
interactions,therelevantcollectivemodesappearatmod-
estenergy,typically1to100meV(seeFigures32and36).
ModerninelasticX-rayorelectronscatteringspectrometers
lackthecombinationofphotonfluxandenergyresolution
requiredtomeasureS(q,ω)inthisrange.Theabsenceofa
meanstomeasurecollectivemodesrepresentsanenor-
mousgapinourunderstandingofquantummaterials.
–40
1
–2 0 2 4
0.0000.0100.0200.0120.0160.020
2
1
-1
0
-2
0.0 0.5
0.0
-0.2
-0.4
-0.60.3 0.4 0.5
1.0
(a)
(b)
(c)
qs qe q
Figure32 Two examples of theoretical predictions of collective excitations in S(q,ω). Left: Collective mode predicted by a unified field the-ory of the Mott state.1 Right: Collective mode characteristic of superfluid defects in a smectic, stripe state.2 Because S(q,ω) has never been measured in a quantum material in the relevant energy regime, none of these (or any other) predictions have ever been tested.
58
3 . SCIENCE DRIVERSQUANTUM MATERIALS
gaps,itisdifficulttodistinguishsuperconductinggaps
fromthoseresultingfromotherbroken-symmetrystates,
suchaschargeandspindensitywaves.Infact,exactly
thisdifficultyhasledtotheprolonged“pseudogap”con-
troversy in high-Tc superconductivity, where we are
unabletodistinguishbetweennascentfluctuatingsuper-
conductivityandcompetingformsoforder.
Superconductingcoherenceappearsinthe“anoma-
louspropagator”—ratherthanthesingle-particlepropa-
gator described above.The anomalous propagator is
related to the probability amplitude that the system
remainsinitsgroundstateifweremoveanelectronfrom
thestate|k↑>attimezeroandanotherfrom|-k↓>ata
latertime.Aspectroscopybasedonnon-lineartwo-pho-
tonARPEScandirectlyprobetheanomalouspropagator
andtherefore,two-electronquantumcorrelationssuchas
superconductingcoherence.Thebrightness,pulsedura-
tion,repetitionrate,andwavelengthtunabilityofNGLS
makeitaperfectplatformfromwhichtocarryoutsuch
measurements.
Ouranalysisoftwo-photonARPESfollowsdirectlythe
standardtreatmentoftwo-photonabsorptioninnonlin-
earoptics.Two-photonabsorptionproceedsfromaground
statetoafinalstateviaanintermediatevirtualstate,with
thefirstphotoncreatingthe
virtual intermediate state,
andthesecondphotonpro-
motingthesystemfromthe
intermediatetofinalstate.
Inthesecond-ordernonlin-
earARPESprocessillustrated
3.7.1 Understanding Charge Pairing: Two-photon Nonlinear ARPES Spectroscopy
Thecomplexityofquantummaterials,aswellastheir
potentialutility,canbetracedtothepresenceofcompet-
inginteractionsbetweenspin,charge,andlatticedegrees
offreedom.Theseinteractionsgenerateamultiplicityof
broken-symmetryphases,suchascharge/spindensity
wavesandsuperconductivity, aswell asmoreexotic
phasesthathaveyettobeobserved,suchasd-density
waveandcurrentlooporder.Inthissectionwedescribea
new spectroscopy termed “two-photon nonlinear
ARPES”toprobemany-bodyquantumsymmetrybreak-
ing,whichwillbecomepossiblewiththeuniquecombi-
nation of (controllable) high power density, high
repetition rate, and tunable soft X-rays provided by
NGLS.
Asdiscussedearlier,ARPESdirectlymeasures the
one-electronspectraldensityfunction,A(k,ω),whichis
relatedtothesingleparticle-propagator.Fromthespec-
tral intensity,allsingle-particlepropertiessuchasthe
momentum-resolveddensityofstates,quasiparticlelife-
time,anddispersionrelation(renormalizedbandstruc-
ture) are obtained. However, because A(k,ω) is a
one-particlefunctions,ARPESdoesnotdirectlyprobecol-
lectivemodesandmulti-particlecorrelations,andiseffec-
tively blind to a wealth of two- (and multi-) electron
propertiessuchassuperconductingcoherence,exciton
pairing,spindimerization,andlocalvalencebondforma-
tion.WhileARPESdoesobservetheopeningofenergy
Correlated Phenomena
Control/Design
Cooper pairformation
Charge density/orbital waves
Antiferromagnetism Ferromagnetism
Spinliquid
Stripes/Checkerboardorder
Orbitalwaves
Electronic phaseseparation
Superconductivity andmagnetism
Coupled charge/Spin order
Opto-magnetics
Ferroelectricity andferromagnetism
UnconventionalSuperconductivity
ColossalMagnetoresistanceMulti- ferroics
Rich and Novel Electronic Phenomena
Figure33 Collective electronic states and dynamical modes that lead to enhanced material properties for next generation applications.
Time-resolved
non-linear ARPES
59
3 . SCIENCE DRIVERSQUANTUM MATERIALS
delayshouldthenyieldtheadditionalinformationabout
thequasiparticlecoherencetime.
TheconditionforobservingnonlinearARPESisthat
thesecond-photonmustfollowthefirstwithinaspace-time
intervaldeterminedbythequasiparticlemean-freepath
andlifetime.Forhigh-Tcsuperconductorstheseparameters
are~100nmand~1psrespectively.Therequiredminimum
peakfluenceistherefore~1022photons/cm2/s,whichcor-
respondstoaninstantaneouspowerof~100kW/cm2.
Thispowerrequirementiswellabovethedamagethresh-
oldforaCWsource,orevena~100pspulsedsource.
Withpulsesofupto500fsdurationfromNGLS(~10meV
energyresolution,usingatime-compensatedmonochro-
mator),focusedto100µm,~106photonsperpulseare
neededtoapproachthecoherentnonlinearregime.TheinFigure34,thegroundstateistheBardeen-Schrieffer-
Coopergroundstate,|BCS>.Inthevirtualintermediate
state,onephotoelectronisejected,leavingbehindaqua-
siparticle,orunpairedelectron.Thesecondphotonthen
ejectsthisunpairedelectrontoreachafinalstatewith
twophotoelectrons.Thefinalstateisthegroundstateof
thesuperconductor,albeitwithonelessCooperpair.The
keypoint is that two-photonabsorptionprovides the
extraenergytobreaktheCooperpair,withtheexcess
photonenergysharedbythetwoejectedelectronsina
coherentprocess.
Figure35contraststhephotoemissionsignalfroma
superconductorinthecaseoflinear(one-photon)and
nonlinear(two-photon)ARPES.InlinearARPESonlythe
occupiedstatesbelowtheFermienergyareobserved.
The spectral density is zero at the Fermi energy and
appearsatEF-Δ.Inadditiontothegap,onemayobservea
faint“backfolding”oftheband,whichisaweaksignature
ofpairing.Incontrast,thereisaclearsignatureofsuper-
conductingcoherenceinnonlinearARPES,showninthe
right-handplotofFigure35.Wepredictapeakinspectral
densitysharplylocalizedinbothenergyandmomentum.
Thespectraldensityappearsatthemid-gapenergyand
inanarrowrangeofmomentum,Δk,ofordertheinverse
ofthesuperconductingcoherencelength.
Thereareavarietyofwaystodistinguishthelinear
andnonlinearARPESsignalsexperimentally.Perhapsthe
mostdirectandinformative is tomeasuretheARPES
spectraldensityresultingfromapairofX-raypulsesasa
functionoftheirrelativedelay.Whenthetwopulsesare
coincidentintimethereisanadditionalnonlinearsignal
resultingfromtheircoherentsuperposition.Measuring
thechangeinthenonlinearARPESsignalasafunctionof
|BCS > +2 photoelectrons(final state)
|BCS > +1qp + 1 photoelectron(intermediate state)
|BCS > +(intial state)
∆
EF
EF
kF
Unoccupied
Correlatedstates
Spectral density—linear ARPES
Spectral density—nonlinear ARPES
Spectral density at Fermi level reveals superconducting coherence
Ener
gy
Occupied
Momentum
εκ
Ener
gy
Momentum
Figure34 Illustration of 2-photon ARPES spectroscopy of super-conductors: initial ground state, intermediate state, and final states with two emitted electrons.
Figure35 Contrasting the ARPES spectral density in linear (top) and nonlinear ARPES (bottom).
60
3 . SCIENCE DRIVERSQUANTUM MATERIALS
(seeSection3.8foracomparisonofacquisitiontimesfor
time-resolvedARPES).Laser-basedharmonicsourcesare
notcontinuouslytunable,andwhiletheycanprovidethe
requisitepeakpower,orthehighrepetitionrate,theycannot
providebothsimultaneouslywithresolutioninthe10meV
range.*
3.7.2 Collective Excitations: Energy-Domain Resonant Inelastic X-Ray Scattering (RIXS)
ResonantinelasticX-rayscattering(RIXS)isapower-
fulapproachwiththepotentialtoprobecollectivecharge
excitations,revealingacompletemapofS(q,ω)withboth
energyandmomentum(q)resolutionspanningtheentire
Brillouinzone(BZ).9-11InasimplesemiconductorRIXS
revealsexcitationsacrossabandgap,showingthekinetic
regimethatcanbeaccessedviachemicaldoping.When
quantumstructureandnanoscaleinhomogeneityareadded
tothesystem,newclassesofcollectiveexcitationsappear
atlowenergycorrespondingtothebreakingofquantum
entanglementormodificationofthespatialdistribution
ofcharge(“chargetransfer”excitations).8-13Furthermore,
RIXSisanelement-specificprobeofbulkproperties,with
sensitivitytothealteredelectronicandstructuralenvi-
ronmentatinterfaces,12anessentialcapabilityforunder-
standingcompositesandnano-structuredmaterials.
However,RIXScapabilitiesareseverelylimitedbythe
fluxofpresentX-raysourceswhichprovideonlysparse
‘slices’ofS(q,ω)coveringasmallpartoftheBZwithrath-
ercoarseenergyresolution(~100meV).Whilespectrom-
etersatmodernsynchrotronscanachievemeVenergy
resolutionwithhigh-orderBraggmonochromators(and
gratingmonochromatorsinthesoftX-rayrange),this
comesattheseverepenaltyofdiscarding99.998%ofthe
beamintensity,leavingonly108or109photons/secfor
experiments,fluxcomparabletothatfromalab-scale
rotating anode X-ray source; and requiring weeks to
monthofdataacquisitionforacomprehensivedataset
evenatcoarseenergyresolution.Thisisnotnearlysuffi-
cientforstudyingcollectiveelectronicexcitations~kBT,
andisadirectconsequenceofthefactthatsynchrotron
sourcesaretemporallyincoherent.
high-repetitionrateofNGLSinconjunctionwithanarray
ofmomentum-andenergy-resolved3D time-of-flight
(TOF)analyzers(seeSection3.8)willenablerapiddata
acquisition and discern small nonlinear signals from
background,whiletheshortpulsesreducetheaverage
poweronthesampletoonly~100mW/cm2whichistypi-
callybelowthedamagethreshold.Thecombinationof
MHzrepetitionrate,timeandenergyresolution,andtun-
ability(importantforoptimizingphotoemissioncross-
sectionswithsufficientmomentumtospantheentire
Brillouinzone)arenotavailablefrompresentsynchro-
tron,X-rayFEL,orlaser-HHGX-raysources.
Two-photonnonlinearARPESenabledbyNGLSwillbe
apowerfulnewtooltounderstandsuperconductivityin
complexcorrelatedmaterials.Keyattributesinclude:
• The appearance of a new spectral feature at the
chemicalpotentialthatsignalstheonsetofsupercon-
ductingcoherence.
• Thespectralweightofthisfeatureisdirectlypropor-
tionaltothesuperfluiddensity.
• Themomentumspacewidthofthenonlinearspectral
densitymeasures thesuperconductingcoherence
length.
• Cross-correlationoftwoX-raypulsesyieldsadirect
measureofthesuperconductingquasiparticlecoher-
encetime.
Requirementstwo-photonARPESasdescribedabove
arebeyondthecapabilitiesofcurrentsynchrotrons,X-ray
FEL’s,andlaser-basedharmonicsources.Theyinclude:
• moderatepeakpowers—minimumoftwophotons
withinaquasi-particlelifetimeandmean-freepath
• transform-limitedpulses—toachieveresolutionin
the10meVrange
• highrepetitionrate—toavoidspace-chargebroad-
ening,andtodiscernthespectralsignatureofthe
Cooperpairsfromthebackground(viaenergyand
momentum-resolvingtime-of-flightspectrometer)
• tunability—tooptimizephotoemissioncrosssection
forsensitivitytocoherentstates
Synchrotronsourcesprovidetherequisiterepetition
rateandtunability,butcannotprovidethenecessarypeak
power.X-rayFEL’sprovidetherequisitepeakpower,but
onlyafractionofthisisusable(inordertoavoidspace-
chargebroadening),andthelowrepetitionratesofcur-
rentFEL’sleadstoimpracticalacquisitiontimesforARPES
*Forexample,toachieve>106photonsonthesample(at100eV,10meVbandwidth)at>100kHz,onerequiresanominal1kWaveragepowerlaser—assuming10-5conversionefficiency(perharmonic),0.4eVnominalharmonicbandwidth,andx100lossinatime-compensatedsoftX-raymono-chromator.
61
3 . SCIENCE DRIVERSQUANTUM MATERIALS
3 .7 .2 .1 Collective Excitations that Define a
Superconducting Gap
Insuperconductors,theenergyandmomentumquan-
tizationofcollectiveexcitationsatthesuperconducting
gapencodes the fundamental interactions that cause
superconductivity.13Theseexcitationscanbecreatedby
perturbingthephasecoherenceinthesuperfluidofelec-
trons,takingtheformofawhirlpool-likevortex(forexam-
pleseeFigure40,right)oraripple-likegapexcitationsuch
asismodeledinFigure38.Theenergyscaleofthesuper-
conductinggapinnoteworthyhigh-Tccupratesandiron
pnictidescanrangefrom20-50meV,14,15whichcannotbe
measuredwithadequateenergyresolutionandmomen-
tumrangeatexistingRIXSbeamlines,butwillbecom-
pletely characterized with an array of q-resolved,
high-resolutionspectrometers(seeFigure46)inconjunc-
tionwithhighaveragefluxavailableattheNGLS.
Mappingthemomentumdependenceof thesuper-
conductinggapcollectivemodescanqualitativelyreveal
thesymmetryof theorderparameter (e.g.s-waveor
d-wave)andthelengthscaleofCooperpairing.Witha
material-specificmodelsuchasshowninFigure38,one
candirectlyfittheinteractionsthatcreatesuperconduc-
tivity,withthegoalofrelatingthemtochemicalcomposi-
tioninamaterialclass.Agreatdealofrecentinterestin
thehigh-Tc“pseudogap”phaserelatestotheunresolved
question of whether quantum fluctuations above the
superconductingtransitionareaprecursorofsupercon-
ductivity or represent a competing form of quantum
order.16Understandingpseudogapbehaviormaybean
importantroutetodevelopingmaterialswithhighercriti-
caltemperatures.Currenttechniques(e.g.visiblelight
spectroscopiesandSTM)lackmomentumresolutionand
observeonlyasinglestructuredenergygapinboththe
pseudogapandsuperconductingphase.Incontrast,RIXS
providesatwo-dimensionalenergyvs.momentummap
ofthegapcollectivemodesandstructureofeachphase
revealingessentialinsighttotheinteractionsthatunder-
pinsuperconductivity.Mostfieldtheoriesofcorrelated
electronsystems,inparticularthedopedMottinsulator,
involvespecificpredictions forS(q,ω) atmeVenergy
scales.Theabilitytomeasurethisquantitywillunravel
theintricacyofemergentphenomenaandrevolutionize
condensedmatterphysics.
NGLSsoftX-raylaserswillovercometheseexperi-
mentalbarriersbyprovidingthreetofourordersofmag-
nitudehigheraveragephotonfluxinpulsesclosetothe
Fouriertransformlimit(i.e.longitudinallycoherent,with
narrowbandwidth,seeSection5,Table2).Forexample,
NGLSbeamline1willprovide1011photons/pulse,with
<50meVbandwidthpriortoanymonochromator.The
highrepetitionrateenablesexperimentsatmoderate
peakpowerstoavoiddamagetothesample.
3 eV
1 eV
Mott Gap,C-T Gapd-d excitations,Orbital Waves
PseudogapsOptical phononsMagnonsLocal Spin–FlipsSuperconducting gapSpin resonance modes
0
100 meV
–3 –2 –1
Energy loss (eV)
0 1 2
(e) 2008 E = 0.13 ev
RIXS spectra of La2CuO4 at Cu L3 -edge
Figure36Energy scales of some important types of collective excitation. All of these modes can be probed by RIXS and ARPES using the high energy resolution proposed for NGLS.
Figure37 Resolution sets the paradigm: RIXS measurements with 0.1eV resolution currently allow separate charge excitations to be resolved in a cuprate superconductor. With meV resolution at NGLS, low energy excitations such as magnons, superconducting gap excitations and many others will be visible, as well as line-shape features from the entanglement between those modes and electronic excitations. (Figure courtesy of G. Ghiringhelli and L. Braicovich)
62
3 . SCIENCE DRIVERSQUANTUM MATERIALS
functionality,8,18suchasthequantumcomputingcapa-
bilitiesillustratedinFigure40(right).Thesearelikelyto
definefuturegenerationdevicearchitectures,andunder-
standingtheirelectricandmagneticfieldresponsewith
chemicalspecificityisthekeytonotonlydefinetheirper-
3 .7 .2 .2 Emergent Behavior at Interfaces
Whilemultifunctionalmaterialsdevelopedfromcom-
posites and nano-structured material interfaces are
emergingasthemostpromisingcandidatesfornext-gen-
erationelectronics,welacktherequisitetoolstoprobe
suchheterogeneousandentangledelectronicstates.As
showninFigure39,X-rayspectroscopieshavetheunique
capabilitytoselectivelymeasureelectronicpropertiesat
aninterface.Forexamplethelifetime,energy,andspatial
propertiesofcollectiveexcitationssuchasinteraction-
dressedelectrons,charge-transferexcitations,andspin
wavescanbedirectlyprobedviaRIXS.9,10Collectiveexci-
tationsinvolveatransientdipolemomenttowhichX-rays
couple,withawiderangeofcharacteristiclengthscales.
UsingshortwavelengthX-raysatNGLSwillmakeitpos-
sibletoprobecollectiveexcitationsonatunablelength
scalesapproachinginteratomicspacings,whichiscriticalfor
materialpropertiesbutcannotbeaccessedwithvisibleor
ultravioletlight.Probingwithresonance-tunedX-raysalso
targetsachemicallyspecificlocation,12makingitpossi-
bletoisolatehowmaterialpropertiesarelinkedtothe
quantumstateofelectrons,andtotracetheconnection
betweenchemicalcompositionanddesiredphenomena.
Thelocalchargegradientanddistinctsiteenergiesat
interfacesmakeitpossibletotargetspecificsitesbytun-
ingthephotonenergyandscatteringgeometry.Ithas
beenshownthatsuperconductorsandmagnetsfabricated
withinheterostructuresandcompositesexhibitenhanced
A
FY
YBCO cap layer
LCMO cap layer
TEY
Mn edge
Cu edge
e-
e-
h
C
H
v
b
Interface cluster
c
a
MnO(1)
O(2)
O(3) Cu
Ba
Ba
Y
ba
Figure39 Targeted measurements (A) X-rays with energy tuned for element and orbital sensitivity can be targeted to observe the electrons at an interface. (B) An atomic structure studied by X-ray scattering at a manganite-cuprate interface. (Graphic from Reference 17)
2.8002.2751.9501.6251.3000.97500.65000.32500
0 4 5 6 70.0
0.5
1.0
1.5
2.0
2.5
3.0
1 2 3
Figure38Contour plot of the predicted RIXS spectral function for a collective excitation of the cuprate superconducting gap for a range of q from (0, π) to (π, π) versus energy (given in units of ~5 meV). (Image from Reference 13)
Momentum
Majorana
Superconductor
Ener
gy
Tim
e
Figure40Low energy collective modes define new functional materials: Recent measurements have shown that collective elec-tronic vortex modes on the surface of a superconducting doped topological insulator (CuxBi2Se3, bands in left panel) are a natural platform for quantum computing. (right) A quantum computing operation can be performed by “braiding” vortices. Band structure theories suggest that these “Majorana fermion” collective vorti-ces exist, but current RIXS spectrometers lack the time and ener-gy resolution to study collective modes linked to the supercon-ducting phase. (CuxBi2Se3 data from Reference 8)
63
3 . SCIENCE DRIVERSQUANTUM MATERIALS
orbitalhybridization,etc.)asthecorrelationdevelops.
Suchstudieswillopenanewdimensionandnewunder-
standingofmaterialpropertiesbeyondthatpossiblewith
static(ortime-averaged)measurementsasafunctionof
temperature,pressure,doping,appliedmagneticfield,
isotopesubstitution,etc.Itisalsoacrucialsteptoward
addressingtwograndchallenges:(1)tounderstandcom-
plexmaterialsystemsoutofequilibrium,and(2)touse
tailoredexcitationtocontrolemergentbehaviorincomplex
correlatedsystemsinordertoachievedesiredproperties.
Inmany ionicsolids,photoexcitationresults in the
generationofacharge-transferexcitoninwhichchargeis
partiallytransferredfromoneatomicsitetoaneighbor-
ingone.Animportantexampleofthisisthecupratefam-
ily towhich thehigh-Tcsuperconductorsbelong.The
enhancedmobilityofpairedchargesrequiresthatoptical
excitationsinsuchdopedMottinsulatorsbetreateddif-
ferentlyfromtheirbandcounterparts. Inthisareaour
presentunderstandingiscriticallylacking.Figure41(left)
showsthenearbandgapabsorptionofSr2CuO2Cl2,a
two-dimensional,spin-1/2Heisenbergantiferromagnet,
and the corresponding real-space excitation. Due to
strongCoulombcorrelations,eachCu3dorbitalisoccu-
piedbyasingleholeinequilibrium.Theinsulatorexhibits
a2eVabsorptionpeak,wherephotoexcitationcorre-
sponds toÅscalespatial transferof theCuhole toa
superpositionofsurroundingO-2porbitals.
Oneexample(ofanentireclassofexperiments)isto
coherentlydrivecharge-transfer(CT)excitonswithafew-
formancefactorsbutalsoimprovetheminaninformed
waythroughthedevelopmentofphysicalmodels.Inaddi-
tion,externalelectricalormagnetic fieldsare readily
incorporatedinRIXSexperimentsasameanstotune
material properties and introduce desired collective
behaviors(e.g.electricfieldstopolarizeorbitalorder,or
andmagneticfieldstocreatevorticesinasuperconductor).
3.7.3 Collective Excitations: Time-Dependent (Pump-Probe) Approaches
3 .7 .3 .1 Pump-Probe Attosecond Dynamics of Collective
Excitations in Complex Materials
Thepropertiesofstrongly-correlatedelectronmateri-
alsemergefromcomplexelectronicgroundstateswhere
strongCoulomb interactionsbetweenchargecarriers
givesrisetoanon-rigidbandstructure,i.e.theenergy/
momentumdistributionofelectronicstatesdependson
theoccupationsofspecificatomicorbitals.Theenergy
scaleoftheseinteractionsrangesfrommeVtoeV,corre-
spondingtodynamicsthatdevelopon100’soffemtosec-
ondstoattosecondtimescales.Here,NGLScanprovidea
revolutionarycapabilitytounderstandthesesystemsvia
ultrafastmeasurements,inwhichtailoredcoherentexci-
tationsperturbthesystemoutofequilibriumontime-
scalesshorterthantheunderlyingcorrelations.NGLS
ultrafastX-rayscanthenbeusedtodisentangletheinter-
actions by probing the evolving electronic structure
(bonding,chargedistribution,spin/magneticmoment,
Sr2CuO2CI2
Photon energy (eV)
1.6
1.5
1.0
0.5
520
-8 -4-4Time (fs)
Ener
gy (e
V)En
ergy
(eV)
0 4 8
540
560
900
920
940
960Copper
Cu-O charge state
Oxygen
0.01.8 2.0 2.2
Abso
rptio
n α
(105 c
m-1
)
15 K
400K
X-ray O K-edge X-ray O K-edge
X-ray Cu L-edgeX-ray Cu L-edge
Figure41Left: Charge-transfer exciton peak in Sr2CuO2Cl2.19 Middle: Illustration of the related attosecond real-space transfer in the CuO2 plane probed via two-color X-ray pulses. Right: Simulated differential absorption spectra at the Cu and O X-ray L-edges, revealing the coherent Cu-O polarization directly on the sub-femtosecond timescale with element specificity.
64
3 . SCIENCE DRIVERSQUANTUM MATERIALS
tionandspectralresolution(withinthetransformlimit)
arecriticalcapabilitiesthatarealsounavailableusing
presentultrafastsources.Moreover,forprobingcorrelat-
edelectronstructureitis,moreover,essentialthatthe
highaveragefluxbedeliveredathighrepetitionratesin
ordertokeeptheflux/pulseinatolerablerangesoasnot
toadverselyaffectthecollectivestatesbeingmeasured
(seeAppendix1forfurtherdiscussion).
3 .7 .3 .2 Pump-probe: Control of Time-Reversal
Symmetry in Topological Insulators
The prediction20,21 and subsequent experimental
observations22-27ofso-called3Dtopologicalinsulators,
buildingonthetheoreticalunderstandingofthe2Dquan-
tumspinHalleffect,28havestartedanavalancheoffer-
vent activity in both theoretical and experimental
investigationsofthesematerials.Thesesystemsrepre-
senttheexistenceof“topologicalorder”inthesolidstate
(asopposedtomorecommonsymmetry-breakingorder)
anddependonthetime-reversalinvariancepresentin
non-magneticmaterials.Theresulting2Dlinearmetallic
surfacestatesofmasslessDiracfermionsarestrongly
robustagainstmanyperturbations,withtheirgapless
naturefullyprotectedbythetime-reversalinvariance.7
Thedemonstratedmomentumspin-lockingofthesurface
state,23wherestatesarestronglyspinpolarizedalonga
spatialdirectiondeterminedbythedirectionoftheircrys-
talmomentum(depicted inbothrealandmomentum
spaceinFigure42),enhancetheexcitementoverthefun-
damental physics present. Of equal interest are the
wealth of potential device applications ranging from
spintronicstoquantumcomputing.
Thecreationof a transientnearbymagnetic state,
breaking time-reversal symmetry, provides a unique
opportunitytodirectlyobserveandunderstandthere-
establishmentoftopologicalorderfromabrokensym-
metrystate.Whilehigh-resolutionspin-resolvedARPES
experimentshavemadestrongprogressinexploringthe
energy,momentum,andspindependenceoftheelec-
tronicstructuresintopologicalinsulators,21-26littlehas
beendonetodirectlyexploretheirtimedependenceand
interactionswithsurfaceinhomogeneity.Inadditionto
fundamentalunderstanding,thetemporaldependenceof
theseelectronicstructuresawayfromequilibriumiscen-
traltothedevelopmentofdeviceapplication.Thepossi-
bility of pump-probe based time-resolved RIXS, and
time-andspin-resolvedARPESprovidestheuniqueability
cyclenear-IRpulse.Onatto-
second time scales, the
oscillatingIRelectricfieldis
almoststationary.Thus,the
effect of light waves as a
time-dependent perturba-
tionof thecorrelatedelec-
tronic structure will be
directlyobservableviatran-
sientsoftX-rayabsorption
spectroscopyand/ortime-
resolvedRIXS.Absorption
edges can be exploited to
providetheelementalspeci-
ficity, forexamplebetweenCuandOusingtwo-color
attosecondpulses.Thiswillenableforthefirsttimethe
directobservationofthecoherentpolarizationbuildup
anddephasingduringcharge-transferandstabilizationin
theexcited-stateofahighlycorrelatedmaterial.These
willbemanifestasquantumbeatsontheattosecondtime
scale(Figure41,right)andascoherentRabioscillations
atopticalfieldintensitiessufficientforcompletetransi-
tions between ground-state and CT-exciton states.
Resolving the charge-transfer dynamics will provide
important new insight to the Cu-O correlations and
Coulombinteractionsrelevantforawiderangeofcom-
plexmaterials.
Theimportanceofultrafasttime-resolvedmeasure-
mentsinunderstandingthesesystemsderivesfromthe
capabilitytousetailoredexcitationtoperturbsystems
outofequilibriumontime-scalesshorterthantheunder-
lyingcorrelations,andthendisentangletheinteractions
byprobingtheirtimeresponseasthecorrelationdevelops.
TheunimaginablyshorttimescaleaccessiblewithNGLS
willprovideuniqueinsightintothefundamentaldynam-
icsofelectronicwavefunctionsandinteractionsinsolids
bytracingdirectlyelectronicpolarizations,populations,
andbandstructurewithelementalspecificityas they
evolveonattosecondandfew-femtosecondtimescales.
AtNGLS,thestabilityaffordedbyasuperconducting
linac(operatinginCWmode)willenablesynchronization
ofthesub-femtosecondFELpulsestowithinanoptical
cycleoftheCEPstabilizedexcitationpulses.Thelowaver-
agefluxofcurrenttable-topultrafastsoftX-raysources
presentsaseverelimitationforbothoptical-pumpX-ray
probe, and X-ray pump X-ray probe experiments.
Polarizationcontrolandanabilitytotradeofftimeresolu-
Two-color X-ray probe
X-ray pump, X-ray probe
High-resolution RIXS
Stimulated X-ray Raman (CXRS) – wave mixing
Core-hole correlation – wave mixing
see Section 4.3
65
OVERVIEW OF REVOLUTIONARY X-RAY SCIENCE TOOLS AT NGLS
insulatorcrystalswithmagneticimpurities.26Figure43(a)
and(b)showthelinear,masslessDiracdispersionwith-
outmagneticdopingandthemassiveandgappedDirac
dispersions with magnetic doping, respectively.This
methodofbreakingthetime-reversalsymmetry,however,
isintrinsicallyastatictechnique,andthereforecannot
allowtheobservationofthesystemsasthetime-reversal
symmetryphaseisre-establishedfromanearbybroken-
symmetryphaseonfundamentaltimescales.Suchchem-
icaldopingalsointroducesadditionalimpurityscattering
andchangesthechemicalpotential.
Highintensitypulsesofcircularlypolarizedlightpro-
videameans tobreak time-reversalsymmetryonan
ultrafasttimescale.Thismethodhasdistinctadvantages
overmagneticchemicaldopingasitavoidsthecomplica-
tions of impurities and modification of the chemical
potential.Additionally,theshorttime-scaleofthepulses
immediately allows the direct study of the dynamics
betweenphaseswiththeuseofX-rayprobepulsesand
RIXSortime-andspin-resolvedARPES(asdiscussedin
Sections3.8.2.2and3.8.3).Suchanexperimentisdepict-
edinFigure44.Spinresolutionisfundamentalinthese
materialsforprobingthecharacteristicspin-momentum
texturesofthetopologicalstate,andtheresultingspin
dynamicsinthepresenceoftransientsymmetrybreak-
ing. Preliminary measurements show that significant
timereversalsymmetrybreakingispossiblewithalaser
pumpperturbation.Collectiveexcitationsbetweenthe
surfacestateDiracconesarehighlysensitivetotheX-ray
polarizationandscatteringgeometryattheL2/3andM2/3
resonanceedges.Thesefactorsreflectthespinpolariza-
tionandcanbeusedtoidentifyagapopenedthrough
timereversalsymmetrybreaking,inlargepartbecause
spinsneartheDiracpointflipdramaticallyfromin-plane
toout-of-planeperpendicularorientationswhenagapis
openedfrommagneticperturbation.8
Magneticsymmetrybreakingwillreconfigurethespin
orientationoftopologicalDiracsurfaceelectrons,chang-
ingthebalanceofinteractionsthatsetstheenergyof
excitations that involve the simultaneous collective
motionofmanyelectrons(plasmons).Thestrongentan-
glementbetween2DDiracelectronsandplasmons(com-
biningtoform“plasmarons”)isthereforeexpectedto
generateastrongsignaturevisibleinhighfluxX-rayscat-
teringmeasurements.TheNGLSistheonlyproposed
sourcewithsufficientaverageflux,repetitionrate,and
versatilityforthistypeofinvestigation.Thecreationofa
to directly probe the energy, momentum, and spin-
dependentdynamicsofelectronicexcitationsintopologi-
calinsulators.
Thetime-reversalinvarianceofthesesystemsiscen-
traltothesurfacestatesofmasslessandmetallicDirac
fermions. Breaking the time-reversal symmetry with
magneticfluxcanthereforeresultinmassiveDiracfermi-
onsinstead,withagapformingattheDiracpoint,and
canleadtofunctionalenhancementssuchastherecord-
ingofstablequantuminformation(Q-bits,seeFigure
43c). Such breaking of time-reversal symmetry has
recentlybeenachievedbychemicallydopingtopological
a Real space b Momentum (k) space
Figure42 Unique spin texture of the topological insulator surface state. (a) In real space, electrons flow along the sample surface with their spins locked perpendicular to their direction of travel, and oppositely flowing electrons have opposite spins. (b) In momentum space, the surface state electronic bandstructure forms a linear Dirac-like cone. The electrons have spin polarization locked tan-gential to constant energy contours, as depicted at the Fermi level.
Perturbed TI surface Superconductor
a
Bi2Se3 Dirac cone
.4
–1 10
Momentum (Å–1)
.2
0
Bind
ing
ener
gy (e
V)
b c
B
Topo surface
Figure43 (a) ARPES measured spin-integrated dispersion of the topological insulator Bi2Se3, showing a linear, massless, Dirac cone dispersion, with the intersection at the Dirac point ensured by time-reversal symmetry. (b) Chemically doped with magnetic atoms, time reversal symmetry is broken causing a gap at the Dirac point. (c) When magnetic perturbations and superconductiv-ity are combined on a topological insulator, vortices at the surface act as stable bits of quantum information, Q-bits. (Images from Reference 8)
66
OVERVIEW OF REVOLUTIONARY X-RAY SCIENCE TOOLS AT NGLS
states,suchasentangledplasmonsandelectrons(“plas-
marons”)ingraphene,29andatthesurfacesoftopologi-
calinsulators.Theselow-energyexcitationscomedirectly
fromthequantumelementsthatareofinterestfordevice
applications.
Some of the most immediate questions that time-
resolvedRIXScouldanswer,using thecapabilitiesof
NGLS,havetodowithhowtheelectrondynamicsand
entanglementinaquantumsystemchangewithtimein
pump-probeexperiments.Ithasbeenwellestablished
that ultrafast X-ray pulses can be used to measure
(“probe”)themeltingandreformationoflowtempera-
tureorderedstatesafterexposuretoan intense laser
“pump”.However,itcanbechallengingtoobtainmean-
ingfulscientificinformationfromthesemeasurements,
becausethequantumstateafterlaserexposureisnearly
impossibletopredictormodelfromfirstprincipletheo-
ries.Analyzingthetime-evolutionofRIXSscatteringfol-
lowingalaserpumpwillgreatlyclarifythepicture.When
valenceelectronlevelsaredepopulatedbyalaserpulse,
thevacantstatesareexpectedtobevisibleaslowenergy
peaksintheRIXSspectrum,30,31givinganimmediate
metricofhowtheelectronconfigurationhasbeenper-
turbed.Throughsuchmeasurements,themeltingandre-
establishmentoforderedstatescanbedirectlycompared
totheevolutionofthelowenergywavefunction,giving
muchmoretractiontotheoreticalmodels.
Manyinterestingquantumstatessuchassupercon-
ductivityandtopologicalinsulatorordercannotbestud-
ieddirectlyinpresent-daytimeresolvedX-rayscattering
experimentsbecausetheirfunctionalstatesaredistin-
guishedbycollectiveelectronicbehaviorsthathaveno
correspondingstructuralphasetransition.Inthesecases,
themostdirectwaytoobservethetimeevolutionwillbe
tomeasuretime-resolvedchangesininelasticcollective
modeswithhighfluxtechniquessuchasRIXSand2-pho-
tonARPES.
RIXSspectroscopicmeasurementsprovidemultifac-
etedinformationabouttheelectronicstatethatisinde-
pendent of any detailed theoretical model. The
momentum-andenergy-axiswidthofRIXSfeaturespro-
videameanstoevaluatethetimeevolutionoftheelec-
tronicmeanfreepathandscatteringrate.Thedegreeto
whichtheenergyofRIXSfeaturesdependsonmomen-
tumrepresentshowstronglyelectronsarelocalizedina
material(e.g.byspinororbitalorder).TheRIXSsignalat
largemomentainparticularisthoughttobedominated
nearbymagneticstate,breakingtime-reversal,starting
fromatopologicalinsulator,isjustoneexampleofhow
NGLSwillrevealthephysicsofcompetingorprecursor
phasesincomplexmaterials.
3 .7 .3 .3 . Pump-probe RIXS
Theavailabilityofultrafastpulsesathighrepetition
ratefromNGLSwillopenentirelynewapproachesfor
understandingcollectivedynamicsinthetimedomain.
Forexample,ultrafasttime-resolvedRIXSmeasurements
performed at NGLS will measure collective electron
dynamicsinresponsetotailoredexcitations:vibrational
excitations,THzexcitations,transientquasiparticlecre-
ation,andcharge-transferexcitations.Thisissubstantially
moreinformativethanpresenttime-resolveddiffraction
(elasticscattering)studiesthatprobeonlyasinglelong-
rangeorderparameter,andwillmakeitpossibletostudy
the ordering and perturbation-response dynamics of
functionalstates ina farmore flexibleandphysically
informativeway,evenformaterialssuchastopological
insulatorsandsuperconductorsthathavenostaticorder.
Following inelasticmodes in the timedomainwill
extendpump-probetechniquestoavarietyofordered
statesandquantumpropertiesthatcannotcurrentlybe
trackedinthetimedomainduetoalackofchargeorlat-
ticesuperstructure.Tonamejustafewpossibilities,atthe
10meV energy scale one could measure momentum-
resolvedelectronicexcitationsacrossasuperconducting
gapinhigh-Tcsuperconductors,13ordiscerncollective
excitationfeaturescausedbyunusuallow-dimensional
Optical circular pump:
100 fs
X-ray probe pulse
• Breaks TRS
• Leaves TI in transientnon-zero magnetic state Dynamics:
• Gap?
• Spin texture?
0
Time/Spin ARPES
500
Figure44 Schematic of a pump-probe time and spin resolved ARPES experiment. A circularly polarized pump pulse alters the fundamental symmetry of the system, and a synchronized and variably delayed X-ray pulse will induce photoemission, and the photoelectrons are properly analyzed.
67
3 . SCIENCE DRIVERSQUANTUM MATERIALS
expecttoseethequantumshake-upcauseenergyshifts,
ahigherprevalenceoflowenergyexcitations,andbroad-
erfeaturewidthsdevelopinginthesecondhalfofthe
pulse.The“melting”oforderedstateslikesuperconduc-
tivityormagnetismwillbemanifestbythedisappear-
anceoftheircorrespondingcollectiveexcitations,and
could be monitored in tandem with other collective
modesforafemtosecond-by-femtosecondrecordofthe
quantumstate.
3 .7 .3 .5 Multi-dimensional Spectroscopy of
Collective Excitations
NonlinearX-rayexperimentswillrevealnotonlythe
interactionsthatgovernthecorrelatedgroundstate,but
alsothenatureofnon-equilibriumstatesofrelevancefor
arangeofnext-generationelectronicmaterials.Optically-
drivenexperimentsalongwithmoresophisticatedultra-
fastnonlinearsoftX-rayprobes,suchasCoherentX-ray
RamanSpectroscopy(CXRS),core-holecorrelationspec-
troscopy,four-wavemixing,andrelatedmulti-dimensional
spectroscopytechniquesinthesoftX-rayregimewillpro-
videthefirstcompletepictureofelectroncorrelationsin
thesematerials.Figure45illustratesmulti-dimensional
spectroscopy ina transition-metaloxide.X-raypulse
sequencestunedtotheO1s→2pandCu2p→3dreso-
nancesprobecorrelationsbetweenO-2pandCu-3dlevels
viacore-holecorrelationspectroscopy.35Alternatively,
X-raypulsesmayprobelocalizedd-dtransitions,andfol-
lowvalencechargeflowbetweentheCuandOsitesvia
coherentX-rayRamanspectroscopy.36Thedevelopment
ofnonlinearX-rayscienceisoneofthemostambitious
goalsforNGLS.Thiswilltrulyrevolutionizeourunder-
standingofcomplexmaterialsbyenablingthefullimple-
mentationofmulti-dimensionalX-rayspectroscopyasa
probeofmany-bodycorrelationsinquantummaterials.A
moredetaileddescriptionofthesetechniques,andthe
informationtheyprovideisgiveninSection4.3.
Beamlines for Nonlinear ARPES, Ultrafast, and High-resolution Experiments in Quantum Materials
Two-photonARPESexperimentsinquantummaterials
willrelyprimarilyonNGLSbeamline1,providing280eV
photonenergies(betterthan50meVresolutioninlong-
pulseoperationwithoutamonochromator)asdescribed
inSection5(Table2).Theuseofhigh-repetitionrateat
byexcitons(closelyboundstatesofelectronsandholes),
which are sharply defined in energy and should be
strongly affected by the degree of confinement.
Measurementsofthesepropertiesinpump-probeexperi-
ments may provide a time-resolved view of how the
onset of a complex low temperature ordered state
restrictstherelativelyfreeelectronkineticsofdisordered
materials,highlightingthespecificquantumcharacteris-
ticsofacomplexmaterial.RIXSistypicallyseveral(e.g.
3)ordersofmagnitudeweakerthanresonantelasticscat-
tering,withgreatvariabilitydependingonthespecific
material and resonance to be investigated. However,
evenprobingaverylimitedparameterrange,theseprop-
ertiesmakeitacriticalmethodtounderstandthefemto-
secondtimeevolutionofcomplicatedwavefunctions.
3 .7 .3 .4 Time-domain RIXS: Phase Profile
Measurements of Dynamic Structure
Theavailabilityoftunablephase-coherentpulseswith
highaveragebrightnessatNGLSoffersqualitativelynew
approachestoprobecollectiveexcitationsinmaterials.In
particular, nonlinear interference-based pump-probe
techniques suchas frequency-resolvedopticalgating
(FROGandcross-correlated“XFROG”)32,33canbeused
toextractboththeintensityandtemporalphaseofthe
scatteredX-raypulsestodeterminethedynamicstructure
factorwithunprecedenteddetailthatisnotavailablefrom
conventionalRIXSbasedontemporallyincoherentpulses.
Asanincidentphotonprobesacollectiveexcitation
viascatteringinamaterial,theoutgoingphotonisphase
shifted,i.e.thefingerprintofthephase-dependentcollec-
tiveexcitationspectrumS(q,ω,φ)isimprintedonthetem-
poralphaseofthescatteredX-raywave.Thisinformation
canbeextractedbycharacterizingthetemporalphase
andintensityofthescatteredwave.Keyrequirements
includetemporalcoherencethatisrepeatablefrompulseto
pulse,toenabletheaccumulationofsignalovermultiple
pulses.
In recentyears, sophisticated techniques forpulse
characterization(withattosecondresolution)havebeen
developedanddemonstratedintheEUVregime,andare
now being extended to the soft X-ray range.34This
approachprovidesimportantnewinformationaboutthe
“birth”ofcollectiveexcitations,andhowtheyevolveon
theattosecondtofemtosecondtimescales.Forexample,
comparingphaseinformationinthefirst250fsandfinal
250fsofanintense500fsscatteredpulse,onemight
68
3 . SCIENCE DRIVERSQUANTUM MATERIALS
strongnon-resonantscatteringmodes(e.g.phonons)by
atleastthreetofourordersofmagnitude,enhancingthe
contrastofweakresonantchargeexcitationmodesinthe
<100meVrange.
Enhancedenergyresolutioninexistingspectroscopies
hasbeenahallmarkofscientificadvancement,analo-
goustothewaythatMoore’sLawhasdefinedtheadvanc-
ing capabilities of computers. Dramatically higher
averagephotonfluxmakesitpracticaltousehigherreso-
lutionspectrometers,significantlyadvancingthecapabil-
ityofcurrentRIXSmeasurementtechnologiesthatare
limited to~0.1eV resolution.Figure46 illustratesan
advancedhigh-efficiencyRIXSend-stationdesignedto
measuremultiplemomentumdirectionssimultaneously
athighenergyresolution.
NGLSbeamline3inconjunctionwithanarrayofmomen-
tum-andenergy-resolved3DTOFanalyzers(seeSection
3.8)willenablerapiddataacquisitionanddiscernsmall
nonlinear signals from background (while avoiding
space-chargeeffects).Inthiscase,highenergyresolution
willbeprovidedbyamonochromator.RIXSexperiments
onquantummaterialsattransition-metalL-edgesinthe
softX-rayrangewillsimilarlyrelyonthehighenergyres-
olution(andhighaverageflux)ofNGLSbeamline1(with-
out a monochromator) and/or beamline 3 with a
monochromator.Attosecondvisible-pump,X-ray-probe
spectroscopyexperimentswillrelyprimarilyontheseed-
edNGLSbeamlines1and2.Theseexperimentswilluse
one-color(andinsomecasestwo-color)X-rayprobesto
followvalencechargedynamicsviaXASandXESattran-
sition-metalL-edgesandOK-edgeinthesoftX-rayrange.
Multi-dimensionalspectroscopyexperimentswillrelyon
two-colorsub-femtosecondcapabilitiesofbeamline2.
Tunabilityandvariablepolarizationwillbeimportantfor
nearlyallquantummaterialsstudies.
Technical Considerations — RIXS
TheRIXStechniqueisveryphoton-hungry,makinga
highrepetitionratecriticaltoavoiddisruptingthelow-
energycollectivestatesbeingmeasured.Basedonrecent
measurementsattheLCLS,itappearsthatasafeupper
boundforphotonfluxmaybeintherealmof1mJ/cm2,
sufficienttoallowroughly108photonsineachpulseincident
onthesamplewithamoderatelyfocusedbeam(~30μm
X40μm).Ata1MHzpulserate,thisconfigurationwill
produceasignalthatisthreetofourordersofmagnitude
strongerthanexistingbeamlines,*andallowconsider-
ableleewaytoimprovethestateoftheartexperimental
resolution.A90°scatteringbranchanalyzerwillsuppress
O-1s Cu-2p
Valence coupling
Time
Sample
t1 t2 t3
k1
k1
k2
k2
k3
k3
k4
k4
kS
Figure45 Schematic multi-dimensional spectroscopy in transition-metal oxide. X-ray pulse sequences tuned to the O-1s and Cu-2p probe correla-tions between O-2p and Cu-3d levels via core-hole correlation spectroscopy.35 Alternatively, X-ray pulses tuned to the O 1s -2p and Cu 2p-3d transitions may probe localized d-d transitions, and charge flow between the Cu and O sites via cohrerent X-ray Raman spectroscopy.36
–15°0°
15°
Figure46 Schematic of a high efficiency RIXS end-station, config-ured to simultaneously measure multiple momenta with enhanced energy resolution.
* Current spectrometers used for non-resonant measurements can achieve resolution ~1 meV (e.g. at APS Sector 3 among others) by sacrificing roughly four orders of magnitude in scattered signal intensity, which renders them unsuitable for synchrotron-based RIXS.
69
3 . SCIENCE DRIVERSQUANTUM MATERIALS
10. Kotani, A. and S. Shin, Rev. Mod. Phys., 2001. 73: p. 203–246.
11. Abbamonte, P., et al., Resonant Inelastic X-Ray Scattering from Valence
Excitations in Insulating Copper Oxides. Physical Review Letters, 1999.
83(4): p. 860.
12. Chakhalian, J., et al., Orbital Reconstruction and Covalent Bonding at an
Oxide Interface. Science, 2007. 318(5853): p. 1114-1117.
13. Lee, P.A. and N. Nagaosa, Collective modes in the superconducting
ground states in the gauge theory description of the cuprates. Physical
Review B, 2003. 68(2): p. 024516.
14. Carlson, E.W. and e. al., The Physics of Conventional and Unconventional
Superconductors, ed. K.H.B.a.J.B. Ketterson. 2002, Berlin: Springer-
Verlag.
15. Wray, L., et al., Momentum dependence of superconducting gap, strong-
coupling dispersion kink, and tightly bound Cooper pairs in the high-Tc
(Sr,Ba)1-x(K,Na)xFe2As2 superconductors. Physical Review B, 2008.
78(18): p. 184508.
References:
1. Leigh, R.G., P. Phillips, and T.-P. Choy, Hidden Charge 2e Boson in Doped
Mott Insulators. Physical Review Letters, 2007. 99(4): p. 046404.
2. Cvetkovic, V., et al., Observing the fluctuating stripes in high-Tc super-
conductors. EPL (Europhysics Letters), 2008. 81(2): p. 27001.
3. Geim, A. and K. Novoselov, Nobel Prize in Physics. 2010.
4. Abrikosov, A.A., V.L. Ginzburg, and A. J. Leggett, Nobel Prize in Physics,
2003.
5. Bednorz, G. and K. Müller, Nobel Prize in Physics, 1987.
6. Hasan, M.Z. and C.L. Kane, Topological Insulators. arXiv:1002.3895v2,
2010.
7. Moore, J., Topological Insulators: The next generation. Nature Physics,
2009.5(6): p. 378-380.
8. Wray, L.A., et al., Observation of topological order in a superconducting
doped topological insulator. Nat Phys, 2010. 6(11): p. 855-859.
9. Hasan, M.Z., et al., Electronic structure of Mott insulators studied by
inelastic X-ray scattering. Science, 2000. 288(5472): p. 1811-1814.
RIXS Experiments at 1 meV
Thefollowingestimatesillustratethescientificneedforahigh-repetition-rateultrafastXraylasertoinvestigate
low-energycollectiveexcitationsincorrelatedmaterialsviaRIXS:
Required integrated flux on the sample: ~1018 photons
ph/pulse (usable) Rep . rate [Hz]Time to do experiment
Time resolution
StorageRing 102[2] 5x108 200 days 100ps
PulsedFEL 108[1] 102 1000 days ~ps
NGLS 108[1] 106 3 hrs ~ps
[1] Fluence limit:~1mJ/cm2toavoiddisruptionofelectronicproperties
(e.g.1keV,108ph/pulse,50μmfocalspot⇒ ~1mJ/cm2)
includesmonochromatorlosseswithSASEFELoperation
Fouriertransformlimit:1meV⇔2psec
[2] Bandwidth limit:1meVBWand~100xlossesfrommonochromatoroptics
Nominal Storage Ring Source:
Flux ~5x1015ph/s/0.1%BW@1keV(withoutmonochromatorlosses)
Rep.rate 5x108Hz
Pulseduration 100ps
Nominal Storage Ring Source with Bunch Tilting:
Flux ~6x1012ph/s/0.1%BW@1keV(~106ph/pulse/0.1%BW)(withoutmonochromatorlosses)
Rep.rate 6x106Hz
Pulseduration ~1ps
70
3 . SCIENCE DRIVERSQUANTUM MATERIALS
27. Kane, C.L. and E.J. Mele, Quantum spin Hall effect in graphene. Physical
Review Letters, 2005. 95(22).
28. Moore, J.E. and L. Balents, Topological invariants of time-reversal-
invariant band structures. Physical Review B, 2007. 75(12).
29. Bostwick, A. and e. al., Science, 2010. 328: p. 999-1002.
30. Tohyama, T., K. Tsutsui, and S. Maekawa, Theory of RIXS in strongly cor-
related electron systems: Mott gap excitations in cuprates. Journal of
Physics and Chemistry of Solids, 2005. 66(12): p. 2139-2144.
31. Li, Y.W., et al., X-ray imaging of dispersive charge modes in a doped Mott
insulator near the antiferromagnet/superconductor transition. Physical
Review B, 2008. 78(7): p. 073104.
32. Trebino, R., Frequency-Resolved Optical Gating: The Measurement of
Ultrashort Laser Pulses. 2002: Springer.
33. Trebino, R. and e. al., Rev. Sci. Instrum., 1997. 68: p. 3277.
34. Thomann, I., et al., Characterizing isolated attosecond pulses from hol-
low-core waveguides using multi-cycle driving pulses. Opt. Express,
2009. 17: p. 4611-4633.
35. Schweigert, I.V. and S. Mukamel, Coherent ultrafast core-hole correla-
tion spectroscopy: X-Ray analogues of multidimensional NMR. Phys Rev.
Lett., 2007.99(16): p. 163001.
36. Tanaka, S. and S. Mukamel, Coherent X-ray Raman spectroscopy: A non-
linear local probe for electronic excitations. Phys Rev. Lett., 2002. 89(4):
p. 043001.
16. McElroy, K., Nature Physics, 2006. 2: p. 441-442.
17. Damascelli, A., Z. Hussain, and Z.X. Shen, Reviews of Modern Physics,
2003. 75: p. 473-541.
18. Fert, A. and P. Grünberg, Nobel Prize in Physics, 2007.
19. Lövenich, R., et al., Evidence of phonon-mediated coupling between
charge transfer and ligand field excitons in Sr2CuO2Cl2. Physical Review
B, 2001. 63(23): p. 235104.
20. Fu, L., C. Kane, and E. Mele, Topological insulators in three dimensions.
Physical Review Letters, 2007. 98: p. 106803.
21. Hsieh, D., et al., Observation of Time-Reversal-Protected Single-Dirac-
Cone Topological-Insulator States in Bi2Te3 and Sb2Te3. Physical Review
Letters, 2009. 103(14): p. 146401.
22. Hsieh, D., et al., A topological Dirac insulator in a quantum spin Hall
phase. Nature, 2008. 452(7190): p. 970-U5.
23. Hsieh, D., et al., A tunable topological insulator in the spin helical Dirac
transport regime. Nature, 2009. 460(7259): p. 1101-U59.
24. Hsieh, D., et al., Observation of Unconventional Quantum Spin Textures
in Topological Insulators. Science, 2009. 323(5916): p. 919-922.
25. Chen, Y.L., et al., Experimental Realization of a Three-Dimensional
Topological Insulator, Bi2Te3. Science, 2009. 325(5937): p. 178-181.
26. Chen, Y.L., et al., Massive Dirac Fermion on the Surface of a Magnetically
Doped Topological Insulator. Science, 2010. 329(5992): p. 659-662.
71
3 . SCIENCE DRIVERSSPIN AND MAGNETIZATION AT THE NANOSCALE
digmsthatwilldefineinformationandenergytechnolo-
giesofthe21stcentury.
Toaccomplishthis,criticalgapsinourunderstanding
ofspinandmagnetizationdynamicsmustbebridged,
andforthispurposethecapabilitiesofNGLSsoftX-ray
laserwillbeindispensible.Inparticular,NGLSwillenable
incisiveprobingofspindynamicsonthefundamental
timescalesofexchangeinteractions(1-100fs)andspin-
orbitcoupling(~1ps).Mostimportantly,thecapabilityfor
probingthesefundamentaltimescaleswillbecombined
with:
• nanoscalespatialresolution—imaginglocalspin
structures
• momentumandenergyresolution—understanding
k-dependentspinscattering
• elementspecificity—distinguishingtherolesofspe-
cificinnercoreelectrons,e.g.intransition-metalions
• magneticorderingsensitivity—distinguishingferro-
andanti-ferromagneticorderviaX-raymagneticlin-
eardichroism(XMLD)
Magnetisminnovelmagneticmaterialsarekeycom-
ponentsofmoderntechnologiesrangingfromadvanced
permanentmagnetsinelectricitygenerationanduse,to
computerharddrives,toscientificandmedicalimaging.
Themanipulationofspinandchargeonananometer
lengthscaleformsthefoundationformoderninforma-
tionprocessingandstoragetechnology.Todate,thevora-
cious demand for higher-speed and higher-density
informationprocessing,storage,andretrievalhasbeen
metbyexponentialimprovementsintechnologyoverthe
pastseveraldecades.Thispaceofadvancementisnow
approachingsignificantlimitationsthattestourfunda-
mentalunderstandingofspinandmagnetizationdynam-
icsatthenanoscale.Atthesametime,therolesofspin
andmagnetization,andthecouplingofspinandcharge
incomplexmaterialsgivesrisetonewphenomenaand
newmaterialpropertiessuchasunconventionalsuper-
conductorsandtopologicalinsulators.Akeychallengeis
tounderstand,manipulate,andexploittheseproperties
toprovideafoundationfornewdevicesandnewpara-
Achieving a fundamental understanding of spin and magnetization dynamics at the nanoscale is essential to meet the future demand for higher-speed and higher-density information technologies. New concepts to manip-ulate the spin of the electrons on nanometer length scales will provide a foundation for new devices and new paradigms that will define information and energy technologies of the 21st century.
NGLS will be indispensible to bridge critical gaps in our understanding of spin and magnetization dynamics. In particular, NGLS will enable incisive probing of spin dynamics on the fundamental time scales of exchange inter-actions (1-100 fs) and spin-orbit coupling (~1 ps) in combination with element-specific nanoscale spatial resolu-tion, and momentum and energy sensitivity. The high repetition rate, high energy resolution, and ultrafast pulses at NGLS will bring unprecedented experimental capabilities to perform time resolved spin-polarized ARPES in spin flip processes. Femtosecond two-color experiments with polarized soft X-rays will reveal the distribution of spin and angular momenta and experiments are envisioned, where one of the two pulses can be utilized as a source for generating spin accumulations with a subsequent probing of the spin dynamics in non-magnetic metals.
NGLS will provide unique insight into the fastest manipulation of spins by photons and will open a path to utilize pure spin currents in loss-free future electronic devices.
3.8 SpinandMagnetizationattheNanoscale
72
3 . SCIENCE DRIVERSSPIN AND MAGNETIZATION AT THE NANOSCALE
alsomissing.Directmeasurementofthetransientnon-
equilibriumelectronicstructure,withfullenergy,momen-
tum,spin,andtimeresolutioniscriticalforelucidating
theseissues,andtime-andspin-resolvedARPESoffers
exactlythesecapabilities.Time-resolveARPESmeasure-
mentswillprovidethefirstreal-timeglimpseofspin-flip
scatteringprocesseswhicharebelievedtoproceedvia
“hotspots,“i.e.regionsintheBrillouinzoneofhighspin-
orbitcoupling(seeFigure48).
• spin/orbitsensitivity—quantifyingandseparating
magneticgroundstatepropertiesviaX-raymagnetic
circulardichroism(XMCD).
3.8.1 Ultrafast Manipulation of Magnetism and Spin
Prospectsforcontrollingmagnetismontheultrafast
time scale were highlighted by seminal experiments
morethanadecadeagothatdemonstratedlight-driven
sub-picosecond demagnetization of a ferromagnetic
metal.1Thesesurprisingresultschallengedconventional
paradigms,andevidencedourlackofunderstandingof
spinandmagnetizationdynamicsonfundamentaltime
scales. Moreover, they sparked new interest into
approachesforultrafastopticalcontrolofmagnetism.2,3
Theconceptofall-opticalswitchingofmagnetization(see
Figure47)illustratestheprospectsforpushingmagnetic
data storagespeeds into the fs region.However, the
microscopicmechanismbehindthisswitchingprocess
remainselusive.Otherintriguingneweffectsappearon
thehorizonsuchastherecentlyreportedchangeofthe
magnetic anisotropy energy barrier during ~200 fs
intenseelectricalfieldpulses.4However,itsmicroscopic
originisevenlessexploredandunderstood.
Theessenceofmagnetismisangularmomentumas
demonstratedclearlybytheEinstein-deHaaseffect.5We
needtounderstand:
• Whatarethechannelsforultrafastangularmomen-
tumtransferto,fromandwithinthespinsystem?
• Doestheangularmomentumcomefromlightorisit
providedbyotherreservoirssuchasthelattice?
3 .8 .1 .1 Ultrafast Manipulation of Magnetism and Spin:
Time- and Spin-Resolved ARPES
Whileall-opticalpump-probetechniquesareableto
takestroboscopicimagesoftheactualmagnetization,
keyquestionsremainaboutthemicroscopicspinand
electrondynamicsontime-scalesrangingfromtheultra-
fastcontrolpulse to thecompletionofmagnetization
reversal.Forinstance,theexactmechanismforthebal-
anceofangularmomentumwhichmustoccurbetween
the light, the electron spins, and the lattice remains
unknown.Afullunderstandingofhowthesystemisable
tocompletethemagnetizationreversalthroughademag-
netizedstate,longafterthecontrolpulseisremoved,is
Figure47Demonstration of all-optical magnetization reversal on GdFeCo alloy where the magnetization of adjacent regions can switch only by changing the helicity of the incident fs laser pulse.6
K
+q
K
X-L
spsp
d
Kx (Å-1)
K y (Å-1
)
Figure48Region of the Ni bulk Brillouin zone as measured by synchrotron based ARPES (H. Durr, unpublished) showing the regions of spin-orbit “hot spots”, i.e. those points where spin up and down bands cross, leading to reduced magnetic moments and increased spin-orbit coupling. It is believed that spin-flip scattering into such “hot spots” provides an efficient avenue to spin angular momentum transfer to lattice excitations,2 q. Spin and time-resolved ARPES at NGLS will provide the first time access to observing such processes in real time.
73
3 . SCIENCE DRIVERSSPIN AND MAGNETIZATION AT THE NANOSCALE
showsthefirstreportedbranchingofspinandorbital
momentevolutionsfollowingfemtosecondlaserheating
ofCoPdalloys,usingultrafastX-raydichroismtofollow
thespinandorbitevolution.8Thistypeofexperimental
probewilldirectlyviewthemechanismswithintheelec-
tronic structure that provide the required angular
momentumbalanceandallowultrafastmagnetization
control.Thisinturncanprovideinsightforoptimizing
materialdesignstoimproveperformance,andincrease
spatialdensityforeventualdeviceapplications.
In complex materials, a powerful experimental
approachistransienttwo-colorscatteringorholographyas
illustratedinFigure50.ThisexploitssoftX-raydichroism
andsourcecoherencetofollowspinandorbitalordering,
andtheirevolutioninresponsetotailoredmaterialexcita-
tion.Inmultiferroicsforexample,thisapproachwillpro-
videinsighttothecoupledorderparameters.Thistypeof
experimentsaimsatspatiallycorrelatingcoupledorder
parametersandobtainingsnapshotsoftheirtemporal
evolution.Becauseofdynamicfluctuationsoftheorder
parameters, thecorrelationhastobeperformedona
pulsebypulsebasisusingtwoX-rayprobecolors(see
Figure50)ordifferentX-raypolarizations.
3.8.2 Spin Accumulations and Currents
Presentsemiconductorelectronicdevicesanddata
storagetechnologiesrelyprimarilyonchargecurrentsto
transmitandstoreinformation.Powerconsumptionand
A time-delayed ultrafast X-ray pulse, following an
ultrafastopticalorTHzexcitationpulse,withfullanalysis
oftheresultingphotoelectronenergy,angle,andspin,
will capturenot just the time-resolvedmagnetization
state,but reveal theentirespin-dependentelectronic
structure.Suchmeasurementswillenableadirectcor-
roborationofcontroversialinitialreportsonlaser-induced
localizationofthevalenceelectronicstructureinmaterials
likemetallicNi.7
3 .8 .1 .2 Ultrafast Manipulation of Magnetism and Spin:
Dynamic Soft X-Ray Scattering
X-raysoffertheuniqueadvantageofallowingusto
probespinandorbitalmomentumseparately.Figure49
Sz
Lz
T=280 ± 20 fs
–55
–67
0.2
0 1Delay (ps)
2
0.4
0.6
0.8
T=280 ± 20 fs
Orbits Spins
Time
Spin
and
orb
ital m
omen
ts (ħ
per
ato
m)
Figure49 Materials composed of alternate layers of Co and Pd introduce a spin orientation perpendicular to the layers which is preferred for magnetic data recording. Femtosecond optical puls-es were used to alter the electronic level population leading to a change in electron orbits (blue arrows and ellipses) and essential-ly quenching the magnetic anisotropy responsible for the perpen-dicular spin orientation. Femtosecond X-ray pulses were used to probe the change in orbital motion(blue symbols) and follow the spin rotation into the layer plane (red symbols).8
Probe spin & orbital distributions
Holography
Electrons
Lattice
Spins
THzpump
X-rayL3probe
X-rayL2probe
Figure50Two-color coherent scattering experiments to probe spin and orbital contributions to magnetic order, and their evolu-tion in response to tailored excitation, e.g. THz excitation of the electronic system (figure courtesy H. Durr).
74
3 . SCIENCE DRIVERSSPIN AND MAGNETIZATION AT THE NANOSCALE
tingisrathersmall.10NGLSX-raylaserswillachievethe
goalofdirectimagingofspinaccumulationduethecom-
binedcapabilitiesofhighrepetitionrate(forhigh-sensi-
tivitymeasurementsandhighaverageflux)withtime
structurefortransientspectroscopymeasurements.
Furthermore,theabilitytoperformtwo-colorX-raypulse
experimentswillenablemeasurementsanalogoustoopti-
calpump-probeexperimentsinsemiconductors,sincea
circularpolarizedX-raybeamcanalsobeutilizedasasource
forgeneratingspinaccumulations.Thelatterisparticularly
interesting,sincethesub-pspulselengthswillenable
directinvestigationofspindynamicsinnon-magneticmet-
als,wherethespinrelaxationtimeistypicallyafewps.14
ThusfutureX-rayinvestigationsofspinaccumulations
inmetalswillresultinasignificantlymoredetailedunder-
standing of spin-dependent effects.This is important
sincetheimpactofspincurrentsgoesfarbeyondthe
spintronics paradigm. Namely, spin currents can be
observed in insulators,19 theycanbecoupledtoheat
transport,20andtheycanresultinangularmomentum
transport,whichimpactsmechanicalmotion.21Thusthey
offerawidevarietyofnovelopportunitiesforenergycon-
versionatthenanoscale.
3 .8 .2 .1 Spin Accumulations and Currents:
Magnetic Nano-spectroscopy
Currentapproachestostudyspinandmagnetization
dynamicson fundamental lengthand timescalesare
inherentlyandseverelylimited.Thefastestfemtosecond
heatdissipationarenowmajorobstaclestofurthermin-
iaturization,withpowerdensitiesofmodernprocessors
exceeding100W/cm2.Moreover,theprojectedenergy
demandfromconsumerelectronicsin2030,basedon
today’stechnologies,willrequiretheequivalentof230
additionalnuclearpowerplants.Onepromisingcandi-
date to replace existing charge-based technologies
(CMOS)exploitsspincurrentsandaccumulations,which
isthefoundationforspintronicsdevelopment.9
Mostpracticalimplementationsofactualspintronic
devicesaremetal-based,butatthesametime,adirect
imagingofeitherspincurrentsoraccumulationsinthese
structuresremainselusive.10Additionally,theconceptof
purespincurrents,whicharenotaccompaniedbyanet
chargecurrent,hasrecentlyreceivedincreasedatten-
tion.11Fromafundamentalpointofviewthesepurespin
currentsprovidedirectinsightintospin-dependentphys-
ics and are completely undisturbed by charge trans-
port.12-14 For technological applications, they offer
significantpotentialadvantages,suchasreducedpower
dissipation,absenceofOerstedstrayfields,anddecou-
plingofspinandchargenoise.
Purespincurrentscanbegeneratedthroughnon-local
electricalinjection,opticalinjection,spinpumpingfroma
precessingferromagnet,andviatheandspinHalleffect.15
Tounderstandthespindynamicsofsuchnon-equilibrium
spinaccumulations,adirectimagingcapabilityenabled
byNGLSX-raylaserswillbeamajoradvance.Thisisevi-
dent frompast researchonsemiconductingsystems,
wheresuchimagingispossibleintheopticalregimedue
tostrongmagneto-opticeffectsandlongspindiffusion
lengths.16-18Questionsofparamountsignificancetoboth
thebasicunderstandingandeventualtechnicalutilization
ofspincurrentscouldbeimmediatelyanswered:
•Howdospincurrentscoupletochargecurrents?
•Dospincurrentsinteractwithheattransport?
•Canweexploitangularmomentumtransfer?
•Whatisthespatialandtemporalspinflowcreatedby
spinpumping?
X-rayimagingwithelementspecificmagneticcontrast
through X-ray circular dichroism is a very promising
approach.Theachievablespatialresolutioniscomparable
orbetterthaneventheshortestspindiffusionlengths
encountered inmostmetals.However,so far there is
insufficientX-rayspectralsensitivitytospinaccumula-
tions,sincethespin-dependentchemicalpotentialsplit-
Current CMOS technology– inherent energy losses
Log
(l S-D
)
VT VG-S
CMOS
Mott-FET
Figure51 I-V characteristic of current CMOS technology illustrating the relatively large required switching voltages (and inherent energy losses) in comparison with Mott-FET transitions.
75
3 . SCIENCE DRIVERSSPIN AND MAGNETIZATION AT THE NANOSCALE
spinaccumulationswithasubsequentprobingof the
spindynamicsinnon-magneticmetals,wherethespin
relaxationtimeistypicallyafewps.
Avarietyofimagingtechniques,whichhavealready
beendevelopedatcurrentsources(X-rayholography23
orzoneplatebasedfullfieldX-raymicroscopy24),canina
similarwaybeimplementedatNGLStotakesnapshot
imagesofnanoscaleultrafastspindynamics.
EstimatesfortheheatloadontoaFresnelzoneplate
objectivelenstoachievehighspatialresolutioninasin-
glepulseexperimentindicatethatthetemperaturerisein
theopticalelementwillbelessthan100degrees,which,
basedontheU41microscopeatBESSY,canbeeasilytol-
eratedbyexistingzoneplatetechnologies.24
3 .8 .2 .2 Spin Accumulations and Currents:
Time- and Spin-resolved ARPES
Inthepastdecades,angle-resolvedphotoemissionspec-
troscopy(ARPES)hasprovedtobeanextremelysuc-
cessfulandpowerfultechniqueforadvancingthe
understandingofcomplexcorrelatedelectronsystems,
includinghigh-temperaturesuperconductors(HTSC),25-30
colossalmagnetoresistive(CMR)manganites,31-33gra-
phene,34-37andtopologicalinsulators.38-42Thedevelop-
mentofspin-resolvedARPEStechniqueshassparked
rapidlygrowinginterestinthelastfewyearsduetoits
relevanceforprobingspinphysicsforpotentialdevice
applications,andagrowingpoolofhigh-impactresults
laserexperiments,limitedbythewavelengthofoptical
light,lacksufficientspatialresolution.Thehighestresolu-
tionmicroscopiesaremanyordersofmagnitudeaway
fromaccessing fundamental timescales. Inaddition,
opticaltechniqueslacksensitivitytoseparatespinand
orbitalmoments,andareunabletodistinguishbetween
different typesofmagneticordering.SoftX-ray tech-
niquesareaperfectmatchintermsofelementspecificity,
sensitivitytospinandorbitalmoments,andspatialreso-
lution.Forexample, the latestdevelopmentsofX-ray
opticshasdemonstrateda<10nmspatialresolution.22
Slicingexperimentsatcurrentsynchrotronscanaccess
thefew100soffemtosecondregime,however,thelossin
intensityistremendousandinsufficient(bymanyorders
ofmagnitude)toenablesnap-shotcapabilitiesforimag-
ingorspectroscopicanalyticaltoolsatthenanoscale.This
gapcanonlybefilledwithanextgenerationsoftX-ray
source.
Thehighrepetitionrate,thehighenergyresolution
andtheultrafastpulsesatNGLSwillbringunprecedent-
edexperimentalcapabilities.Timeresolvedspin-polar-
izedARPESwillallowthestudyofspinflipprocesseswith
energyandmomentumresolution,whichareessentialto
identifyspin-flipprocessesresponsibleforthemagneti-
zationreversal inanall-opticalprocess.Femtosecond
two-colorexperimentswithpolarizedsoftX-raysincom-
binationwithimagingandmicro-spectroscopywillreveal
thedistributionofspinandangularmomenta.Oneofthe
twopulsescanalsobeutilizedasasourceforgenerating
FM
Spin
wav
e
NMFigure52 Spin Hall effects generate transverse spin currents from charge currents even in non-magnetic materials.
Figure53 Pure spin currents generated in nonmagnetic metals (NM) via spin pumping from spin waves in a ferromagnet (FM).
76
3 . SCIENCE DRIVERSSPIN AND MAGNETIZATION AT THE NANOSCALE
exampleseeReferences59,60),aswellasthefunda-
mentallengthscalesofthemanynanoscaleandnano-
structured systems of current interest. Physical
restrictionsonthelocationsofthezoneplatesorother
opticsrequiredforthenanoscalebeamsrequirephoton
energies≥100eV,wherehigh-harmonicgenerationlaser
basedsystemsaresimplyunabletoprovidethehigh
averagefluxrequiredforARPESexperimentswithtime,
spin,andspatialresolution.
Thefullpotentialoftime-,space-,andspin-resolved
ARPEStoattackthekeyquestionsintheelectrondynam-
icsofnextgenerationmaterialsrequiresasourcewith
shortpulsesatthetimescalesinquestion(10-500fs),
tunabilityintherangeof280–1000eV,highaverageflux,
andhighrepetitionrates(~1MHz).Thesestrictrequire-
mentsarefulfilledonlybyNGLSspecifications.Thegain
inourabilitytodirectlyobservechargeandspindynam-
icsandtheirresponsestoexternalstimuliwillbepara-
mountforourgoalsofdevelopingmoreprecise,faster,
moreefficient,andmorereliablecontrolovermaterials
andtheircapabilities.
3.8.3 Instrumental and Technical Considerations for Time- and Spin-Resolved ARPES
Spin-resolvedARPESisinherentlyalowcount-rate
experimentaselectronspin-polarimetersarequiteineffi-
includingthediscoverybyspin-ARPESoftopological
insulators.39,40,43-48
AswithX-rayscattering,highphotonfluxalongwitha
highrepetitionratefromNGLSX-raylaserswillallow
spin-resolvedARPEStobeperformedwithtime-resolu-
tion through pump-probe techniques. Initial research
developing (non-spin resolved) pump-probe time-
resolvedARPESstronglysuggestthatitcanbeasuperb
probeofelectrondynamics,49-51althoughthelimitations
ofthecurrentprobesourcesarerestrictive.
Initialtime-andspin-resolvedexperimentshavebeen
performedwithtable-toplasersystemsprovidingboththe
pumpandprobepulses(forexample,seeReferences52
and53),howevertheseexperimentsareseverelylimited
bytheprobesourceintermsoflowphotonenergyand
lackof tunability.HighresolutionARPESexperiments
alsorestrictthenumberofusablephotonsperpulsedue
tosignificantspace-chargebroadeningoftheoutgoing
photoelectrons.54-58Combinedwiththerequirementof
highaveragefluxforsuchcount-starvedexperiments,
thisresultsintheabsoluteneedforahighrepetitionrate
source,farbeyondwhatlaser-basedsystemsandother
FELsourceswillbeabletoprovide.Theseissuesarenot
minor, but become full“show-stoppers” for the vast
majorityofexperimentsthatcanbeenvisioned.
TheconceptofspatiallyresolvedARPESexperiments
performedwithnanoscalefocalspotshasalsobecome
importantduetothelargespatialinhomogeneityofmany
“correlatedelectron” systemsof current interest (for
FEL
Plane mirror
Sample stage
CCDdetector
Condenser
Zone plateobjective
Figure54 X-ray optical setup of a full-field X-ray microscope for single pulse imaging. The FEL radiation is collected by a special condenser — a beam shaping diffractive optical device about 1 mm in diameter — which forms a homogeneous illumination of the object field. (Figure courtesy G. Schneider, BESSY)
77
3 . SCIENCE DRIVERSSPIN AND MAGNETIZATION AT THE NANOSCALE
anarrayofmultiple(possiblyupto10)analyzersmount-
edatdifferentemissiondirectionsforsimultaneoususe.
Theresultingmassivelyparalleldataacquisitionwould
greatly improve the speed and scope of successful
experiments.TheultrafastpulsewidthofNGLS,com-
pared with current synchrotron sources, could even
allowforimproveduseoftheTOFtechniqueovercurrent
setupsthatarelimitedbythepulsewidthsofthelight
source.Withmuchshorterpulsewidthsprovidedby
NGLS,andimprovedelectrondetectors(e.g.Reference
64),significantlysmallerTOF-basedanalyzerscouldbe
utilizedwhilestillachievingthesameenergyresolution.
Thispossiblesizereductioncouldbequiteimportant,
especiallywhenconsideringtheconceptofanentire
arrayofmultiplexedanalyzers.
Beamlines for Studies of Spin and Magnetization Dynamics
Visible/THz-pump,X-ray-probestudiesofmagnetiza-
tionandspindynamicswillrelyprimarilyontheseeded
NGLSbeamlines1and2asdescribedinSection5(Table2).
Theseexperimentswilluseone-color(andinsomecases
two-color)X-rayprobestofollowmagnetizationandspin
dynamicsviaX-raydichroismanddichroicscattering
effectseffectsattransition-metalL-edgesinthesoftX-ray
cient.Inadditiontohighrepetitionrateandothercapa-
bilitiesprovidedbyNGLSX-raylasers,theseexperiments
willalsorequireinstrumentationthatmaximizesdetec-
tionefficiency.ThetimingstructureofNGLSpulses,in
additiontoprovidingtimeresolutionthroughpump-and-
probetechniques,naturallyallowstheuseoftime-of-
flight(TOF)basedphotoelectronanalyzersthatcanbe
moreefficientthanthemorefrequentlyusedhemispheri-
calanalyzers,duetomultiplexingdetectionintheenergy
dimension.61-63
AnewlydevelopedTOF-basedspin-ARPESanalyzer61
(Figure55)thatincludesaphotoelectronbandpassfilter
allowsforhigherrepetitionratesources(>10MHz)and
betterenergyresolutions(<10meV)thanaretraditionally
possible,andillustratesanidealinstrumentalapproach
forthistypeofexperiment.Theenhancedefficiencyofthe
uniquespinpolarimeterincludedinthisdesign61furthers
improvestheseexperiments.Indeed,thisinstrumenthas
alreadysuccessfullytakenspin-resolved(nottime-resolved)
ARPESdataatbeyondstate-of-the-artresolutionswhile
makinguseoftheALStwo-bunchmodeat~3MHzrepeti-
tionrate,61thusdemonstratingthefeasibilityofthistype
ofexperimentwithinNGLSrepetitionratecapabilities.
Akeyconceptinmaximizingthepossiblethroughput
ofsuchexperimentswouldbetoadditionallymultiplex
detectionintheangular(momentum)dimensionthrough
e–
Figure55 Left: Schematic of a time-of-flight (TOF) based, high efficiency spin-resolved photoemission spectrometer, recently developed at the ALS. The spin resolution is obtained through low energy exchange scattering, depicted in the inset. From Reference 61. Right: Depiction of a high efficiency ARPES endstation with multiplexed TOF analyzers for massively parallel data acquisition over an extremely wide angu-lar range. Such a setup could provide the efficiency required in such count-starved experiments, where the allowed photon flux per pulse is also restricted by space charge considerations.
78
3 . SCIENCE DRIVERSSPIN AND MAGNETIZATION AT THE NANOSCALE
withoutamonochromator,and<10meVresolutionwith
amonochromator,asdescribedinSection5(Table2).The
highrepetitionrateprovidedbybeamline3willbeimpor-
tantforhigh-sensitivitymeasurements(whileavoiding
space-chargeeffects).Inthiscase,highenergyresolution
willbeprovidedbyamonochromator.
range.Tunabilityandvariablepolarizationwillbeofpara-
mountimportancefornearlyallstudiesofmagnetization
andspindynamics.
Time-andspin-resolvedARPESexperimentswillrely
primarilyonNGLSbeamline1,providing280eVphoton
energies—<50meVresolutioninlong-pulseoperation
Time- and Spin-Resolved ARPES Experiments at 10 meV
Thefollowingestimatesillustratethescientificneedforahigh-repetition-rateultrafastX-raylasertoinvestigate
spinandmagnetizationdynamicsviatime-andspin-resolvedARPES:
Required integrated flux on the sample: ~1017 photons
ph/pulse (usable) Rep . rate [Hz]Time to do experiment
Time res .solution
StorageRing 104[2] 107[3] 10 days 100ps
PulsedFEL 106[1] 102 10,000 days ~fs
NGLS 106[1] 106 1 day ~fs
[1] Fluence limit:space-chargelimitations(distortionofphotoemissionspectra)
includes10xmonochromatorlosseswithSASEFELoperation
Fouriertransformlimit:10meV⇔200fs
[2] Bandwidth limit:10meVBWand~10xlossesfrommonochromatoroptics
[3] Rate limit:<107Hz,determinedbytime-of-flightenergyanalyzer
Nominal Storage Ring Source:
Flux ~5x1015ph/s/0.1%BW@1keV(withoutmonochromatorlosses)
Rep.rate 5x108Hz
Pulseduration 100ps
Nominal Storage Ring Source with Bunch Tilting:
Flux ~6x1012ph/s/0.1%BW@1keV(~106ph/pulse/0.1%BW)(withoutmonochromatorlosses)
Rep.rate 6x106Hz
Pulseduration ~1ps
79
3 . SCIENCE DRIVERSSPIN AND MAGNETIZATION AT THE NANOSCALE
24. Schneider, G., et al., X-Ray Microscopy at BESSY : From Nano-
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27. G.H. Gweon, et al., An unusual isotope effect in a high-transition-temper-
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28. H. Ding, et al., Spectroscopic evidence for a pseudogap in the normal
state of underdoped high-Tc superconductors. Nature, 1996.382: p. 51.
29. T. Kondo, et al., Competition between the pseudogap and superconduc-
tivity in the high-Tc copper oxides. Nature, 2009. 457: p. 296.
30. Z.X. Shen, et al., Anomalously large gap anisotropy in the a-b plane of
Bi2Sr2CaCu2O8+δ. Phys. Rev. Lett., 1993. 70: p. 1553.
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12. G. Mihajlovic, et al., Absence of the Giant Spin Hall Effect. Phys. Rev. Lett. ,
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16. J. M. Kikkawa and D.D. Awschalom, Lateral drag of spin coherence in
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45. Hochstrasser, M., et al., Phys. Rev. Lett., 2002. 89: p. 216802.
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81
3 . SCIENCE DRIVERSBIOLOGICAL SYSTEMS: IMAGING DYNAMICS AND FUNCTION
decadesintime,frompicosecondstomilliseconds.Such
dynamicsarecentraltothefunctionofbiologicalsystems
andmacromolecularmachines,rangingfromtheenzymes
responsibleforDNArepairandreplication,toribosomes
responsibleforproteinsynthesis,todynamicmembranepro-
teinchannelsandsignalingcomplexes,toorganellsandthe
hierarchicalstructuresofthecell—thefundamentalunitoflife.
Inordertoadvancetheunderstandingofbiological
processesatacellularlevel,detailedknowledgeofthe
structureofverylargemacromolecularmachinesisan
absolutenecessity.Thehigh-repetition-rateX-raylasers
atNGLSwillenableentirelynewmethodsforimaging
biomolecules innativeenvironments (solution rather
thancrystal),capturingthedynamicsofmoleculesduring
theirfunctionalcycles,andvisualizingofcellularcompo-
nentsintheirnativecontext.Thedreamof“imagingbio-
logicalfunction”willberealizedforthefirsttime.
In the following, we present a few representative
examplesofkeyareasofbiologythatwillbetransformed
Modernsynchrotronsandmacromolecularcrystallog-
raphymethodshaverevolutionizedthefieldofstructural
biologybyenablingtheroutinestructuredeterminationof
isolatedorsimplecomplexesofmacromolecules.These
structuralmodels1arethefoundationforunderstanding
fundamentalprocessesinbiology,2,3andthedevelop-
mentofnewtherapeuticdrugs4,5andnovelclassesof
nano-materials.6Thesignificanceofthisisevidencedby
severalNobelPrizesinrecentyears.
However,conventionalstructuraldeterminationsof
biologicalmacromoleculesdisregardthepresenceofhetero-
geneousconformationsbyassumingidenticalobjects.
Moreover,currentX-rayandelectronmicroscopy(EM)7
structuraldeterminationmethodsdependongrowingordered
crystalsofproteins,whichnecessarilyinhibittheinvesti-
gationofdifferentconformations,andthedynamicmotions
thatconnectthem.Atthesametime,itiswellknownthat
biologicalfunctionisprofoundlyinfluencedbysubtle(and
notsosubtle)changesinconformationthatspanmany
Biological function is profoundly influenced by changes in molecular conformation that span many decades in time, from picoseconds to milliseconds. Such dynamics are central to the function of biological systems and macromolecular machines, ranging from the enzymes responsible for DNA repair and replication, to ribosomes responsible for protein synthesis, to dynamic membrane protein channels and signaling complexes, to organ-elles and the hierarchical structures of the cell — the fundamental unit of life. An enhanced understanding of the role of dynamics in biological function has both fundamental and practical importance, ranging from under-standing how cells work to the design of better therapeutics.
Modern synchrotrons and scattering / diffraction techniques have revolutionized the field of structural biology by enabling the routine structure determination of isolated or simple complexes of macromolecules. The high-rep-etition-rate of NGLS will enable entirely new methods for imaging biomolecules in native environments (solution rather than crystal), capturing the dynamics of molecules during their functional cycles, and visualization of cellular components in their native context. Of paramount importance will be the ability of NGLS to deliver ultrafast X-ray pulses at high repetition rate. This, in combination with newly developed sample delivery systems and X-ray detectors, will make it possible to collect billions of images of single molecules and whole cells. From these images it will be possible to probe thousands of conformational states accessible to biomolecules in solution.
NGLS will provide for a greater understanding of biomolecular dynamics and structure in native environments, and a wealth of information to guide the development of better therapeutics.
3.9 BiologicalSystems:ImagingDynamicsandFunction
82
3 . SCIENCE DRIVERSBIOLOGICAL SYSTEMS: IMAGING DYNAMICS AND FUNCTION
However,thismethodprovidesonlystaticsnapshotsof
reactionstatesthathappentocrystallize,andhenceis
incapableoffullymappingoutthecompleteconforma-
tionalevolutionofintact,functionalholoenzymes.The
approachesofferedbyNGLSoffer anunprecedented
opportunitytorisetothischallenge.Furthermore,suc-
cessfulapplicationofthesemethodstotopoisomerases
willopenanewwindowin to thestudyofany large,
dynamicmacromolecularcomplex.Suchassemblieslie
attheheartofthemolecularoperationsofthecellandlife
asweknowit.
An example of another such system is the“initio-
some,”alargecomplexresponsibleforassemblingthe
machinery(the“replisome”)necessarytocopyDNAfor
cellproliferation.13Thesecomplexesassembleinastep-
wisemanner,againdependentonATP,butoveramuch
largerregionofDNA.Theyfurtherwrap,deform,andmelt
DNAasanintegralpartofthepathwaytowardreplisome
assembly.Thestructuresformedduringthisprocessare
large(borderingon1MDa),dynamic,14and—giventhe
flexibilityofDNA—impossibletocrystallize.Anabilityto
imagethetransient-assemblystatesonthewaytorepli-
someformationwillonlybepossiblebydynamicsingle
moleculeimagingmethodssuchasdescribedinSection
4.1basedontheuniquecapabilitiesofNGLSX-raylasers.
bythecapabilitiesofNGLS
forimagingheterogeneous
ensembles and conforma-
tionaldynamicsofbio-mole-
culesinnativeenvironments.
InSection4.1wedescribein
further detail the enabling
techniques of fluctuation
X-ray scattering (fXS), bil-
lion-snapshot coherentdif-
fractive imaging, and
associated advanced com-
putationalmethodsofmani-
fold mapping, that will be
realized for the first time
using the high-repetition-
rateX-raylasersatNGLS.
3.9.1 DNA Repair
TypeIItopoisomerasesareessentialenzymesrequired
forunknottinganddisentanglingDNAinthecell.8Both
reactionsarecarriedoutbythetopoisomerase-mediated
breakingofoneDNAsegment,andthepassageofasec-
ondDNAthroughthebreak.Followingpassage,thebro-
kenDNAisresealedtopreventdamagetothegenome.
Howtype II topoisomerasescarryoutstrandpassage
faithfullyandrapidlyisnotunderstood.Theenzymemust
coordinatelarge-scaleatomicmovements(ontheorder
of30-150Å)tosuccessfullynavigateoneDNAthrough
another.Achemicalco-factor,ATP,isrequiredforactivity.
HowchemicalenergyreleasedfromthehydrolysisofATP
ischanneledintomechanicalmotionisnotunderstood.
Sucheventsarecentralnotjusttotopoisomerases,but
alsototheoperationsofallmolecularmachines.
TypeIItopoisomerasesalsoarethetargetofnumer-
ousantibiotics9andanti-canceragents10usedintheclinic.
HowthesedrugsactattheactivesiteforDNAcleavage
andATPhydrolysishasbeenestablished.Whatisless
clearishowdrugsaffecttheconformationalcyclingof
theprotein.Thereisanintriguingpossibilitytodevelop
newclinically-valuableagentsthatinterferespecifically
withtopoisomeraseandDNAmovement,asopposedto
chemistry.
New imaging methods are needed to tackle these
problems.Crystallographyhasbeeninvaluableindeter-
miningatomic-resolutionmodelsoftopoisomerasepieces.11,12 Figure56 Hexameric helicases.
Coherent diffractive imaging
Fluctuation X-ray scattering
Giga-shot diffractive imaging
Heterogeneous ensembles
Spontaneous dynamics
Native environments
Sample replacement between pulses
see section 4.1
83
3 . �SCIENCE�DRIVERSBIOLOGICAL�SYSTEMS: �IMAGING�DYNAMICS�AND�FUNCTION
The interface between the two ribosomal subunits, the
30S and 50S subunits in bacteria, consists of salt water to
a large extent.15 The number of direct interactions
between the two subunits, which are primarily RNA, was
much lower than expected. This may explain why ribo-
some function is incredibly sensitive to the salts in the
solution around it. The ability to image the ribosome and
its dynamics, under physiological conditions in solution,
will provide a crucial understanding of the effects of local
environment on structure and stability. The ability to
image thousands of conformational states will show the
full trajectory of ribosome movement during its cycle, and
potentially the production of the growing protein chain.
3.9.3� Membrane�Proteins
Cells are surrounded by membranes that separate the
cellular interior from the outside world. The lipids and
proteins that dominate the composition of membranes
exhibit a characteristic architecture in which the lipids
adopt a bilayer arrangement (~40 Å thick) penetrated by
integral membrane proteins. The flows of molecules,
energy and information across this barrier are mediated
by the integral membrane proteins embedded in the
bilayer. Membrane proteins represent a fertile area for
structural study; while an estimated 25% of the proteins
encoded in the genomes of organisms are membrane
proteins, they constitute less than 1% of the current
entries in the Protein Data Bank.16 The biological signifi-
cance of membrane proteins is mirrored in their pharma-
cological significance, since membrane proteins
represent the targets for a majority of the most popularly
prescribed drugs.17
3.9.2� Protein�Synthesis
Proteins are manufactured by ribosomes, molecular
machines that translate mRNA into a chain of amino acids
specified by the nucleotide sequence originating from the
DNA. The molecular machinery involved in this produc-
tion process, from transcription to translation, has been
extensively researched and has led to two Nobel prizes
over the last five years. However, a full understanding of
the entire protein production process, involving the inter-
actions between many molecular machines is lacking. In
order to understand the protein synthesis cycle, many
groups have been striving to obtain atomic-resolution
images of the intact ribosome synthesizing a protein.
Cate et al., have determined the first “frames” of the reac-
tion cycle by solving the crystallographic structures of
two intact ribosomes from the model organism
Escherichia coli.15 The ribosome crystals contained two
ribosomes per asymmetric unit, which provided two dif-
ferent snapshots of the molecule’s reaction cycle. This has
provided clues as to how the ribosome moves along mes-
senger RNA (mRNA), the genetic template for protein
synthesis. Comparison of the two ribosome structures
revealed movement of the head of the small ribosomal
subunit that appears to helps complete one translocation
step along the mRNA. Using models and lower-resolution
structures of the ribosome, it has been possible to pro-
pose a sequence of steps in translocation that finishes
with the swiveling of the small subunit head to allow the
mRNA and the transfer RNAs to move by one step.
Swiveling of the head may be driven by elongation-factor
G (EF-G), which uses one guanosine triphosphate (GTP)
molecule to catalyze the stepping along the mRNA.
CPS13
S19
L9
E
E
P AL11
5
L11L1
Head
Head
3
Figure57Ribosome�ratcheting.
84
3 . SCIENCE DRIVERSBIOLOGICAL SYSTEMS: IMAGING DYNAMICS AND FUNCTION
between‘open’and‘closed’states.Thisconformational
switchingmaybegatedinresponsetochangesinmem-
brane potential,22 ligand binding,23 or application of
mechanicalforces.24Structuralstudieshavebeenableto
visualizeendpointsand,rarely,intermediatestatesin
conformationalswitching.(e.g.Figure58)
Voltage-gatedchannels20representoneofthebasic
circuitelementsofneurobiology,andtheirresponseto
changesinmembranepotentialservesasakeyeventin
thepropagationofelectricalsignalsthroughthenervous
system.Fortypicalpotentialsof~0.1V,significantelectric
fields,inexcessof107V/m,aremaintainedacrossthe
membraneduetothethinnessofthebilayer.Overthe
pastdecade,thestructuralframeworkforaddressingthe
openingandclosingofvoltage-gatedchannelshasbeen
developed,20butthedynamicsofthisprocess(thetem-
poralresponseofthestructuralchanges)hasbeenlack-
ing. It shouldalsobenoted that the influenceof the
membranepotentialonaproteinrequiresamembrane,
and these effects cannot be studied with detergent
extractedproteinsintheabsenceofamembrane.Time-
resolvedstructuralstudiescharacterizingtheresponseof
membraneembeddedvoltage-gatedchannels(andthe
membrane)tochangesinmembranepotentialwillbe
essentialinestablishingthemechanisticdetailsofthis
fundamentaleventincellsignaling—howdoesaprotein
domainmovethroughthemembranetocreateanall-or-
nonechangeinchannelconductancewithoutcompro-
misingtheintegrityofthemembrane?
3.9.4 Nanogeobiology
Recentadvancesinmicrobiologyhavedemonstrated
thatmicro-organismsareintimatelyinvolvedinthetrans-
formationofinorganicmineralsatornearthesurfaceof
theEarth.25Microbe-catalyzedelectrontransfer(ET)to
metalionsinthesemineralsliesattheheartofmanyof
thesetransformations,whichareperformedbyredox-
activeproteinsthatefficientlytransportelectronsover
longdistanceswithminimallossoffreeenergy.26These
redoxproteinsareexquisitelytunedforfacileETtodifferent
solid-phaseelectronacceptors.Understandingthenatural
diversityandimpactofmicrobialredoxproteins,how
microorganismshaveshapedtheEarthovergeologic
periodsandhowtheycontinuetodoso,isabasicresearch
goalinbiogeochemistry.Becausemicroorganism-mineral
ETprocessesinfluencethedistributionandmobilityof
Thesamepropertiesthatenablemembraneproteins
tofunctioninthisheterogeneousmilieualsohavepro-
found consequences for their structural analysis.
Reflectingthesmallmembranesurfaceareatocellular
volumeratio,membraneproteinsaregenerallypresentin
lowabundance,andextractionfromthemembranetypi-
callyinvolvestheuseofdetergentstosolvatethehydro-
phobicsurfaces.Asaconsequence,themostdetailed
structuralinformationonmembraneproteinsistypically
intheirdetergentextractedstate.18Whiledetergentshave
beenextremelyuseful,theyarenotalwaysfaithfulmim-
icsofthemembranebilayerandcanperturbthestructure
and dynamics of membrane proteins. New methods
enabledbyNGLSX-raylasers,thatprovideinformation
onthestructureanddynamicsofmembraneproteinina
native-likebilayerenvironment, includingproteolipo-
somesorsupportedbilayers,willbetransformative.
Thisinterplaybetweendynamicsandfunctionisbeau-
tifullyillustratedbyionchannels:acollectionofintegral
membraneproteins thatmediate the transmembrane
passageofionsintheirenergeticallyfavoreddirection.19
Ionchannelsarekeyelementsofsignalingandsensing
pathways, including nerve cell conduction, hormone
response, and mechanosensation.The characteristic
propertiesofionchannelsreflecttheirconductance,ion
selectivity,andgating.Ionchannelsareoftenspecificfor
aparticulartypeofion(suchaspotassium20orchloride21)
oraclassofions(suchasanions)andaretypicallyregu-
latedbyconformationalswitchingoftheproteinstructure
Step 1
Step 2
α = -63Expanded: η = 49 R = 3.0
α = -42Closed: η = 30 R = 1.2
α = -68Open: η = 52 R = 11.0
TM1-
TM1´
cro
ssin
g an
gle,
|α| (
º)
TM1 tilting angle, η (º)Pore
radiu
s, R (Å
)
75
70
65
60
55
50
45
3035
4040
5055 0
24
68
1012
14
SI–Sn
40
StepSteppS
p 1S
p
StSStep 2Step 2
α = -63Expanded: η = 49 R = 3.0R
α = -42Closed:o η = 3030 RR = 1.21RR
O
SI–SnS
Figure58 Different conformational states of a mechanosensitive ion channel moving from closed to open24.
85
3 . SCIENCE DRIVERSBIOLOGICAL SYSTEMS: IMAGING DYNAMICS AND FUNCTION
betweenproteinsandsolidsistounderstandhowprotein
structureandconformationaldynamicsmediateintrapro-
teinandinterfacialET.Becauseofthedearthofcomputa-
tional and experimental tools, there is virtually no
mechanisticinformationoninterfacialET.
Multi-hemec-typecytochromesexpressedbyFe(III)-
reducingbacteria(Geobacterspp.,Shewanella oneiden-
sis) are among the best-characterized redox-active
microbialproteinsthatinteractwithmineralsandsoluble
metalions.29Physiologicalstudiesofthesebacteriahave
identified a number of outer membrane multi-heme
c-typecytochromesthatinteractwithmineralsurfaces.
OmcA and MtrC, decaheme c-type cytochromes
expressedbyS. onedensis,havebeenshowntobindtoa
hematitesurface,andratesofelectrontransfertothe
mineral surface have been measured.30 However no
high-resolution structure of MtrC or related proteins
exists to rationalize thepathwayofelectron transfer.
Consequently,thedetailedmechanismsofETareoften
derivedfrommodelstudiesonsimplerredox-activepro-
teinsthatareeasiertohandleandforwhichcrystallo-
graphicstructurescanbeobtained31(Figure59).Such
modelsystemscannotaddressstructure—activityrela-
tionshipsinrelevantsystems.Forexample,itisknown
thatsubtlechangesintheaxialcoordinationgeometryof
theFecenterinthehemegroup,causedbyaminoacids
substitutionsremote fromtheaxialhemeaminoacid
ligands,mayexertsignificantchangesintheredoxprop-
ertiesandratesofelectrontransfer.32
Amajorcontributiontoourunderstandingofthese
complexnanogeobiologicalsystemswillbenewmeth-
ods,enabledbyNGLSX-ray lasers, todetermine the
structureofenvironmentallyandtechnologicallyimpor-
tant electron transfer proteins, including multi-heme
contaminants,andthecarboncycle,understandingthem
iscrucialforsafeguardingournaturalenvironment.
Moreover,thereisanenormouspotentialforharness-
ingETproteinsformyriadbiotechnologies.Biological
engineersatLBNLhaverecentlyusedthesenaturally-
occurringproteinstorelayelectronicsignalsfromliving
systemstosyntheticinorganicmaterials.27Byelucidating
theprinciplesunderlyingprotein-inorganicET,thiseffort
will specifically contribute to one of the DOE Grand
Challenges:“totaptheexistingworldofbiologicalnano-
technologybyconstructingmolecularlevel,functional
interfacesbetweenlivingsystemsandsyntheticsoft-mat-
terandsolid-statetechnology.”28Themostsignificant
challengeforunderstandinganddirectingelectronflow
Figure59Soluble tetraheme c-type cytochrome used as model for electron transfer at a protein-mineral interface.31
Optical pump
Photoactivatedelectron donor
Single protein-clustercomplexes
θ
Electrontransfer protein
Polyoxometalateelectron acceptor
X-ray probe
Figure60 Schematic representation of optical pump-probe experiment with sensitizer-protein-acceptor bioconjugate.
86
3 . �SCIENCE�DRIVERSBIOLOGICAL�SYSTEMS: �IMAGING�DYNAMICS�AND�FUNCTION
low resolution electron microscopy images. The dynamic
imaging capabilities of NGLS X-ray lasers present
unprecedented opportunities to characterize whole func-
tional carboxysomes; observe dynamic structural changes
during catalysis; understand the cellular context in which
carboxysomes assemble; and track changes in carboxy-
some structure in response to environmental changes.
Carboxysomes may be viewed as complex CO2 fixa-
tion machines that self-assemble into an architecture that
is reminiscent of icosahedral viral capsids. The carboxy-
some shell is composed of thousands of copies of hexa-
meric shell proteins that tile to form the facets of the
shell.36,37 A second protein forms pentamers; 12 of these
provide the “defects” necessary to close a hexagonal
layer into an icosahedral shell38 (Figure 61c). Packaged
within the carboxysome shell are approximately 250 cop-
ies of L8S8 RubisCO,34 a highly abundant but inefficient
enzyme that utilizes Ribulose 1,5-bisphosphate (RuBP) as
a substrate to convert CO2 into 3-phosphoglycerate, the
key step in the Calvin cycle39 (Figure 61d). Also contained
within the carboxysomes are approximately 80 copies of
a carbonic anhydrase (CA) which converts bicarbonate
into CO2.36 The encapsulation of RubisCO with CA inside
of the carboxysomes enhances CO2 fixation by elevating
the local CO2 concentration around RubisCO and mini-
mizing the competing reaction of RubisCO with oxygen.
Because carboxysomes are found in all cyanobacteria
and many chemoautotrophic bacteria, they are a key
component of the global carbon cycle.
New time-resolved X-ray imaging techniques are
required for a full understanding of the structure and
catalytic dynamics of CO2 fixation by the carboxysome.
For example, structural data at nanometer resolution for
a single carboxysome will reveal how RubisCO and
c-type cytochromes such as OmcA. Dynamic pump-probe
studies that visualize the structural changes that occur on
the timescales of intraprotein and interfacial ET are
essential.
Experiments that can visualize the structural dynamics
of the protein-inorganic material interface before and
during intermolecular electron transfer will vastly expand
our understanding of ET. To determine how the kinetics
and pathways of electron transfer are altered by associa-
tion with a mineral surface, the sensitizer-protein biocon-
jugate could be complexed to a polyoxometallate cluster,
which can serve as a model for a metal oxide mineral
(Figure 60). These kinds of studies will provide the first-
ever systematic understanding of the structure and kinet-
ics of interfacial ET. We envision that this fundamental
science will enable rational engineering of the redox pro-
tein / crystal interface, and in turn have significant applica-
tions in biosynthesis, bioenergy, and biosensing.
3.9.5� Carboxysomes
Carboxysomes are self-assembling proteinaceous organ-
elles that play a key role in bacterial CO2 fixation (Figure
61). They range in size from approximately 90-170 nm in
diameter and in mass from 100-350 MDa33-35 and encap-
sulate hundreds of copies of two key enzymes of CO2 fix-
ation: RubisCO and Carbonic Anhydrase (CA) (Figure 61c
and d).34,36 Given the urgent global need to reduce CO2
emissions and develop CO2 sequestration technologies,
there is great interest in understanding carboxysomes
and in utilizing these organelles to enhance CO2 capture
and fixation rates in bio-engineering applications. Thus far,
structural information about the carboxysomes has been
limited to structures of isolated component proteins or
A B C D O2
HCO3 -
HCO3 - CO2
3-PGA
RuBP
2PG
Figure61�(A�and�B)�Transmission�electron�micrographs�of�carboxysomes�in�a�dividing�cyanobacteria�(A)�and�single�carboxysome�(B)�Scale�bars,�50nm�(Figures�from�Reference�37)�(C)�Predicted�model�of�a�whole�carboxysome.�RubisCO�(green),�Carbonic�Anhydrase�(red),�single�domain�hexameric�shell�proteins�(dark�blue)�tandem�domain�shell�proteins�(light�blue)�and�pentameric�shell�proteins�(yellow)�(D).�Schematic�of�CO2�fixation�reactions�inside�the�carboxysome.�CA,�carbonic�anhydrase;�PGA,�phosphoglycerate;�RuBP,�ribulose�bis-phosphate.
87
3 . �SCIENCE�DRIVERSBIOLOGICAL�SYSTEMS: �IMAGING�DYNAMICS�AND�FUNCTION
events during neurotransmission are highly regulated
and subject to stimulated changes.41 In the presynaptic
terminal, these changes modulate the releasable pool
and the release probability of synaptic vesicles. The
molecular components involved in neurotransmitter
release interact in a hierarchical fashion: Some compo-
nents have mutual pairwise interactions, some compo-
nents have interactions that are restricted to adjacent
partners, and some components or groups of compo-
nents are spatially separated by compartments. In addi-
tion, some of the interactions are sequential. This
complexity allows the neuron to create multiple regula-
tory mechanisms for neurotransmitter release.
Brunger and colleagues have applied a combination of
structural and biophysical studies to understand the
molecular basis for neurotransmitter release. Structural
information about complexes between the individual
molecular components has been obtained by X-ray crys-
tallography,42 NMR,43 and electron microscopy44 methods.
This information has provided the framework for investi-
gations targeted at the functional and dynamic aspects of
the system, using single-molecule fluorescence micros-
copy techniques.45-47 The experimental studies have then
been used to guide computational simulations designed
to capture intermediate conformations in fusion, other-
wise inaccessible experimentally.48
Despite this wealth of experimental and computational
information the detailed interactions between this complex
machinery is still not understood in the context of the cell.
Therefore, a complete understanding of the fusion process
requires the visualization of membranes fusing in the
presence of the driving proteins in the cell. Cellular imaging
methods that can focus into similar regions of cells from
many snapshots are needed to capture this process and
describe the conformational changes involved. The capa-
bilities of NGLS will make it possible to image entire syn-
aptic vesicles, synaptosomes (synaptic vesicles docked to
plasma membrane fragments), and of the minimal fusion
machinery that exhibits calcium-triggered complete fusion
activity with fast kinetics in an in vitro system recently
developed by Brunger and colleagues. Of key importance
will be the ability to collect billions of images of fusing
cells and “zoom in” on topologically similar regions in
order to generate a consensus image and the conforma-
tional variability around this average (see Section 4.1.2).
CA pack together inside a functional carboxysome to
mediate substrate and product channeling. Docking
atomic level structures of RubisCO, CA and shell proteins
into a nanometer scale resolution map of the whole car-
boxysome will provide a detailed view of how RubisCO
and CA are arranged for CO2 fixation, and how they interact
with the protein shell. We also anticipate that the internal
structure of the carboxysome may change with available
light, the metabolic state of the cell or the state of biogene-
sis; a powerful result of single particle imaging will be the
ability to compare reconstructions of carboxysomes from
many cells, or cells grown under different environmental
conditions. This will make it possible to characterize con-
served and heterogeneous features of carboxysome
structure and to identify structural changes that occur in
response to changing environmental conditions.
Nanometer resolution structures of carboxysomes will also
provide an initial foundation to understand how the struc-
tures of carboxysome proteins change during catalysis.
Moving to the cellular level, how are these organelles
integrated into the metabolic and regulatory processes of
the cell and which structural elements in the cell help pro-
mote and stabilize their assembly? This is particularly rel-
evant for bioengineering experiments because
understanding macromolecular self-assembly is one of
the frontiers of synthetic biology. Knowing how carboxy-
somes self-assemble and identifying associated cellular
changes that enhance assembly will guide experiments
to reconstruct or even improve these CO2 fixation
machines in vitro. Studying cells growing under different
environmental conditions using experimental techniques
that do not require subsequent freezing or fixation could
also led to significant advances in understanding the bio-
logical conditions that lead to carboxysome assembly.
Given their regular architecture and intermediate size
between whole cells and single particles, carboxysomes
offer an ideal system for developing organelle and cellular
imaging methods that could later be applied to more
complex or designed systems for encapsulating other
metabolic processes in carboxysome shells.
3.9.6� Membrane�Fusion
The protein-mediated fusion of lipidic membranes is a
critical process in biology.40 The pre- and postsynaptic
88
3 . SCIENCE DRIVERSBIOLOGICAL SYSTEMS: IMAGING DYNAMICS AND FUNCTION
X-rayFELbaseddiffractionorscatteringexperiments
onisolatedmolecules,complexesorwholecellstherefore
mustrelyontheuseofultrashortX-raypulsesthatareable
toovercomethedamagelimit.Inthelasttenyearstheo-
reticalcalculationshaveindicatedthatultrashortpulses
couldbesuccessfullyusedtoobtaindiffractionpatterns
beforeradiationdamagedestroysthesample(“diffract
anddestroy”).54,55RecentexperimentsatFLASHand
LCLSusingpulsesinthe10to100fsrangeappeartocon-
firm this theory for macromolecular nanocrystals
(J.Spence,unpublished).Imagingofbiologicalsamples
atNGLSwillrelyonitsabilitytogenerateextremelyshort
X-raypulsesinthefemtosecondregimeatMHzrate.
Beamlines for Imaging Biological Dynamics and Function
Biologicalimagingresearchwillrelyon“diffractand
destroy”methodsusingthe3rdand5thharmonicswith
thehighestfluxperpulseontheseededNGLSbeamline1
at100kHzrepetitionrates,andontheun-seededSASE
beamline3,atMHzrepetitionrates(ashigh-speeddetec-
torsallow)asdescribedinSection5and6.6.Choiceof
wavelengthwillbedeterminedbybalancingthescatter-
ingefficiencyandtherequiredresolutionforparticular
samples.Akeycomponentoftheseexperimentswillbea
high-speedparticleinjectorsynchronizedtotheCWSCRF
linac(seeSection4.1.3),provideforsamplereplacement
onapulse-by-pulsebasis.
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New techniques enabled by NGLS4
revealawealthofadditionalstructuralinformation,con-
tainedinfluctuationsaboutthemeanSAXSsignal(i.e.
modulationsinthenominallyradially-symmetricSAXS
signal),thatcannotbeobtainedbyconventionaltech-
niques.UnlikeSAXS/WAXS,whichprojectsthreedimen-
sionsontoone,thesnapshot“WAX”patternsfromNGLS
willshowtwo-dimensionalvariation,andsoprojectthree
dimensionsontotwo,providingmuchmoreinformation.
Thepowerfulapproachdescribedabove,isknownas
“fluctuationX-rayscattering”(fXS)andwasoriginally
proposedbyKamthirtyyearsago.3,4Todate,thismethod
hasbeenconsideredintractable—notduetothespeedof
thepulseperse,northedifficultyoftheanalysis,butthe
requirementofmeasuringatleastonescatteredX-rayper
particlepersnapshot,withinarotationaldiffusiontime.A
recentexperimentaltest5attheAdvancedLightSource,
usingafixeddistributionofgoldnano-rodsasamodel
two-dimensional system has shown that an ab-initio
reconstructionispossiblewithoutrelyingonanyofthe
modelingassumptionsnormallyrequiredfortheanalysis
ofSAXSdata.However,themodelexperimentusingTMV
(whichisstillahighlyfavorablecase)inwateratroom-
temperatureisstillunfeasibleonexistingstoragerings.
ThecapabilitiesofNGLSX-raylaserswillfinallyenablethis
powerfulfXSapproachfordetermining3Dstructuresof
macromoleculesintheirnativeenvironments.
Theory: TheresurgenceofinterestinKam’scorrelation
averagingapproachhasnothappenedinavacuum.The
long-plannedsinglemoleculediffraction(SMD)method
(diffract and destroy) poses significant data analysis
4.1.1 Fluctuation SAXS — Molecular Structures in Native Environments
Understandingbiologicalprocessesatacellularlevel
requiresdetailedknowledgeofthestructureanddynam-
icsoflargemacromolecularmachines.Wepresentlylack
thenecessarytoolsfordynamicimagingofsuchbiologi-
calmachines—inoperation,innativeenvironments,and
atthenanoscale.Small-angleandwide-angleX-rayscat-
tering (SAXS/WAXS) techniques1 offer tremendous
potential,andarepresentlybeingusedtogreatadvan-
tageincombinationwithcrystallography.2However,the
fullpotentialofthesescatteringtechniquesremainsunre-
alizedbecausetherequiredexposuretimesatpresent
X-raysourcesareordersofmagnitudelongerthanthe
rotationaldiffusiontimesofthesampleparticles.Thusthe
scatteringdataaresphericallyaveraged,ineffectproject-
ingthethreedimensionsofthestructureontoonedimen-
sion.The restricted information available from these
one-dimensionalprojectionsisinsufficientforunambigu-
ousstructuredetermination.
NGLSX-raylaserswillprovidethefirstcapabilityfor
high-quality SAXS/WAXS with exposure times much
fasterthanrotationaldiffusiontimescales,therebyelimi-
natingthesphericalaveraginglimitation(e.g.<140nsfor
atestsampleoftobacco-mosaicvirus,TMV,size~300nm
x20nm,inpureroom-temperaturewater—rotational
diffusiontimeswillbeps-nsforsmallermacromolecules).
Thehigh-speedSAXS/WAXSscatteringsnap-shotswill
4.1 Imagingstructureandfunctioninheterogeneousensembles
92
4 . �NEW�TECHNIQUES�ENABLED�NY�NGLSIMAGING�STRUCTURE�AND�FUNCTION�IN�HETEROGENEOUS�ENSEMBLES
recorded of different areas and the autocorrelation
around a single q ring was calculated for each pattern. The
averaged autocorrelations “converged” toward the auto-
correlation of one particle. This derivation of a one-parti-
cle pattern from a measurement of an ensemble of
particles is the essence of correlation averaging (Figure
63). The final step consisted of phasing this data to recon-
struct a two-dimensional image of one typical gold rod.
This work represents the first successful ab initio experi-
mental demonstration of the Kam fXS method, albeit in
two dimensions.8,9
A more ambitious experiment was carried out by
Howells et al.,10 at the TROÏKA hard X-ray beam line at the
European Synchrotron Radiation Facility. This was an
attempt at a full 3D fXS experiment but using highly vis-
cous solvents and low temperatures to slow the rotation
of the particles. Goethite and TMV samples were used and
correlations were recorded. Data analysis is in progress.
4.1.1.2 WhatHasBeenDone?Theory:
A full 2D theory was described and used successfully
to reconstruct a set of simulated diffraction patterns of
K-channel membrane protein molecules in a membrane,
again with orientations random about one axis.5 The
same theory was used to reconstruct the set of patterns
from the gold rod samples in the experiment cited above. A
3D theory, still using spatial correlation averaging, was also
developed for the SMD experiment and used successfully
to reconstruct the 3D image of a protein molecule.11
Recent theoretical developments show that ab-initio
image reconstruction of three-dimensional objects (ran-
domly oriented) is possible from fXS data, using algo-
rithms developed for protein crystallography and
coherent diffractive imaging. Furthermore, current algo-
rithms available for shape reconstruction from SAXS
data can be extended to incorporate fXS data. Although
the computational complexity of the problem is signifi-
cant, modern computational approaches, such as hard-
ware acceleration of specific time-consuming operations,
are expected to eliminate the most significant computa-
tional bottlenecks.
In order to determine the structure of intermediate
states during the reaction cycle of macromolecular
problems that have been under study. The idea to use cor-
relation averaging in that case was rediscovered indepen-
dently by Saldin and coworkers.6 Indeed it may be that
SMD data can only be reconstructed by correlation aver-
aging.7 The reconstruction in fXS requires the solution of
two phase problems, one to get from the experimentally
determined autocorrelation function to the 3D single-par-
ticle diffraction pattern and a second one to invert the dif-
fraction pattern to retrieve the 3D image (as in coherent
diffraction imaging). However, in spite of this increase in
activity, the latest round of publications has not included
a solution of the full 3D reconstruction problem for a
modern fXS experiment, even in simulations.
4.1.1.1 WhatHasBeenDone?Experiment:
Good progress has been made on the two-dimension-
al analogue of fXS, in which identical particles lie in a
plane differing only by rotation about an axis parallel to
the X-ray beam. An fXS experiment has been done at the
ALS in which 80-nm-long gold rods were dispersed on
membranes, the rod orientations being random about
one axis only. About a hundred diffraction patterns were
SimulationData
20
10
0
-10
0˚ 90˚ 180˚ 270˚ 360˚
SimulationData
20
10
0
-10
0˚ 90˚ 180˚ 270˚ 360˚
Figure63�The�upper�panel�shows�the�q�ring�autocorrelation�of�a�single�snapshot�of�many�80�nm�cylinders�(dotted)�and�the�calculat-ed�autocorrelation�of�a�single�cylinder.�The�lower�panel�shows�the�effect�of�averaging�together�121�different�measured�snapshots�and�again�the�calculated�autocorrelation�of�a�single�cylinder.�The�aver-age�over�snapshots�converges�toward�the�single-particle�pattern.�(Figure�from�Reference�8)
93
4 . �NEW�TECHNIQUES�ENABLED�NY�NGLSIMAGING�STRUCTURE�AND�FUNCTION�IN�HETEROGENEOUS�ENSEMBLES
• High average photon flux, and high flux / pulse:
Number of scattering particles and scattering efficiency
is limited. Proposed NGLS flux per pulse is compara-
ble to exposing roughly one second on state of the art
SAXS beamlines, indicating that with sufficient data,
high resolution 2D fXS patterns can be efficiently
obtained. Scattering patterns from multiple expo-
sures can be summed together (with appropriate pro-
cessing of individual patterns).
• Coherence: A fully coherent beam provides significant
simplifications in data analysis of mixtures and
doesn’t impede analysis of samples with a single
structural species.
The proposed parameters of NGLS X-ray lasers fit the
above requirements. The optimal choice of energy for an
fXS experiment will be determined by the nature of the
problem investigated. The available energy ranges from
hard X-rays (using harmonics) to the water window
(wavelength approximately ~2 nm) allow for the imaging
of relatively small systems such as enzymes (using hard
X-rays) or very large systems such as viruses or large
macromolecular assemblies, such as polysomes.
NGLSCapabilitiesforfXS
• Fast X-ray pulses to exploit the “diffract and destroy”
method for single molecules or ensembles.
• Samples can be in buffer in physiological conditions,
without viscosity enhancers.
• Samples can be at room temperature, no need for
cooling to slow the particle diffusion.
• Higher X-ray flux at NGLS means that fXS can be
effectively applied to smaller and more symmmetric
macromolecules.
• Can study time dependent or triggered processes in 3D.
• Can deal more effectively with heterogeneity (non-
identical particles).
• Resonant scattering (near edges) possible in principle.
• The amount of information provided by an fXS measure-
ment is roughly equivalent to knowing the 3D autocor-
relation of the unknown structure at SAXS resolution.
This will be orders of magnitude more information
than that provided by the spherically-averaged pat-
terns of standard SAXS, allowing one to determine
many more parameters with higher accuracy.
machines, the theory of fXS can be extended following
the lines of Andersson et al.,1,3 The latter work demon-
strates that a time resolved series of WAXS data from an
evolving mixture of species, as observed in time resolved
measurements, can be decomposed into curves of indi-
vidual species of (meta) stable intermediates (Figure 64).
It can be shown that a similar technique can be applied to
fXS data, enabling the investigation of time resolved
structural changes of large macromolecular machines in
near native environments.
4.1.1.3 ExperimentalConsiderationsforfXS
SourceRequirements:
• Ultrafast X-ray pulse durations: scattering patterns
must be measured with X-ray exposures that are fast-
er than molecular rotational diffusion times (typically
sub-picosecond under native conditions)
-1ns100ps316ps1ns3.16ps10ns31.6ns100ns316ns1us3.16us10us31.6us100us316us1ms3.16ms10ms
0.0x100
-3.0x104
-6.0x104
-9.0x104
-1.4x105
a
b
0.0 0.5 1.0q[A-1]
1.5 2.0 2.5
0.0 0.5 1.0q[A-1]
1.5 2.0 2.5
Mb†•COMb*•COMb•COMbThermal
Mb*•CO-Mb†•COMb•CO-Mb*•COMb-Mb•COThermal(static)
Figure64Time�dependent�evolution�of�SAXS/WAXS�patterns�of�CO-Myoglobin�dissociation�(a).�Numerical�analyses�of�these�pat-terns�results�in�curves�of�individual�species�(b).12
94
4 . NEW TECHNIQUES ENABLED NY NGLSIMAGING STRUCTURE AND FUNCTION IN HETEROGENEOUS ENSEMBLES
machines,andprovideunprecedented,statisticallyvali-
datedaccesstotheoperationofmolecularfactories.
Radiationdamageseverelylimitstheinformationthat
canbeobtainedfromasinglecopyofsoft-matterobjects
beforetheyaredestroyed.Toboosttheweakscattered
signal,currentstructuraltechniquesaveragedatagath-
eredfrommanyobjectsassumedtobeidentical.This
includeswell-establishedtechniquessuchascrystallog-
raphyandcryo-EM,15aswellas“scatter-and-destroy”
methodsrecentlyenabledbyX-rayFEL’s.7,16-19
Thereismountingevidence,however,thatstructural
variabilityisnotonlycommonatthemolecularlevel,but
alsoimportanttofunction,andthatinsoft-matter,“struc-
ture”isneitherstatic,norimmutable.20-26Understanding
structuralvariabilityanddynamics,whilevitalforcon-
trollingsoft-matterprocesses,hasremainedelusive.
Molecularmachines,suchasenzymes,undergo—
ofteninitiate—conformationalchanges.Snapshotsof
molecularmachinesthusnecessarilyrepresentultralow-
signalsightingsofmembersofconformationallyhetero-
geneousensembles,whichcannotbesimplyaveraged.
Understandingthestructureanddynamicsofheteroge-
neousensemblesrepresentsakeyfrontierinsoft-matter
science.
The ability to collect and comprehend information
from systems spanning a broad heterogeneity landscape
is essential, if we are to control complex dynamic pro-
cesses for efficient energy conversion, carbon sequestra-
tion, and enzymatic reactions honed by nature over
millennia.
4 .1 .2 .1 Accessing Dynamic, Heterogeneous Systems by
Manifold Mapping
Randomsnapshotsobtainedfrommembersofahet-
erogeneousensemblearecorrelated—otherwisethere
wouldbenothingtocharacterizetheset.Intheabsence
ofnoise,thesecorrelationsforcethesnapshotstolieona
hypersurface—amanifold—whosedimensionalityis
determinedbythenumberofdegreesoffreedomavail-
abletothesystem.Arotatingrigidbodyobservedinfar-
fielddiffraction,forexample,hasthree(orientational)
degreesoffreedom,andthusproducesathree-dimen-
sionalmanifold.19Theadditionaldegreesoffreedomofa
dynamic,non-rigidsystemgiverisetohigherdimensional
manifolds27(Figure65).Similarly,thereactioncoordi-
natesofanongoingprocessarereflectedinthedimen-
sionalityofthedatamanifold.27
Sample Environment and Data Aquisition .
Utilizingaliquidwatermixingjet13runninginasingle
fileacrosstheNGLSbeam,combinedwithsuitablechoices
ofsubstrate,proteinconcentration,jetvelocityanddelay
lines,wecanobtainfXSdataatintermediatetimepoints
withinthedutycycleofamacromolecularmachinethat
canspanfromsub-nanosecondstoseconds.Thehigh
repetitionrateofNGLSwillallowustocollecthundreds
ofthousandsofpatternsatsuitablychosentimepoints
alongthecontinuousordropletbeam.NGLSwilloperate
inthe“diffract-and-destroy”mode,inordertoavoidany
effectsofradiationdamageonthediffractionpatterns.14
Givendataratesassociatedwiththeseexperiments,
efficientdetectortechnologyisinstrumentaltothesuc-
cessoftheseexperiments.Anexcitingpossibilityisthe
developmentofhardwaresolutionsfordatareduction
(seeSection6.6).ObtainingfXSdatainvolvesthecompu-
tationofangularcorrelationsofthescatteringpatterns.If
thisessentialstepcanbeperformedonthedetectorwith
dedicatedhardwarewhilethedataiscollected,asignifi-
cantinfrastructurebottleneckisresolved.
4 .1 .1 .4 Outlook
ThefXStechniqueoverlapswithsinglemoleculedif-
fractionstudiesthattypicallyprovidemoredetailedinfor-
mationthantheensemble-basedfXSmethod.Duetothe
underlyingexperimentaldesignofanfXSexperiment,
thetechniquelendsitselftoahigherlevelofautomation.
Assumingaconservative,particleinjectorlimited,data
acquisition rate of 10 kHz, 10 million images can be
obtainedwithin20minutes,sufficienttoassembleahigh
resolutionfXSdataset.Giventhehigh-throughputnature
oftheexperiment,akintoindustrystandardproteincrys-
tallographicdataacquisition,automated,high-through-
putfXScanbeanessentialtoolinthediscoveryofnovel
drugsactingonmembraneproteinsorotherlargemac-
romolecularmachinesnotamenableforroutinebiophys-
icaltechniquesemployedinstructurebaseddrugdesign.
4.1.2 Giga-shot Imaging of Heterogeneous Ensembles with Manifold Mapping
NGLSwillproduce~1010diffractionsnapshotsperday.
Combinedwithadvancedmanifold-basedanalyticaltech-
niques,thiswillelucidatetheroleofheterogeneityinbio-
logical systems, enable 4D imaging of molecular
95
4 . NEW TECHNIQUES ENABLED NY NGLSIMAGING STRUCTURE AND FUNCTION IN HETEROGENEOUS ENSEMBLES
fundamentalunderlyingsymmetries.29Theseso-called
isometries,previouslyassociatedwithcertainmodelsof
theuniverseingeneralrelativity,36directlyrevealthenat-
ural,physicallymeaningfuleigenfunctionsofdatamani-
folds produced by scattering. Projection on these
eigenfunctionsallowssuperbnoisediscrimination,while
theisometriesthemselvesprovidea“compass”fornavi-
gationonthemanifold.Thisunprecedentedcapability
canbeusedtoefficientlyidentifydatamanifoldsinthe
presenceofoverwhelmingnoise,andextractphysically
meaningfulinformationfromthem.
Manifolds swept out by random sightings of heteroge-
neous dynamic systems can be used to compile 4D maps
(“3D movies”) of these systems. Key to this approach is
the availability of a sufficient number of snapshots to
define the manifold to the required granularity. The
unique characteristics of NGLS, particularly the combina-
tion of high flux per pulse and its high repetition rate, are
essential to this capability.NGLS, combined with mani-
fold-based methods, offers unprecedented access to
structural heterogeneity in complex dynamic systems.
4 .1 .2 .2 Conformational Heterogeneity and Dynamics:
Molecular Machines
Molecularmachinesundergodiscreteand/orcontinu-
ousconformationalexcursionsandinducesimilarchanges
inthesubstrate.21Usingultralow-signalsnapshotswith
no orientational or timing information, these can be
mappedbymanifold-basedanalyticaltechniques.29
Thedatamanifoldcontainstheentireinformationcon-
tentofthedataset.Often,thismanifoldmustbedeter-
mined in the presence of overwhelming noise.
Noise-robustmanifoldmappingalgorithmshavebeen
demonstrated with simulated and experimental data
downto~10-2scatteredphotons/pixelinthepresenceof
backgroundscattering,withsignals-to-noiseratiosaslow
as~-20dB.19,26,29
Comprehendingtheinformationcontentofthedataset
istantamounttobeingableto“navigateonthemanifold”
soastoreachanydesiredpoint.Forexample,onemay
wishtoreconstructthe3Dstructureofanenzymeata
particularpoint in itsconformational landscape.A3D
modelisequivalenttotheabilitytoproduceany2Dpro-
jectionatwill.Onthedatamanifold,thisinvolvesnavigat-
ingfromacertainpoint(agivenconformation)along
directionsofpureorientationalchangeonthemanifold,
and thus producing any desired 2D projection of the
givenconformation(Figure65).Similarly,onemaywish
toextracttheconformationalchangesobservedfroma
certaindirection—equivalenttomovingalongthemani-
foldinadirectionofpureconformationchange.
Graph-theoreticmeansforidentifyingdatamanifolds
arewell-established,30-34but ithasproveddifficult to
assignphysicalmeaningtothedimensionsofso-called
embeddedmanifolds35andthuspurposefullynavigate
onthem.Recentworkhasshownthatdatamanifoldspro-
ducedbyanyscatteringprocess—elastic,inelastic,kine-
matic(single)ordynamical(multiple)scattering—possess
200
400
600
800
1000
1200
1400
1600
1800 Figure65 Manifold traced out by simu-lated diffraction snapshots of a molecule of adenylate kinase as it unfolds due to heating.27 Insets show the molecular structure at points on the manifold. Advanced analytical techniques offer a route to reconstructing the 3D structure of the molecule during it evolution — essentially a 3D movie of the unfolding process.29
96
4 . NEW TECHNIQUES ENABLED NY NGLSIMAGING STRUCTURE AND FUNCTION IN HETEROGENEOUS ENSEMBLES
whicheachclassoriginatedidentified.Thisisequivalentto
attachinganaddresslabeltoeachclass.Thenumberofbits
requiredforeachaddressisdeterminedbythesizeofthe
object,thenumberofdistinguishablestatesaccessibleto
thesystem,andtheresolutionofthemethodofobserva-
tion.7,19,27,38Whenthesignalissolowthattheinforma-
tioncontentofthedatasetissmallerthanthenumberof
bitsrequiredtolabelitsclasses,theprocessbreaksdown.
Downtothisverylowlimit,38onecantradetotalaccumu-
latedsnapshotsagainstthedoseperpulse,i.e.,compen-
sate forextremely lowscatteredsignalsbycollecting
moresnapshots.
Thepracticallimitissetbythenumberofsnapshotsin
thecollection,which,inturn,dependsonthesourcerepeti-
tionand/ordetectorread-outrates.Asaspecificexample,
recoveringthestructureofarigidobjectto~1nmresolu-
tiontypicallyrequires~104snapshots.19Distinguishing
between100differentconformationsofamoleculewitha
singleconformationaldegreeoffreedomincreasesthe
numberofsnapshotsto~106(Reference27).TheLCLS
produces~107snapshotsperday,limitingthesize,com-
plexity,andresolutiontowhichtheconformations,con-
figurations,anddynamicsofasystemcanbestudied.
NGLS will be capable of generating more than
1010 snapshots per day. Assuming a one-day experiment
and 104 snapshots to recover the 3D structure of each
configuration to ~1 nm resolution, one can study systems
with ~106 conformational states. The largest number of
conformational states mapped so far is ~ 50 (Reference
21).NGLS represents a spectacular advance in the study
of molecular machines and processes at the nano-atto
scale.
The projected capabilities outlined above must be
comparedwith thoseexpected fromalternative tech-
Whenamolecularmachinepossessesdiscreteconfor-
mationalstates,suchastheclosedandopenconforma-
tionsoftheenergy-relevantenzymeadenylatekinase,
simulationsshowthatsuitablealgorithmsareabletosort
thesnapshotsintotwodifferentmanifoldsanddetermine
theorientationofeachsnapshot.Thesortingconfidence
canbeextremelyhigh(~8.5σ)evenatverylowsignal
(~4x10-2scatteredphotonsperShannonpixelinthepres-
enceofPoissonnoise)27asshowninFigure65.Theability
toseparatediscreteconformationswithhighconfidence
anddeterminetheorientationfromwhicheachsnapshot
emanatesisanindicationoftheefficientusemanifold-
basedapproachesmakeoftheinformationcontentofthe
entiredataset.
Inmanyinstances,molecularmachinesundergocon-
tinuousconformationalchanges,sweepingoutamani-
foldwithdimensionscorrespondingtoorientationaland
conformationalchanges(Figure65).Suchmanifoldsare
bestregardedfromthepointofviewofanantcrawling
onthemanifold,withacompassprovidedbythesymme-
tries underlying image formation by scattering.29
Specifically,theisometrieshencethenaturaleigenfunc-
tionsof thescatteringmanifoldallowone to identify
directionscorrespondingtoorientationalandconforma-
tionalchanges.Inthisframework,onecancompile4D
maps(3Dmoviesintime)ofcontinuousconformational
changes in molecules and their assemblies. Such
approachesarecurrentlybeingusedtomapconforma-
tionsofmolecularmachinesusingcryo-EMimages,21,26
andX-rayFELdiffractionsnapshots.26
Thefundamentallimittothisapproachissetbyinfor-
mation-theoreticconsiderations.Attheconclusionofthe
analysis,theexperimentalsnapshotshavebeen“sorted”
intoclasses(“bins”),andthestateofthesystemfrom
CLOSED
OPEN
LID 4πSignal: 4x10-2ph/pixel
2π
2π0 Correct orientation
Dedu
ced
orie
ntat
ion
LID
NMP
NMP
CORE
CORE
AP5A
Manifold 1
Manifold 2
Figure66Manifold mapping separates snapshots from different conformations and determines the snapshot orientations within each set with no a priori knowledge. When a mixture of diffraction snapshots from the molecule adenylate kinase in its open and closed conformations is presented to noise-robust versions of graph-theoretic techniques at sig-nal levels corresponding to 0.04 ph/pixel at 0.18 nm, the snapshots are automatically sorted into different manifolds and their orientations determined.27 The 8.5-σ separation between the two manifolds implies extreme fidelity in separating different conformations.
97
4 . NEW TECHNIQUES ENABLED NY NGLSIMAGING STRUCTURE AND FUNCTION IN HETEROGENEOUS ENSEMBLES
torwith~109pixels,a30xincreaseinlineardimensions
comparedwithdetectorscurrentlyinuse.Theobjectis
firstreconstructedinto83=512voxels,withthevoxelof
interestsuccessivelysubdividedinto512voxels.Starting
witha~10μmfieldofview,fourlevelsofzoomsufficeto
reach~1nmresolution(Figure67).3Dreconstructionat
eachlevelneedsonly~5x104snapshots.
Akeyissueconcernsthenumberoftopologicalstates
availabletosuchcomplexsystems.Thefollowingsum-
marizesthenumberofstatesavailabletoobjects,whose
topologiescanbedescribedintermsofgraphsoforder8
(graphswith8nodes,or“anchorpoints”).41,42Thelarge
numberoftopologicalstatesissubstantiallyreducedas
soonasasingletopologicalclass(familyofgraphs)is
considered.Representingthetopologyoftheregionof
interestateachlevelofzoomasagraphoforder8(i.e.,
assuming that the topology of 512 voxels can be
describedby8anchorpoints) reduces thenumberof
topologicalstatestobeexploredateachlevelofzoom
from~6x108to~2x105.Thislevelofheterogeneitycanbe
easilyexploredby the~1010snapshotsavailable.The
numberoftopologicalstatesexploredinfourlevelsof
zoom,however,exceedsAvogadro’snumber,indicatinga
toopermissiveestimateof thenumberof topological
statestobeconsideredateachlevel.Inpractice,itmaybe
niqueswhenNGLScommencesoperation.Thereisno
doubtthatcryo-EMwillcontinuetomakeprogressin
investigatingmolecularmachines.Cryo-EMdatasetscur-
rentlyspan~106snapshots.Whilefurtherprogressispos-
sible in key steps such as sample preparation and
microscopeoperation,itispresentlydifficulttoenvisage
increasesofmorethan100xinthesizeofcryo-EMdatas-
ets, which will still be 100x smaller than capabilities
offeredbyNGLS.Moreimportantly,ithasnotbeenpos-
sibletointroducetimeresolutionincryo-EMotherthan
byslowingreactionschemically.
The high repetition rate and the time-resolved capabil-
ities of NGLS represent an inherent advantage in study-
ing dynamic systems on the nano-atto scale.
4 .1 .2 .3 Cellular Imaging
Topological heterogeneity: Molecular factories
Beyondconformationalheterogeneity,complexsys-
temsofagivenclasscandifferinconfigurationandinthe
numberofcomponentstheycontain.Molecularfactories,
suchasthoseinvolvedincarbonsequestrationandbio-
logicalcellsthusdisplayconformational,compositional,
andtopologicalheterogeneity.
A10μm-diameterobjectcontains~1012nanometer-
sizevoxels.3Dreconstructionofsuchanobjecttonano-
meter level is computationally intractable, and
unnecessary.Inpractice,onebeginswithinitialscrutiny
atlowmagnification,successively“zooming”intoselected
regionsofinterest.Zoomingcanbeachievedbynear-
fielddiffraction,39orviaholography.40Bothapproaches,
which have been demonstrated experimentally for
X-rays,encodepositionalinformationinthesnapshot.
The largenumberofsnapshotsprovidedbyNGLS
enableadecisiveadvance.Insteadofrelyingontheinfor-
mationfromasingleobject,whichisquicklydestroyed
byradiationdamage,onesuccessivelyintersectseachof
~1010membersofaheterogeneousensemblewithan
intensepulse.Dataarecollectedbeforeonsetofdamage
andsubsequentsampledestruction.Asoutlinedbelow,
manifold-basedapproachesallowonetousetheinfor-
mationcontentofthedatasetfromtheentireensembleto
reconstructeachmemberof theensemble,providing
unprecedentedandstatisticallyvalidatedinformationon
suchheterogeneoussystems.
Onebeginsbyrecordingsnapshotswithsufficiently
largemomentumtransfertoallowreconstructionto~1nm
level.Fora~10μmdiameterobject,thisrequiresadetec-
y
Root cell0 1 2 3 4 5 6 7
x z
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7
0 1
131 1 1 1 10 1 2 3 4
130 1 1 1 0130
zyx 1 1 1 0
7 7 7 7 4
2 3 4 5 6 7
Figure67Taming voxels by repeated “divide and conquer”. Reconstruct the macroscopic (e.g. 10 μmØ) object from the coher-ent diffraction patterns, initially with 83=512 voxels. Zoom in to a single voxel, and then reconstruct again with 83=512 voxels (8x zoom) — repeat. Four zooms span from μm to nm scale.
98
4 . NEW TECHNIQUES ENABLED NY NGLSIMAGING STRUCTURE AND FUNCTION IN HETEROGENEOUS ENSEMBLES
4.1.3 Diffract and Destroy at High Repetition Rates
An important consideration fordiffractive imaging
applicationsdescribedinSections4.1.1and4.1.2isthat
theywillrelyonfundamentallynewimplementationsof
the“diffractanddestroy”method thathasbeenpio-
neeredatFLASHandLCLS.BothfXSandbillion-shotdif-
fractiveimagingdealwiththeproblemofrandomsample
orientationinnewwayswhichcapitalizeonthecapabili-
tiesofNGLSX-raylasers.Bothoftheseapproachesrely
onsamplereplacementbetweenX-rayshots,withthe
expectationthat thesample isdamagedordestroyed
eachtime.However,theX-rayexposurepermolecule
is limited (distributed among many molecules either
seriallyinthesingle-moleculecase,orinparallelinthe
fXScase),insuringthatscatteringoccursbeforedisrup-
tionofthesamplestructure.Importantly,thehigh-repeti-
tion-rate of NGLS (and advanced computational
algorithms19,27,29,37,44) are exploited to both address
theissueofrandomsampleorientation,andtoprovide
sufficienttorestrictthegraphordertoalowervalue,per-
hapsaslowasfive.Theseconsiderationsleadtotheview
thatthezoom/octalsearchapproachismorethanade-
quate tomapthe topologicalheterogeneity in typical
objectsofinterest.
Thedatamanifoldnowreflectsconformationaland
configurational (“topological”) degrees of freedom
(Figure68).Navigatingsuchamanifoldwillallowoneto
exploretheconformationalandconfigurationalspace
availabletoanyregionofinterestinamolecularfactory.
Specifically,~1010differentsamplesofagivenregioncan
be investigated.As~104snapshotsareneededfor3D
reconstruction,thismeansthe3Dstructureofselected
regionsfrom~106differentmolecularfactoriescanbe
explored.
Cryo-EMandtraditionalX-raytomographyarealterna-
tivetechniquesforstudyingmolecularfactories.Cryo-EM
requiresthinsectionsbecauseofstrongmultiplescattering
ofelectronsinmicron-thickobjects.X-raytomographyis
limitedbyradiationdamage.43Theweakscatteringof
X-rays,thepossibilitytocollectinformationbeforeonset
ofradiationdamage,18andtheabilitytoexplorealarge
numberofheterogeneousobjectsmakeNGLSanunri-
valedinstrumentforinvestigatingmolecularfactories.
Giga-shot digital cellular microscopy with NGLS offers an
unprecedented capability to reconstruct selected regions of
a large number of objects with orientational, conforma-
tional, and configurational degrees of freedom, providing
a route to statistically validated examination of molecular
factories beyond the limits set by radiation damage.
V
Figure68The manifold (in multi-dimensional data space) repre-sents the information from the entire ensemble of ~1010 particles, spanning all orientations, conformations and configurations (topologies). To reconstruct the 3D structure of all conformations and configurations, one zooms in to a region of interest. Navigating along the manifold corresponding to the selected region allows one to image all such regions sampled during the experiment, and thus reach statistically validated conclusions on heterogeneous systems.
Figure69Hydrated bio-particle jet for interaction with the FEL beam. The jet provides a controlled chemical environment, e.g. for living cells or membrane proteins. A coaxial high-pressure gas sheath focuses the entrained liquid from a nozzle large enough to avoid clogging. The concentration of the particles is arranged to ensure 100% hit rate — with each FEL pulse strikes one particle, or several particles in the case of fXS. (From DePonte et al.13)
99
4 . NEW TECHNIQUES ENABLED NY NGLSIMAGING STRUCTURE AND FUNCTION IN HETEROGENEOUS ENSEMBLES
10. Howells, M.R., et al., Attempt to demonstrate collection of biological
solution SAXS data suitable for 3D structure determination by spatial
correlation averaging. 2009, ESRF: Grenoble.
11. Saldin, D., et al., Structure of isolated biomolecules obtained from ultra-
short x-ray pulses:exploiting the symmetry of random orientations. J.
Phys.: Condens. Matter, 2009. 21: p. 134014.
12. Cho, H.S., et al., Protein structural dynamics in solution unveiled via 100-
ps time-resolved x-ray scattering. Proceedings of the National Academy
of Sciences, 2010. 107(16): p. 7281-7286.
13. DePonte, D.P., et al., Gas dynamic virtual nozzle for generation of micro-
scopic droplet streams. Journal of Physics D: Applied Physics, 2008.
41(19): p. 195505.
14. Chapman, H.N., X-ray imaging beyond the limits. Nat Mater, 2009. 8(4): p.
299-301.
15. Frank, J., Three-Dimensional Electron Microscopy of Macromolecular
Assemblies, in Three-Dimensional Electron Microscopy of
Macromolecular Assemblies. 2006, Oxford University Press: New York.
16. Solem, J.C. and G.C. Baldwin, Microholography of living organisms.
Science, 1982. 218(4569): p. 229-235.
17. Neutze, R., et al., Potential for biomolecular imaging with femtosecond
X-ray pulses. Nature, 2000. 406(6797): p. 752-757.
18. Gaffney, K.J. and H.N. Chapman, Imaging atomic structure and dynamics
with ultrafast X-ray scattering. Science, 2007. 316(5830): p. 1444-1448.
19. Fung, R., et al., Structure from fleeting illumination of faint spinning
objects in flight. Nature Phys., 2009. 5: p. 64-67.
20. Ludtke, S.J., et al., De Novo backbone trace of GroEL from single particle
electron cryomicroscopy structure. Structure, 2008. 16(3): p. 441-448.
21. Fischer, N., et al., Ribosome dynamics and tRNA movement by time-
resolved electron cryomicroscopy. Nature, 2010. 466: p. 329-333.
22. Scheres, S.H.W., et al., Disentangling conformational states of macro-
molecules in 3D-EM through likelihood optimization. Nature Methods,
2007. 4: p. 27-29.
23. Brink, J., et al., Experimental verification of conformational variation of
human fatty acid synthase as predicted by normal mode analysis.
Structure, 2004. 12(2): p. 185-191.
24. Yu, I.M., et al., Structure of the immature dengue virus at low pH primes
proteolytic maturation. Science, 2008. 319: p. 1834-1837.
25. Levin, E.J., et al., Ensemble refinement of protein crystal structures:
Validation and application. Structure, 2007. 15(9): p. 1040-1052.
26. Shaw, D.E. et al. Atomic-level characterization of the structural dynam-
ics of proteins, Science 2010. 330: p.341-346
27. Schwander, P., et al., Mapping the conformations of biological assem-
blies. New J. Phys., 2010. 12: p. 1-15.
28. Nadler, B., et al., Diffusion maps, spectral clustering and reaction coordi-
nates of dynamical systems. Appl. Comput. Harmon. Anal. , 2006. 21: p.
113-127.
unprecedentednewinformationonsampleheterogene-
ity.27Thisisincontrasttosingle-molecule“diffractand
destroy”asoriginallyproposed,17,18inwhichtheorienta-
tionofthesamplemustbedeterminedonashot-by-shot
basis(settingalowerlimitontherequiredfluxperpulse),
whileatthesametimetryingtoavoiddisruptingthemolec-
ularstructureduringthepulseduration(settingastrict
upperlimitonthefluxperpulseand/orpulseduration).
DiffractiveimagingstudiesatNGLSwillexploitthetre-
mendousadvancesinexperimentalcapabilitiesthathave
beenbroughaboutbyfirst-generationX-rayFELs.Inpar-
ticular,asshowninFigure69,DoakandSpenceetal.,
havedevelopedaliquid“aerojet”injectorforgenerating
a1MHzstreamof~1micronsizeliquiddropletsatveloci-
tiesof~10m/s.13Thishasbeensuccessfullydemonstrat-
ed in initial experiments at LCLS (using protein
nano-crystalsinsolution).Thetemporalstabilityanduni-
formpulsespacingderivedfromNGLSsuperconducting
linacoperatinginCWmodewillbeessentialforsynchro-
nizationwithfuturehigh-speedaerojetinjectors.
References:
1. M. Andersson, et al., Structure, 2009. 17: p. 1265-1275.
2. Putnam, C.D., et al., X-ray solution scattering (SAXS) combined with
crystallography and computation: defining accurate macromolecular
structures, conformations and assemblies in solution. Quarterly Reviews
of Biophysics, 2007. 40(3): p. 191-285.
3. Kam, Z., Determination of Macromolecular Structure in Solution by
Spatial Correlation Averaging. Macromolecules, 1977. 10(5): p. 927-934.
4. Kam, Z., M.H. Koch, and J. Bordas, Fluctuation x-ray scattering from bio-
logical particles in frozen solution by using synchrotron radiation. Proc.
Natl. Acad, Sci. USA, 1981. 78(6): p. 3559-3562.
5. Saldin, D.K., et al., Beyond small-angle x-ray scattering: Exploiting angular
correlations. Phys. Rev. B, 2010. 81: p. 174105.
6. Saldin, D., et al., Crystallography without crystals: Structure from diffrac-
tion patterns of randomly oriented molecules, in Coherence 2007. 2007:
Asilomar, USA.
7. Shneerson, V.L., A. Ourmazd, and D.K. Saldin, Crystallography without
crystals. I. The common-line method for assembling a three-dimensional
diffraction volume from single-particle scattering. Acta Cryst. A, 2008. 64:
p. 303-315.
8. Saldin, D.K., et al., Structure of a single particle from scattering by many
particles randomly oriented about an axis: toward structure solution
without crystallization. New Journal of Physics, 2010. 12: p. 14.
9. Saldin, D.K., et al., New light on disordered ensembles: Ab-initio struc-
ture determination of one particle from scattering fluctuations of many
copies. Phys. Rev. Lett., 2010: p. inpress.
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4 . NEW TECHNIQUES ENABLED NY NGLSIMAGING STRUCTURE AND FUNCTION IN HETEROGENEOUS ENSEMBLES
37. Moths, B. and A. Ourmazd, Bayesian algorithms for recovering structure
from single-particle diffraction snapshots of unknown orientation: a
comparison. http://arxiv.org/abs/1005.0640, 2009.
38. Elser, V., Noise limits on reconstructing diffraction signals from random
tomographs. IEEE Trans Information Theory, 2009. 55(10): p. 4715 - 4722
39. Nugent, K.A., Coherent methods in the X-ray sciences. Advances in
Physics, 2010. 59(1): p. 1 - 99.
40. Marchesini, S., et al., Massively parallel X-ray holography. Nat Photon,
2008. 2: p. 560-563.
41. Harary, F. and E. Palmer, G., Graphical Enumeration. 1973, New York:
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42. Sloane, N.A.J., Online encyclopedia of integer sequences, http://oeis.
org/.
43. Le Gros, M.A., G. McDermott, and C.A. Larabell, X-ray tomography of
whole cells. Current Opinion in Structural Biology 2005. 15(5): p. 593-600.
44. Loh, D.N. et al, Cryptotomography: reconstructing 3D Fourier intensities
from randomly oriented single-shot diffraction patterns, (2010) Phys. Rev.
Lett. 104: 225501
29. Giannakis, D., et al., The symmetries of image formation by scattering.
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30. Tenenbaum, J.B., V.d. Silva, and J.C. Langford, A global geometric frame-
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2319-2323.
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linear embedding. Science, 2000. 290(5500): p. 2323-2326.
32. Coifman, R.R., et al., Geometric diffusions as a tool for harmonic analysis
and structure definition of data: Diffusion maps PNAS, 2005. 102(21): p.
7426-7431.
33. Belkin, M. and P. Niyogi, Laplacian eigenmaps for dimensionality reduc-
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1396.
34. Coifman, R.R. and S.e. Lafon, Diffusion Maps. Appl. Comput. Harmon.
Anal., 2006. 21: p. 5-30.
35. Coifman, R.R., et al., Reference free structure determination through
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Equation in a fixed background. Phys. Rev. D, 1973.8: p. 1048-1060.
101
4 . NEW TECHNIQUES ENABLED NY NGLSX-RAY IMAGING: FROM HIGH RESOLUTION TO HIGH SPEED
andparallelworkinvisiblelightopticsbyJimFeinup2in
phasereconstructionalgorithms,thefirstpracticaldem-
onstrationofreconstruction,appliedtoanano-fabricated
2Dpatternofgolddots,wasmadejustoveradecade
ago.3Thesubjectoflenslessimaginghasdevelopedrap-
idlyoverthelasttenyearsandareviewofthecurrent
stateofthearthasrecentlybeenpublished.4
TheoriginalformofcoherentX-raydiffractionmicros-
copyhasbeenappliedtomanysystems,andherewe
illustratethetechniquewithapplicationtoimagingatan-
talum oxide nanofoam.5The general arrangement is
showninFigure70.Thesampleisplacedonathintrans-
mitting membrane and monochromatic transversely
coherentX-raysdiffractfromthesampleandtheresult-
ingdiffractionpatternisrecordedonaCCDcamera.The
realexperimentalsituiationismorecomplexasthedirect
unscatteredradiationhastobeblockedwithastopand
usuallymultipleexposureshavetobetakenatdifferent
intensitylevelstoextendthedynamicrangeofthedetec-
tortocoverthefullrangenecessary,ie.fromtheintense
lowqtoweakhighqranges.
Compared to lens-based transmission microscopy
suchasTXM,thetechniqueeliminatesthelowefficiency
ofaprojectorzoneplateandisnotlimitedinresolution
bythecharacteristicofanX-raylens.Furthermore,unlike
alens-basedsystem,thereisnointrinsiclimitonresolu-
tionsetbydepthoffieldin3Dimaging.Havingobtained
thediffractionpattern,inversiontorealspaceisaccom-
plishedbasedonaniterativephasereconstructionmethod,
whichusesasaconstraintsomeknowledgeoftheobject,
suchasitsphysicalextent.Thissupportfunctioninthe
reconstructioncanbesetbyphysicalpriorknowledgeor
dynamicallyduringreconstructionfromathresholded
autocorrelation.6Thesetechniqueswereappliedinthe
caseofthenanofoamexamplegivenaboveforarangeof
samplerotationangles,toaccomplishafull3Drecon-
struction.Arenderingofthe3Dreconstructionandalocal
regionofthesampleareshowninFigure71.Theresolu-
tioninthestructureshownextendstoaround15nm.
Thisformofdiffractionmicroscopysuffersfromsev-
eralproblems,themostseverebeingthattheobjectmust
beisolated.Toavoidthisissue,anewformoflensless
microscopy,ptychography,hasrecentlybeendeveloped.7
AdefininingcharacteristicofNGLSisthecombination
ofhighpeakpowerandhighaveragecoherentX-ray
power.Thesefeatureswilldramaticallyadvancemany
formsofX-rayimagingoverthatpossiblewithconven-
tional3rdgenerationsynchrotronsources.Hereweout-
line two general areas: first, diffractive imaging of
structures at the nanoscale; and second, continuous
imaging of chemically evolving systems at the mac-
roscale.Manyotheradvancesbetweentheseextremes
willalsobeenabledbyNGLS.
4.2.1 Chemically Specific Nanoscale Imaging
NGLSwillprovideapproximatelyfiveordersofmagni-
tudehigheraveragebrightnessand10ordersofmagntitude
higherpeakbringhtnessthana3rdgenerationundulator
sourcesofsynchrotronradiation.Thiswillenableanew
generationofnanoscaleimagingbasedontheenormous
increaseincoherentpowerwewillhaveavailable.
HighresolutionX-rayimagingwasuntilrecentlybased
onzoneplateoptics,either inScanningTransmission
X-ray Microscopes (STXM) or inTransmission X-ray
Microscopes(TXM).Formsofthesetypesofmicroscopes
mostlikelywillbeusedattheNGLSdependingontheir
matchtoparticularexperimentalneeds.Forexamplethe
STXMhasmanyuniquefeatures—suchastheabilityto
use fluorescence detection, valuable in speciation of
dilute components using fluorescence NEXAFS.
However,oneofthemostexcitingprospectsistheuseof
lenslessformsofimaging,basedoncollectionandinver-
sionofcoherentdiffractionpatterns.Fromthepioneering
workofDavidSayre,whofirstrecognziedthepossibility
forphasingdiffractionpatternsofcontinuousobjects,1
Figure70Geometry for coherent X-ray diffractive imaging. (From Barty et al.5)
4.2 X-rayImaging:FromHighResolutiontoHighSpeed
102
4 . NEW TECHNIQUES ENABLED NY NGLSX-RAY IMAGING: FROM HIGH RESOLUTION TO HIGH SPEED
4.2.2 Cinematic Chemically Specific Macroscale Imaging
Aparalleldevelopmentenabledbytheveryhighaver-
agepowerofNGLSistoperform3Dimagingwithchemi-
calspecificityoverarangeofobjectsizestypicallyupto
themacroscopicsizescaleofmm,withcontinuousobser-
vationofachemicalorstructuralprocess.Applications
range from analysis of combustion chemistry (as
describedinSection3.3),appliedtofueljetsandflames,
analysisofflowthroughporousmediatopolymerpro-
cessing.Thedistinguishingchallengeinthisareaisthat
theobjectisreactive,andisconstantlychangingin3D,
i.e.chemicalreactionsaretakingplacethroughoutavol-
ume.Herewedescribetwomethodsforattackingthis
problem using relatively conventional tomographic
methods,combinedwiththechemicalsensitivityafford-
edbyX-rayabsorptionandfluorescence,andtheunprec-
edentedaveragebrightnessavailablefromNGLS.
X-ray3Dimagingisnormallyachievedthrough
projectionmethods,whereinthesimplestform,direct
transmissionofabeamisimagedontoanX-raysensitive
detector,anddifferentviewsareachievedbysimplerota-
tionoftheobjectaboutoneaxis.Fordynamicallyevolving
objects,onenormallycannotusethissamemethod,due
totheproblemofcoveringanadequatenumberofviews
inatimeshortcomparedtothecharacteristictimescale
fortheevolutionofthedynamicprocess.Inopticalimag-
ing,theproblemhasbeensolvedinavarietyofways,for
exampleconfocalimaging,two-photonconfocal,struc-
turedilluminationimaging,butallthesemethodsrelyin
somewayonveryhighnumericalapertureopticsthat
unfortunatelyarenotavailableintheX-raydomain.
The simplest method of fixed object tomographic
imagingisshownschematicallyinFigure72.Inthismeth-
od,lightiscollimatedintoathinsheet.Itintersectswith
theobjectandlightisgeneratedbyscatteringorfluores-
cenceina2Dplane.Thisplaneisthenimagedontoa2D
detector.Thesheetisthenscannedthroughtheobjectin
adirectionperpendiculartothesheet.Theopticalequiva-
lentof this iswell knownasSelectivePlane Imaging
Microscopy(SPIM).11-13 Inthiscasethe imagingoptic
(imagingthefluorescenceontothedetector)issimplya
highNAlens,andthedetector isaCCD. Ithasmajor
advantagesoverconventional laser-scannedconfocal
microscopyinthatitisfast,andcanbeusedtooptically
sectionverythickobjects.DynamicimagingwithX-rays
Inthiscase,aprobebeamisformedintoarelativelylarge
spotbyanapertureorzoneplatelens,andscannedin
overlappingregionsacrossasamplesothatateachpoint
adiffractionpatterniscollected.
Theoverlapturnsout togiveapowerfulnewcon-
straint in the iterativephase reconstruction,with the
practicalresultthatuniquenessisguaranteed,conver-
gencecanbethousandsoftimesfasterthanconventional
diffraction microscopy, and extended objects can be
imaged.TheoriginaldemonstrationbyRodenburgand
colleagues7hasbeenrefinedtoincludeanintermediate
stepintheiterativereconstructionthatdeterminesthe
probebeamdistributionandfurther improvesresolu-
tion.8LikeconventionalX-raydiffractionmicroscopy,this
morerefinedversionhasrecentlybeendevelopedintoa
3Dimagingmethod,byadditionofsamplerotationso
thattomographicdatasetscanberecordedandrecon-
structed.9
Diffractionmicroscopyasdescribedabove is in its
infancy,buthasalreadyshownthatithasrevolutionary
advantages over conventional X-ray microscopy.
HoweveroneofthemainlimitationsofallX-raymicros-
copywithcurrent3rdgenerationsourcesisthelimited
spatiallycoherentflux.Thisiscombinedwiththefactthat
therequiredfluxscalesastheinverseofthe4thpowerof
the resolution.10This makes high resolution imaging
techniquesslowandsetsapracticallimitofaround10nm
formostimagingmethods.Theenormousaveragecoher-
entfluxadvantagesoftheNGLSmeansthatwewillbe
abletoobtainresolutionclosetothefundamentallimit
setbythewavelength.Inaddition,thehugeincreasein
peakcoherentfluxopensupmanyofthesemethodsin
imagingdynamicprocessesatthenanoscale.
Figure71 Reconstruction of a tantalum oxide nanofoam material; the left panelshows a rendering of the whole 3 micron object and the right panel shows a 0.5 μm segment of the structure. (From Barty et. al.5)
103
4 . NEW TECHNIQUES ENABLED NY NGLSX-RAY IMAGING: FROM HIGH RESOLUTION TO HIGH SPEED
bon,nitrogenandoxygenK-edgeregions,stateoftheart
multilayercoatedmirrorsnowachieveareflectivityof
~10%.15Inthesimplestarrangement,theincidentphoton
energywouldbetunedtoaparticularX-rayabsorption
featureintheNEXAFSregion,forexample,aparticular
molecularorbital feature foradefinedspecies in the
productsfromaflame,andthemultilayerimagingdevice
wouldberequiredtointegrateovertheX-rayfluores-
cencespectrumtolowerenergy.Inordertodothis,the
multilayermightbemadeslightlyaperiodictowidenits
naturalbandwidth.Inthehigherenergyregime,imaging
insuchafashionwillbedifficult,andforimagingthicker
objectswewouldneedtoresorttocomputationalimag-
ingmethods,suchascodedaperturemethods.
Thepromisingapproachofcodedaperatureimaging
iswelldevelopedandhasbeenappliedtoawiderangeof
X-ray imaging applications from X-ray astronomy to
medicalimaging.16-18Codedaperatureshavealsobeen
usedasmultipleobjectreferencesinX-rayholography.19
InsteadofadevicethatfocusesX-raystoanimageplane,
thecodedaperturesimplyconsistsofaseriesofpinholes
withknownlocation.Theimagefromeachpinholeinthis
multiplepinholecameraarrangementfallsonanX-ray
sensitivedetector.Asthenumberofpinholesincreases,
thedetectorsignalincreases,buttheimagesformedby
eachpinholeatsomepointcompletelyoverlap.However,
sincethelocationofeachpinholeisknown,theimage
canbecomputationallyrecovered.Thekeytodoingthis
inthemostefficientwayintermsofsignal-to-noiseratio
istouseaUniformlyRedundantArray(URA)whichhas
thefeaturethatit’scrosscorrelationisadeltafunction,
resultinginasinglepixeleffectivepointspreadfunction.
Togetfromthedetectorimagetotherealspaceimage,a
reconstructionmaskiscreateddirectlyfromtheURAand
thecyclicconvolutionofthemaskwiththedatayieldsthe
realspaceobject.
The stationary object tomographic reconstruction,
therefore,happensinseveralstages:(1)theexcitationis
confinedtoasheetatonetransverselocation,(2)theURL
transmitsanimagetothedetector,(3)theimageiscom-
putationally inverted, and (4) the sheet is translated
acrosstheobject.Thistranslationwouldbesynchronized
withthetimestructureoftheNGLS.Forchemicalimag-
ing,thephotonenergywouldbechosenbasedonfea-
turesintheNEXAFSdatathatprovideafingerprintofa
particularchemicalstate.Severalphotonenergieswould
havetobeusedforthispurpose.Aminimumwouldbe
presents two additional challenges: first, we need to
movetheX-raybeamrapidlyacrosstheobject;andsec-
ond,weneedtoimagetheX-raysoverrelativelylarge
objectsathighnumericalaperture.
InthecaseofNGLS,wecanaccomplishtheformerby
verysmallangularmodulationofaplanemirror.The
sheetisthencreatedasalowapertureconvergentbeam
focusedinoneplaneintothesample.Duetothediffrac-
tion-limitednatureofthebeam(formicron-thicksheets
scanningovermm-scaleobjects)thedeflectionmirror
canbemanymetersupstreamofthesample,implying
verysmalldeflectionangles.Duetotheextremecollima-
tionoftheNGLSX-raylasers,thebeamsizeonthemirror
willbesmall,henceitcouldbelight,andthereforecou-
pledtoahighfrequencyresonantdeflectionstructure.In
thisway,itwillbepossibletomovethebeambyaresolu-
tionelementperpulseoftheFEL(i.e.resolutionelement
per1-10μs).Othermethodsalsoexistforthisdetection
systemincludinguseofacousto-opticallygeneratedgrat-
ings;theperiodwouldbechangedenoughtosweepthe
beamovertheobject.
Toaddressthesecondissue,thetypeofimagingoptic
showninFigure72(labeledURA)needstobetailoredto
theparticularapplication.ForimagingsoftX-rayK-edge
fluorescence,forexample,thedevicecouldbeanormal
incidencemirror,orCassegrainmirrorpair.14Inthecar-
Fuel spray(reactive flow)
Detector
X-rays
URA
Translating sheet
Figure72Fixed object tomography using a transversely scanned sheet.
104
4 . NEW TECHNIQUES ENABLED NY NGLSX-RAY IMAGING: FROM HIGH RESOLUTION TO HIGH SPEED
4. Chapman, H.N. and K.A. Nugent, Coherent lensless X-ray imaging.
Nature Photonics, 2010. 4(12): p. 833-839.
5. Barty, A., et al., Three-Dimensional Coherent X-Ray Diffraction Imaging
of a Ceramic Nanofoam: Determination of Structural Deformation
Mechanisms. Physical Review Letters, 2008. 101(5): p. 055501.
6. Marchesini, S., et al., X-ray image reconstruction from a diffraction pat-
tern alone. Physical Review B, 2003.68(14): p. 140101.
7. Rodenburg, J.M., et al., Hard-X-Ray Lensless Imaging of Extended
Objects. Physical Review Letters, 2007. 98(3): p. 034801.
8. Thibault, P., et al., High-Resolution Scanning X-ray Diffraction
Microscopy. Science, 2008. 321(5887): p. 379-382.
9. Dierolf, M., et al., Ptychographic X-ray computed tomography at the
nanoscale. Nature, 2010. 467(7314): p. 436-439.
10. Howells, M.R., et al., An assessment of the resolution limitation due to
radiation-damage in X-ray diffraction microscopy. Journal of Electron
Spectroscopy and Related Phenomena, 2009. 170(1-3): p. 4-12.
11. Huisken, J., et al., Optical Sectioning Deep Inside Live Embryos by
Selective Plane Illumination Microscopy. Science, 2004. 305(5686): p.
1007-1009.
12. Swoger, J., J. Huisken, and E.H.K. Stelzer, Multiple imaging axis micros-
copy improves resolution for thick-sample applications. Opt. Lett., 2003.
28(18): p. 1654-1656.
13. Verveer, P.J., et al., High-resolution three-dimensional imaging of large
specimens with light sheet-based microscopy. Nat Meth, 2007. 4(4): p.
311-313.
14. Walker, A.B.C., et al., Soft X-ray Images of the Solar Corona with a
Normal-Incidence Cassegrain Multilayer Telescope. Science, 1988.
241(4874): p. 1781-1787.
15. Eriksson, F., et al., 14.5% near-normal incidence reflectance of Cr Sc
x-ray multilayer mirrors for the water window. Opt. Lett., 2003. 28(24): p.
2494-2496.
16. Fenimore, E.E., Coded aperture imaging: the modulation transfer function
for uniformly redundant arrays. Appl. Opt., 1980. 19(14): p. 2465-2471.
17. Fenimore, E.E., Time-resolved and energy-resolved coded aperture
images with URA tagging. Appl. Opt., 1987. 26(14): p. 2760-2769.
18. Caroli, E., et al., Coded aperture imaging in x and gamma ray astronomy.
Space Science Reviews, 1987. 45: p. 349-403.
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IEEE Signal Processing, 2008. 21.
two:oneonaparticularNEXAFSfeature,andonewell
abovetheedgetoprovideforlocalnormalizationofden-
sity.Thiswillallowchemicalspeciationatclosetothe
maximumrepetitionrateoftheNGLS.Ifthephotonenergy
canbescannedthroughtheNEXAFSregionforeachobject
slice,theoverall3Dchemicalvolumemappingwillbe
increasedbyafactoroftypically20.Thiswillbesetbythe
rateatwhichanNGLSX-raybeamenergycanbemodu-
lated.Akeypointisthattheoverallprocesscanberela-
tivelyefficient,andcombinedwiththeveryhighflux/
pulseandthehighrepetitionrate,highspeed3Dchemical
imagingwillbecomepossibleforthefirsttime.
Theabovesectiondescribesfastacquisitionoftomo-
graphicdatausingthehighrepetitionrateofNGLSto
advantageforobjectsthatmustremainstationary.The
spatialresolutionofsuchasystemismodest,~5μmfor
largeobjects,setbythe2Dsheetwidthandthecoded
aperture.To go beyond this, a form of multiple-view
tomographywillbeemployedthatinvolveseitherdirect
transmission and projection, possibly with an X-ray
imagingsystemthatmagnifies,asinnormaltransmis-
sionX-raymicroscopy,ora formofmulti-view-angle
lenslessmicroscopy.Thetechnicalchallengeistosplitthe
incidentX-raybeamintomanybeamlets,deflectthem
throughthesampleatdifferentangles,anddetecteach
beamlet separately.The number of views required
depends strongly on the sparseness of the object.
Methodsbasedoncompressivesensingmaybeusefulin
reducing thenumberofviews required toapractical
value.20Workisunderwaytodesignapracticalimple-
mentationofamulti-viewsystemthatcouldbeusedwith
NGLSX-raylasers.
References:
1. Sayre, D., Imaging processes and coherence in physics, A.e.a.
Schlenker, Editor. 1980, Springer. p. 229-235.
2. Fienup, J.R., Reconstruction of an object from the modulus of its Fourier
transform. Opt. Lett., 1978. 3(1): p. 27-29.
3. Miao, J., et al., Extending the methodology of X-ray crystallography to
allow imaging of micrometre-sized non-crystalline specimens. Nature,
1999. 400(6742): p. 342-344.
105
4 . NEW TECHNIQUES ENABLED NY NGLSMULTIDIMENSIONAL X-RAY SPECTOGRAPHY
complexconsistsofsevencoherently-coupledexciton
states.Themulti-dimensionalspectroscopymap(Figure
73,lowerright)showsthecouplingbetweentheseexci-
tonstates—asnapshotofthecoherentandincoherent
transferofchargepopulationbetweenthesevenstatesat
adelayof1000fsdelayfollowingtheinitialexcitation.
4.3.1 X-ray Multi-Dimensional Spectroscopy
IntheX-rayregion,thetremendouspromiseofmulti-
dimensionalspectroscopyliesinthecapabilitytofollow
coherentchargeflowandenergyrelaxationonfunda-
mental (attosecond to femtosecond) timescaleswith
accesstothefullrangeofvalencestates(unrestrictedby
dipoleselectionrules).Importantly,theelementspecifici-
typrovidedbyX-rays(tunedtocore-levelabsorptions)
willenableusforthefirsttimetofollowchargeandenergy
flow between constituent atoms in materials.These
essentialcapabilitiesarenotattainableusinginfraredor
visiblelaserpulses,andwillprovidecriticalinsighttocor-
relatedelectronsystemsandmolecularcomplexeswith
strongcouplingbetweenelectronicandnucleardynam-
ics. Figure74 illustrates twomulti-dimensionalX-ray
spectroscopyschemes:core-holecorrelationspectroscopy,
andstimulatedX-rayRamanspectroscopy.
4 .3 .1 .1 Multi-Dimensional Spectroscopy —
X-ray Core-hole Correlation
Multi-dimensionalcore-holecorrelationspectroscopy
(Figure74b)3,6isessentiallytwo-dimensionalelectronic
spectroscopyperformedinthesoftX-rayregime.Itexploits
nonlinearinteractionswithcoherent,attosecondX-raypuls-
estoprobecorrelationeffectsbetweenpairsofvalence
electronsexcitedatdifferentatomicsitesinamolecule.
Figure75presentsanexampleandoutlinesthetheo-
reticalbasisforcore-holecorrelationspectroscopy.Here,
anaminophenolmoleculeinteractswithtwoattosecond
pulses,onecenteredat400eV(ωN)andtheotherat535
eV(ωO),inresonancewiththe1scoreexcitationsinNand
Oatoms,respectively.Inthecoherentfour-wavemixing
process,thetargetmoleculeinteractswiththreeX-ray
pulsesseparatedbytimest12andt13andemitsafourth
Overthepastseveraldecades,2ndand3rdgeneration
synchrotronsourceshaveenabledthedevelopmentofa
widerangeof incisive linearprobesof theelectronic,
atomic, and chemical structure of matter. Examples
includeX-rayemissionspectroscopy,X-rayabsorption
spectroscopy,andinelasticX-rayscattering,tonamejust
afew.AsaMHzX-raylaser,theuniquecapabilitiesof
NGLSwillopentheentirelynewfieldsofnonlinearX-ray
scienceandmulti-dimensionalX-rayspectroscopy.
Multi-dimensionalX-rayspectroscopyincorporates
time-orderedsequencesofX-raypulsestogeneratea
signalthatisafunctionofmultipletimedelaysand/or
photonenergies.Thesearenonlinearcoherentwavemix-
ingtechniquesinwhichX-raypulsesareusedasbotha
pump,topreparespecificnear-equilibriumstatesofmat-
ter,andasaprobeoftheseevolvingstates.Thesenew
toolsrelyonsimultaneouscombinationsof:highpeak
power,highaveragepower(highrepetitionrate),spatial
coherence,temporalcoherence,andtunability.
Radio Waves (NMR) ⇒ Infrared ⇒ Visible ⇒ X-rays
Theanalogoustechniqueofnuclearmagneticreso-
nance(NMR)illustratesthetremendouspotentialimpact
ofmulti-dimensionalX-rayspectroscopy.NMRincorpo-
ratessequencesofradio-frequencypulsestogeneratea
two-dimensionalsignal-map(Fouriertransformofthe
timeintervalsbetweenpulses)thatisafingerprintofspe-
cificchemicalstructures,andtheirrelationshipwithina
molecularcomplex.Overthepastdecade,multi-dimen-
sionalspectroscopy(enabledbyultrafastlasersources)
hasbeenextendedtotheinfrared1-3(toprovideafinger-
printofthecouplingbetweendifferentvibrationalmodes
inamolecule)andtothevisibleregime3-5(tomapthe
dynamiccouplingbetweenelectronicstates).Thesetech-
niqueshavebecomeinvaluableforfollowingquantum
coherencesandcharge relaxationbetweenelectronic
statesinsystemsrangingfromchlorophyll(responsible
forlightharvestinginphotosynthesis)toexcitonicstates
insemiconductors(forareview,seeReference3).
Figure73 illustratesamulti-dimensionalelectronic
spectroscopymeasurementofthebacteriochlorophyll
photosynthetic reactioncenter.5 This light-harvesting
4.3 MultidimensionalX-raySpectroscopy
106
4 . �NEW�TECHNIQUES�ENABLED�NY�NGLSMULTIDIMENSIONAL�X-RAY�SPECTOGRAPHY
An important criterion for core-level correlation spec-
troscopy is that the X-ray pulse durations must be faster
than the Auger decay time (~5 fs), since Auger decay sup-
presses the correlation signal of interest.
4.3.1.2 Multi-DimensionalSpectroscopy—Stimulated
X-rayRaman
A complement to core-hole correlation spectroscopy
is stimulated or Coherent X-ray Raman Spectroscopy
(CXRS).7 Whereas conventional optical Raman spectros-
copy techniques exploit visible or infra-red laser fields to
probe lower-frequency vibrational resonances in matter,
pulse with temporal profile S(t, t13, t12). The two-dimen-
sional Fourier transform of this signal with respect to t12
and t13 yields a two-dimensional electronic spectrum in
frequency space. Off-diagonal features in this 2D spec-
trum are present only when there is correlation between
the two excited valence electrons on the N and O atoms;
no signal should be seen in the Hartree-Fock limit of inde-
pendent orbitals. Calculations show that the extent of this
correlation depends not only on molecular structure (i.e.,
it differs in ortho- and para- aminophenol), but also on the
nature of the molecular orbitals excited within the energy
envelopes (∼10 eV) of ωN and ωO.6
0.4
0.3
0.2
0.1
0.0
1
12,000
Frequency (cm-1)
Abso
rptio
n (O
D)
12,300 12,600
2 3 4 5 6 776
543
2
1
1 2 3 4 5 6 7
-12,000 -12,300 -12,600
T = 1000 fsωτ(
cm-1
)
ωτ(cm-1)
Signalamplitude
0.8
0.4
0
-0.4
-0.8
12,600
12,300
12,000
C
B
A
D
Figure73�Top:�Generalized�schematic�of�multi-dimensional�electronic�spec-troscopy�using�a�four-wave�mixing�geometry�with�a�three-pulse�sequence�(k1,�k2,�k3).�The�signal�of�interest�is�the�nonlinear�polarization P(3)
sig�—�shown�here�resolved�in�phase�and�amplitude�via�heterodyne�detection�with�a�local-oscillator�field�ELO.�Bottom:�2D�elec-tronic�spectra�snapshot�(at�1000�fs�delay,�t13)�of�bacteriochlorophyll�pho-tosynthetic�reaction�center5�which�consists�of�seven�coherently-coupled�exciton�states�(lower�right).�The�two�(energy/frequency)�axes�of�the�2D�spectrogram�are�the�Fourier�variables�corresponding�to�the�delay�t12,�and�the�delay�between�k3�and�ELO.
a b c
11
Valance-excitedstates
2 Core-excitedstates
Core levels(e.g. 1s or 2p)Atom 1 Atom 2
Atom 1 Atom 2
Coupledvalencestates
2
1
1
2
2
Figure74Multi-dimensional�spectroscopy�schemes�using�sequences�of�two-color�puls-es�(A).�(B)�Illustrates�core-hole�correlation�spectroscopy�in�which�resonant�core-level�excitation�of�two�atoms�is�used�to�probe�the�coupling�between�their�respective�valence�states�f1�and�f2.�(C)�Illustration�of�stimulated�or�Coherent�X-ray�Raman�Spectroscopy�(CXRS)�in�which�localized�valence�excita-tions�<f1|g1>�and�<f2|g2>�are�created�and�probed�via�resonant�Raman�processes�at�specific�atoms.�This�approach�creates�a�local�valence�excitation,�and�enables�ele-ment-specific�probing�of�charge�flow.�
107
4 . NEW TECHNIQUES ENABLED NY NGLSMULTIDIMENSIONAL X-RAY SPECTOGRAPHY
Thepowerofthismulti-dimensionaltechniqueisthat
itcreatesalocalizedvalenceexcitation,orcoherentelec-
tronicwavepacket,viaastimulatedRamanscattering
processoriginatingfromacorelevel(seeFigure74c).
ThusultrafastX-raypulses,tunedtocoreleveltransi-
tions,provideelementspecificity.Thetimeevolutionof
thewavepackets,andtheflowofvalencechargebetween
differentatomicsites,canthenbefollowedonfundamental
timescales.Inamulti-dimensionalimplementation,the
initialextitationiscreatedbyapulse-pair,andathird
pulse(inaphase-matchedgeometry)readsoutthescat-
teredRamansignal.TheFouriertransformofthesignal
withrespecttothetimedelaysofthepulsescreatesatwo-
dimensionalmapof thevalenceelectronicstatesand
theirevolution.Importantly,sincethefinalstateisnotcore-
excited,butonlyvalence-excited,multidimensionalsig-
nal-mapscanbemeasuredovermuchlongertimescales
thanarepossiblewithcore-holecorrelationtechniques.
4 .3 .1 .3 Multi-Dimensional Spectroscopy — NGLS
Multi-dimensionalX-rayspectroscopyandnonlinear
X-raysciencewillbehallmarksofNGLSastheyrequire
capabilitiesthatarenotavailablefromanyotherX-ray
source.Highpeak-powerX-raypulsesarejustoneofsev-
eralessentialrequirements.Equallyimportantistheabil-
ity to control the degree of X-ray nonlinearity while
resolvingsmallsignalswithhighfidelity.Highrepetition
rate isabsolutelyessential toachieve this inorder to
avoiddisruptingtheelectronicstates(orothersample
attributes) that are being investigated.An important
benchmarktorecognizeisthatthescientificimpactof
multi-dimensional laser techniqueswas realizedonly
after thedevelopmentofmulti-kHzandMHzultrafast
laser sources.These laserscombinedbothhighpeak
powerandhighaveragepowertoenableextremelysen-
sitivity measurements of controlled near-equilibrium
interactionsoflaserpulsesequenceswithmatter.
Followingisabroaderdescriptionofsomeofthecom-
pelling advantages of investigating valence electron
dynamicsandcorrelatedphenomenaviamulti-dimen-
sionalX-rayspectroscopy:
• Temporal (or phase) information —unavailablefrom
conventionalRIXSmeasurements,whichprobethe
spectraldensity-densitycorrelationfunctionS(q,ω)in
thefrequencydomainbutwithoutphaseinforma-
tion.Powerfulcapabilitiesoftime/phasemeasure-
ments include: (1) distinguishing different
CXRSusesX-raystoprobevalenceexcitationsinmatter
(Figure 74c). One may consider CXRS as a powerful
extension (stimulated version) of spontaneous X-ray
RamanprocessessuchaRIXS(asdiscussedinSections
3.1and3.7).AsastimulatedRamanscatteringprocess,
CXRSmeasuresathird-order,χ(3),four-wavemixingpro-
cesswherebyasequenceofthreeincidentpulses(three
fields):En(kn,ωn)|n=1,2,3,generateastimulatedsignal,e.g.,
Esig(-ω1+ω2+ω3),inthemomentum-matcheddirection,
ksig=-k1+k2+k3.
0
3
2
0 1
1
0
-12 3
1 2 3
O1s XANES
HO
OH
NH2
NH2
ortho
para
3
2
0 1
1
0
-12 3
f
gg
O
gO
t3
t2
t1
eN
eN
eO e
N
f
Figure75Top left: para and ortho isomers of aminophenol, and the predicted corresponding 2D X-ray core-hole correlation maps. Top right: Valence and core-excited states of aminophenols. Bottom: double-side Feynman diagram representing one of the contribu-tions to the predicted cross-peak of the 2D X-ray signal map. (From Reference 6)
108
4 . NEW TECHNIQUES ENABLED NY NGLSMULTIDIMENSIONAL X-RAY SPECTOGRAPHY
• Quantum selectivity —pulsesequencesandmomen-
tummatchingallowonetoeffectivelyisolatespecific
termsofthecontributingLiouville-spacepathwaysthat
comprisethetheoreticaldescriptionofcoupledquan-
tumsystemsbasedonatime-dependentperturba-
tionapproach.Thisselectivitymakesitpossibleto
distinguish forexamplecoherentchargecoupling
fromincoherentchargetransfer,electronicrelaxation
fromexcited-stateabsorptionetc.Incombination
withelementspecificityandultrafasttimeresolution,
thiscapabilitywillconstituteamajorbreakthrough
forunderstandingcorrelatedsystems.
References:
1. Hybl, J.D., et al., Two-Dimensional Electronic Spectroscopy. Chem. Phys.
Lett. , 1998. 297: p. 307-313.
2. Asplund, M.C., M.T. Zanni, and R.M. Hochstrasser, Two-dimensional
infrared spectroscopy of peptides by phase-controlled femtosecond
vibrational photon echoes. PNAS, 2000. 97: p. 8219-8224.
3. Mukamel, S., et al., Coherent Multidimensional Optical Probes for
Electron Correlations and Exciton Dynamics: From NMR to X-rays.
Accounts of Chem. Res., 2009. 42: p. 553-562.
4. Li, X., et al., Many-body interactions in semiconductors probed by optical
two-dimensional fourier transform spectroscopy. Phys Rev. Lett., 2006.
96(5): p. 057406.
5. Brixner, T., et al., Two-dimensional spectroscopy of electronic couplings
in photosynthesis. 2005. 434(7033): p. 625-628.
6. Schweigert, I.V. and S. Mukamel, Coherent ultrafast core-hole correla-
tion spectroscopy: X-Ray analogues of multidimensional NMR. Phys Rev.
Lett., 2007. 99(16): p. 163001.
7. Tanaka, S. and S. Mukamel, Coherent X-ray Raman spectroscopy: A non-
linear local probe for electronic excitations. Phys Rev. Lett., 2002. 89(4):
p. 043001.
contributionstothedensity-correlationspectraldis-
tribution,e.g.homogeneousversusinhomogeneous
distributionsofcorrelatedstates,and(2)following
emergentpropertiesastheyevolvefromperturbative
non-equilibriumconditionscreatedbytailoredelec-
tronicorvibrationalexcitationsrangingfromtheTHz
totheX-rayrange(e.g.,modulationormanipulation
ofcorrelatedstatesviacoherentvibrationalmodesor
charge-transferexcitations).
• Element and chemical state specificity —essentialfor
understanding the correlation between valence
statesassociatedwithparticularatomicormolecular
orbitals.Forthefirsttimeitwillbepossibletodirectly
followthecoherentflowofvalencechargebetween
differentatomicsitesintime,energy,andspace.This
abilitywillbeextremelypowerfulforunderstanding
mixed-valencemolecularcomplexes,dilute-magnet-
ic semiconductors, multiferroics, charge-transfer
complexes,cuprates(electronicstatescoupledtoCu
orbitalsversusOorbitals),andmuchmore.Although
conventionalRIXSiselementspecific,itisnotableto
distinguishcoherencesacrossdifferentatoms.
• Symmetry selectivity —sensitivitytospecificvalence
states(e.g.,3dversus2p)andthecapabilitytodistin-
guishspinandorbitalmomentsviapowerfulsoft
X-raydichroismeffects.
• Access to the entire manifold of valence momentum
sates —sincethesoftX-rayexcitationwavelengthis
comparabletotheunit-cell/molecularsize(kvector
largecomparedtotheBrillouinzone),thestrictdipole
selectionrulesthatmediateopticaltransitionsare
substantiallyrelaxed.Themomentumspacespan-
ningtheentireBrilliounzonecanbesampledwith
exquisiteresolution.SoftX-raytransitionsfromcore
levelsdirectlyprobeimportantd-dexcitationsthat
areopticallyforbidden.
5 Proposed facility
InthissectionwedescribetheNGLSfacility.TheNGLS
isenvisionedasahigh-powerX-rayfree-electronlaser
(FEL)facilitythatwillbeamachineofunrivaledinitialper-
formanceandoutstandingfuturecapabilities,preemi-
nentinX-raysciencefordecadestocome.InSection5.1
weoutlinetherequiredcharacteristicsandcapabilitiesof
theNGLS,andcomparetocurrentorunder-construction
light sources.Section5.2presents thecapabilitiesof
potentialalternateapproaches,includingothertypesof
accelerator-driven lightsources, laser-basedhigh-har-
monicgeneration,aswellaswhatperformancemaybe
expectedofthosetypesofsourcesinthefuture;weargue
foracontinuouswave(CW)superconductinglinac-based
arrayofFELsastheoptimalmeanstomeetthescience
needsdescribedinSection3.AnoverviewoftheNGLSis
giveninSection5.3,summarizingFELperformance,facil-
itylayoutandacceleratorparameters.Todeliveraworld-
class lightsourcerequiresanaggressivedesign,and
Section5.4beginswithashortreviewofthechallenges
foranX-raylasersuchastheNGLS,andthendescribesin
somedetailourpre-conceptualNGLSdesignthatenables
thesenewcapabilities.
5.1 CapabilityRequirements
5.1.1 Requirements for the NGLS
ThescientificrequirementsdescribedinSection3,can
beoptimallymetbyanarrayoflinac-driven,high-repeti-
tion-rate,X-rayFELs.Thehighpower,coherence,and
thusbrightness,ofX-rayFELsmakethemuniquetools
fortheexplorationofthestructureanddynamicsofmat-
teratfundamentalscalesoflength,time,momentum,
andenergy.
The NGLS approach combines significant recent
advancesinhighbrightnessphotocathodebeamgenera-
tion,accelerationandtransportwithstate-of-the-artsuper-
conductingRF(SCRF)technologyandundulatordesigns
aswellasrevolutionaryconceptsforseededFELopera-
tion.Theuniformpulsespacingathighrepetitionratewill
provideunprecedentedcapabilitiesatstart-up,accommo-
datingmorediverseorchallengingexperimentsthancur-
rentorplannedsourcesand thepotential to leverage
advancesinavarietyoftechnologiesandnewconcepts,
includingseedlasers,superconductingundulators,X-ray
Injector
Linac 0 Linac 1
Harmoniclinearizer
Linac 2Beam spreader Array of
configurableFELs
X-raybeamline
Endstations
High brightness,high repetitionrate electron gun
Laserheater Bunch
compressor
Figure76 Schematic layout of the main components of the NGLS (not to scale).
110
5 . PROPOSED FACILITYCAPABILITY REQUIREMENTS
• Capabilityforrapidpolarizationcontrol
• Multipleindependentbeamlinessupportingalarge
usercommunity
Inthefollowingsections,wepresenttheNGLSbaseline
parametersforonepreliminarydesignpoint,acknowledg-
ingthesignificantflexibilityaroundthesepointparame-
ters.Nootherexistingorproposedfacilitycandeliverthe
combinationofultrafastcapability,highlongitudinalcoher-
ence,andhighaveragepower,togetherwithmulti-user
operability,flexibilityintimestructure(bothrepetitionrate
andpulseduration),andupgradepotentialtoaddboth
additionalcapacity(uptosevenadditionalFELs),andnew
capability(e.g.higherrepetitionrates,higheraverageand
higherpeakX-raypower,longerpulses,higherresolving
power,shorterwavelengthandlongerwavelengthFELs,
tightersynchronization,X-raypulsefeedbackandshaping
possibilities.)Thedesignoptionspresentedherearemeant
toillustratetheexcitingscientificopportunitiespresented
bytheconvergenceofrapidlyadvancingFELtechnology,
superconductingacceleratortechnology,andsophisticated
micro-manipulationofhigh-energy,high-brightnesselec-
tronbeams.Adetailedoptimizationofcost,performance,
andriskhasyettobeperformed,andwillbuildonthebase-
linepre-conceptualdesignpresentedhere.
5.1.2 Capabilities of Present Facilities
HerewebrieflycomparethecapabilitiesofUVtosoft
X-rayresearchtoolsutilizingring-basedsources,High
HarmonicGeneration (HHG)sources,andFELs.Ring-
basedsources, inparticularstorageringsandenergy
recoverylinacs(ERLs),providemodestaveragepower
withlowpeakpower.Theysupporttunabledevicesthat
providephotonpulsesatveryhighrepetitionrates,and
may be effectively considered CW sources for many
applications. Storage ring-based sources are proven
technology,havewell-establishedusercommunities,and
willremainessentialtoabroadrangeofX-rayscience.
ERLsareanemergingtechnologynotyetoperatinginthe
X-rayrange.HHGsourcesprovidemodestaveragepower
inalmosttable-top-sizedsources,andarerapidlydevel-
opingnewcapabilities,althoughtheyarenotyetavail-
ableintheX-rayrange.FELscanprovideveryhighpeak
power,aswellashighaveragepower,andarenowoper-
atingintheX-rayrangeatlowerrepetitionrates.
Existing storage-ring-based, spontaneous X-ray
sourcesproduceamaximumdegeneracyparameter,or
opticsandFELoscillators.Thedistributedmulti-beam
approachallowsforcapacityincreasethroughongoing
growthinend-stations.Thereisalsotremendousoppor-
tunityfortranslatingadvancesinmachinecontroland
operationintoflexibilityandadditionalcapabilityofX-ray
pulsegeneration.
Figure76showsthemajorcomponentsoftheNGLS;
theinjector,laserheater,CWSCRFlinacsections,linear-
izerandbunchcompressionsystems,beamdistribution,
an array of independent FELs, and X-ray beamlines
willeachbedescribedinmoredetailinlatersectionsof
thisdocument.
The NGLS facility will provide many benefits and
advantagesoverexistingandplannedlightsources,and
willultimatelyfeaturethefollowingcapabilities:
• Highpulserepetitionrates(100kHzorhigherateach
experimental endstation), ultimately 100 MHz at
specificendstations,approachingstorageringrepe-
titionrates
• Very high average flux and brightness (several
orders-of-magnitudegreaterthanthird-generation
ringsandfirst-generationFELs,withpeakpowers
of gigawatts and average powers of up to about
100Wperbeamline)
• Pulsedurationsrangingfromhundredsofattosec-
ondstohundredsoffemtoseconds
• HightemporalcoherenceoftheFELoutputpulses
(closetotheFourier-transformlimit)
• Hightransversecoherence(approachingdiffraction
limits)
• Controlofthetimeduration,bandwidth,andotherlon-
gitudinalfeaturesofthepulses(i.e.,thepossibilityof
envelopeshaping,modulation,orstructuring,andulti-
matelyfeedback-basedcontroloftheseparameters)
• Capability for excellent spectral resolving power,
withouttheneedformonochromators
• SynchronizationoftheFELpulsestoaseedlaserorto
other IRor terahertzsources (with jitteror timing
errorsontheorderof1–10fs)
• FEL output wavelengths (including harmonics)
ultimatelyextendingovermorethantwoorders-of-
magnitude,from~10nmto~1.2Å
• Capability forprecision2-colorX-raypump/X-ray
probeexperiments
• Capabilityforprecisionpump/probeexperimentswith
combinedprobesinthesoftX-rayorEUVandpumps
intheUV,optical,IR,THz,orotherbands
111
5 . PROPOSED FACILITYCAPABILITY REQUIREMENTS
overeachindividualX-raypulse.Forexample,10fspulses
with a large number of X-ray photons can only be
achievedwithFELsources.
Normal-conducting,linac-drivenFELsunderrealistic
operatingconditionsarequitelimitedinrepetitionrate,
andassuchtheiraverageX-raypoweriscomparableto
storagerings.Incomparison,thehighrepetitionrateof
theNGLSwillprovidethreeorders-of-magnitudehigher
averagepowerthannormal-conductinglinacfacilitiesof
comparablebeamenergy, threeorders-of-magnitude
greaterpowerthanstoragerings,andsixorders-of-mag-
nitudegreaterpowerthanHHGsources.TheEuropean
XFELwillbebasedoncryogenicsuperconductingaccel-
erator technology, which will provide average X-ray
powersimilartoNGLS,butbyoperatinginaburstmode
with3000microbunchesspacedby200ns,repeatingat
10Hz.Figure77showsacomparisonofthetimestructure
ofring-basedsources,HHGlasers,andFELs.Acompari-
sonofNGLSparameterswiththoseofotherexistingand
plannedFELfacilitiesisshowninTable1.
HHGsourcescurrentlyprovidenJpulsesuptoabout
100eV,with10kHzrepetitionrate.FutureHHGsources
areexpectedtoimproveinrepetitionrateandenergyper
pulse,and reachsoftX-raywavelengths (seeSection
5.2.4).NGLScapabilitiesinprovidinghighaveragepower
ultrafastpulseswillalsodevelop,bothastheseedlaser
technologyadvances(potentially includingtheuseof
HHGforseeding),andaslow-charge,highrepetitionrate
self-amplifiedspontaneousemission(SASE)operationis
implemented(withbunchchargeofafewpC,atpoten-
tially100MHzrateinadedicatedoperatingconfigura-
tion). In this mode of operation, the short electron
bunchesradiateinasingleorfewopticalmodes,produc-
ing intensecoherentradiationofafewfemtoseconds
duration,andatextremelyhighrepetitionratelimitedby
thetotalelectronbeampower(installed1.8MWcapabili-
ty).Producing~108photonsperpulseat1keV,anaver-
agepowerof~2Wwouldbeproduced(threeordersof
magnitudegreaterthanthatprojectedforHHGsources).
5.2 AlternateApproaches
Inthissectionwediscussfouralternateapproachesto
generatinghighaverage-powerultrafastsoftX-raypuls-
esthateitherexistpresently(storageringsandpulsed
linacs),orareunderdevelopment(energyrecoverylinacs
photon number emitted per“coherence volume,” of
about10-2photonsinasix-dimensionalphasespacecell
whosesizeissetbytheHeisenberg-Fourieruncertainty
principle.Thisdegeneracyparameteris,ineffect,simply
amorefundamentaldescriptionofwhatisoftencalled
brightness,andreflectshowfarthepulseisfromthe
ultimate transformanddiffraction limits inwhichall
photonswouldoccupy thesamemode.FutureERLs
mayproducemorephotonsinsideasmallertotalphase-
spacevolumethanthird-generationsynchrotronsources,
andmayachievedegeneraciesoftheorderof102or10;3
FELsources, throughthecoherentamplificationpro-
cess,producehighlydegenerateX-raypulses,with1010
ormorephotonspercoherencevolume,andwithfur-
ther orders-of-magnitude increase in degeneracy
obtainablefromseededoroscillator-basedFELs.
TheNGLSFELcomplexwillsurpassthecapabilities
ofexistingX-rayfacilitiesinnumerousways:increased
averagephotonfluxandmultiple,simultaneouslyoper-
ableX-raybeamlineswillbeprovided;amoderately
highpeakfluxwillhelptoavoidundesireddamagetoor
perturbationofsensitivetargetmaterials,therebymaxi-
mizingthefractionofusefulphotonsineachpulsein
manyexperiments;thecombinationofveryhighrepeti-
tionrateswithhighphotonfluxesinshortpulseswill
openupentirelynewexperimentalvistas;themultiple
X-raybeamlineswillgivetheflexibilitytoservemany
differenttypesofexperimentsandprovideprobeswith
awidearrayofphotonpulsestructures;thepotentialfor
precisesynchronizationwillallowformultidimensional
spectroscopyandpump-probeexperimentswithmulti-
colorprobesintheX-rayrangeandpumpsintheTHz,IR,
visible,UV,orX-raybands;theuseofseedingschemes
foranFELwillallowforexcellenttunabilityandulti-
matelywillenablefeedback-basedcontroloftheX-ray
pulsestabilityandcharacteristics.
TheFELadvantageinpeakflux(uptosixorders-of-
magnitudegreaterthanstoragerings)andbrightnessor
degeneracy(uptotenorders-of-magnitudegreaterthan
storagerings)arisesbecausetheFELamplificationpro-
cessproducesaverylargenumberofphotonsineach
pulse,andpacksthesephotonsintoasmallopticalphase
space.FELshaveanotheradvantageover ring-based
sources,inthattheirelectronpulsesarepreparedbya
linacandthenusedonlyoncetocreateX-rays.Thisulti-
matelyenablesatransferofbrightnessfromtheelectron
bunchestothephotonpulses,andallowsprecisecontrol
112
5 . PROPOSED FACILITYALTERNATE APPROACHES
structurestorepetitionratesbelowaboutakilohertz.For
example,theSPring-8XFELdesignconsidersanoperat-
ingscenarioinwhichtheirC-bandlinacispulsedatabout
1kHz,atsignificantlyreducedacceleratinggradient,in
ordertoprovidehigherrepetition-ratesoftX-raycapabil-
ity(versus60Hzoperationathighergradientintheircur-
rent design for a hard X-ray range).The maximum
repetitionrateattheLCLS-IIwilllikelybe360Hz.
Supporting a uniform, one-MHz bunch rate with a
SLAC-typelinacwouldrequireCWoperationofthefinite-
conductivity accelerating structures.The 3 m S-band
structureshaveafill-timeofabout700ns(roughlyequal
tothepulseduration)andaninputpowerofabout25MW
andlaser-basedhigh-harmonicgeneration).Weconsider
thesealternativesandtheirabilitiestomeetthescientific
needsdescribedinSection3.Wecontendthatafacility
designbasedonanarrayofFELsdrivenbyaCWsuper-
conductinglinacprovidesthebestchoicetomeetthe
identifiedneeds,andtoprovideaflexibleandupgrade-
ablesourceforthefuture.
5.2.1 Conventional Pulsed Linacs
Warm (finite-conductivity) linacs operating at high
acceleratinggradient (typicallyontheorderof tensof
MV/m)arelimitedbypowerdepositionintheaccelerating
Table1 Comparison of NGLS FEL baseline parameters and technical features to other FEL facilities worldwide.
Wavelength (nm)
Photon Energy (keV)
Pulse duration
(fs, FWHM)
Effective x-ray pulse repetition rate (Hz)
Photons per pulse
Bandwidth (approxi-
mate)
Energy per pulse
(µJ)
Photons per
second
Average photon beam power
(W)
NGLSbaselineparameters
High-power 1 1.2 250 106 1011 10-3 20 1017 19
SASE 3.3 0.38 250 106 1012 10-3 60 1018 61
Seeded,narrow 1.2 1 150 105 1011 10-4 20 1016 2
bandwidth 4.5 0.28 150 105 1012 10-4 40 1017 4
Attosecond 1.2 1 0.25 105 108 10-2 2x10-2 1013 0.002
4.5 0.28 0.25 105 109 10-2 4x10-2 1014 0.004
LCLS&LCLS-II 0.15 8.2 10-100 102 2x1012 10-3 2x103 2x1014 0.24
5 0.25 10-300 102 7x1013 10-3 3x103 8x1015 0.34
FLASH 6.8 0.18 10–50 2x104 2x1012 10-2 60 4x1016 1.15
47 0.026 10–50 2x104 2x1012 10-2 8 4x1016 0.17
XFEL 0.1 12.4 100 3x104 1012 10-3 2x103 4x1016 71
6.4 0.2 100 3x104 4x1014 10-3 104 1019 413
FERMI@elettra 3 0.41 ~40 50 1011 10-4 7 5x1012 0.0003
10 0.12 ~40 50 1012 10-4 20 5x1013 0.001
SPring8XFEL 0.1 12.4 50 60 1011 10-3 2x102 6x1012 0.01
SwissFEL 0.1 12 0.6–28 102 1011 10-3 2x102 1013 0.02
7 0.18 ~1–28 102 1013 10-2 7x102 2x1015 0.07
PohangFEL 0.1 12 ~50 60 1012 10-3 2x103 6x1013 0.12
1 1.2 ~50 60 4x1012 10-3 8x102 2x1014 0.05
ShanghaiFEL 0.1 12 ~75 50 7x1010 10-3 102 3x1012 0.01
9 0.13 ~200 10 5x1012 10-3 102 5x1013 0.001
Note: FLASH and XFEL are based on pulsed superconducting linacs, and utilize trains of bunches (see Figure 2). Effective pulse rate for FLASH is based on 4000 bunches spaced by 333 ns and repeating at 5 Hz; effective pulse rate for XFEL is based on 3000 bunches spaced by 200 ns and repeating at 10 Hz.
113
5 . PROPOSED FACILITYALTERNATE APPROACHES
resultsina2–3kmlinac.Inthesecases,thepowerdissi-
patedintheacceleratingstructureswouldbeapproxi-
mately30kW/m,andwouldrequirewall-plugpoweron
theorder100MW—significantlylessefficientthanthe
CWSCRFproposedfortheNGLS,whichistooperateat
about 10 MW. Operation of a room-temperature RF
machineinpulse-trainmodecouldallowforimprove-
mentinefficiency.However,thisoperatingmodedoes
nothavethebenefitsofCWfeedback,consistentX-ray
pulsespacing,orhighreproducibilitythatcanbeobtained
withanSCRFCWmachine.
Weconcludethatpowerlimitationsofpulsedconven-
tionallinacsprohibitthemfromdeliveringthehighaver-
agepowerX-raypulsesthatarerequiredbythescience
casedescribedinSection3.Reference1summarizesthe
statusandfuturecapabilitiesofFELs,includinguseof
conventionallinacs.
inordertoachieve17MV/mgradientsatarateof60Hz.
TomaintainroughlythesameaverageRFpowerdissi-
pationinthestructureswhenoperatinginCWmode,the
gradientwouldneedtobereducedtoabout0.1MV/m,
sothelinacwouldneedtobeover15kmlongtopro-
duce1.8GeVelectrons(asintheNGLSbaselinedesign).
Otherroom-temperaturestructuresoperatedintrueCW
modewillhavesimilarlimitsonaveragepoweroron
totallength.Onecouldprobablyallowtheheatdissipa-
tion to increase significantlywithadditional cooling
capacity;assumingafactorof100maybeachieved,the
maximumgradientcouldbeincreasedbyafactorof10
comparedtotheaboveestimate,andthusanormal-
conducting,CWS-bandlinacmightberequiredtobe
2–3kmlong.Thepowerconsumptionwouldincrease
proportionally. Similar analysis for an X-band linac
basedonNextLinearCollider (NLC) technologyalso
Ring-basedStorage ringNSLS-II
~10 ps ~ns~nJ, 0.1% BW
ERL JLAB FEL ~100 fs ~0.1 µs
~10 µJ, 0.1% BW
HHG Tabletop ~0.1–10 fs ~100 µs
~nJ, 1% BW
FELXFEL
~100 ms
600 μs
10 to 100 fs
200 ns
~mJ, 0.1% BW
~mJ, 0.1% BW
10 to 300 fs
~1 msLCLS
NGLS
1 μs250 as to 500 fs
~0.1 mJ, 0.1% BW
Figure77 Comparison of X-ray pulse structure of different light source types, based on current capability or near-term capability of facilities under construction. Storage ring and FEL performance is for soft X-rays (around 1 nm). ERLs and HHG sources are not currently operating at soft X-ray wavelengths, and thus perfomance is shown for UV and EUV wavelengths. Pulse energy is in the central cone of undulator radiation, and NGLS values reflect the baseline design SASE FEL repetition rate (the seeded FELs operate at up to 100 kHz, and may produce similar pulse energy in some operating modes; the NGLS may operate in SASE mode at even higher repetition rate with reduced per-pulse energy).
114
5 . PROPOSED FACILITYALTERNATE APPROACHES
opticalcavitybuiltaroundtheFELundulator;theradia-
tionisout-coupledthroughonewindow,andveryhigh
averagepowershavebeenachievedintheIR.Asingle-
pass,high-gainFELcouldbepossible,howeverthedeg-
radationinbeambrightnessfollowinganFELintroduces
significant challenges in implementing multiple FEL
sourcesinanERLconfiguration.Thissingle-FELarrange-
mentofferslimitedflexibilityandpermitsfewerusers.
AparallelarrangementofFELswouldpresenttechnical
challengesandcoststhatareprohibitive.
Weconclude thatERLsarenot flexible,multi-user
sourcesofthehighaveragepower,coherent,ultrafast,
X-ray pulses that are required by the science case
describedinSection3.Reference2summarizesthestate-
of-the-artofERLsaslightsources,andReference1gives
anoverviewofFELperformanceinERLs.
5.2.3 Third- and Fourth-Generation Storage Rings
Storageringshavebeenhighlyproductivefordecades
andarefinelyhonedlightsources.Theyofferreliability,
stability,moderateaverageflux,tuningrange,andpolar-
izationcontrol.Beyondtheexistingthird-generationrings,
the“ultimate”storageringsofferthepossibilityofreduc-
ingelectronbeamemittance(andtherebyraisingX-ray
beambrightness)byafactorof100to1000overexisting
storagerings.Thenewlyapproved,3GeVMAX-IVringin
Swedenwillgoalongwaytowardsachievingthebest
foreseeablestorageringelectronbeamemittance.
StorageringsproduceX-rayfluxestypicallyinthe
range of ~106–108 photons per pulse. Photon pulse
lengthsaretypicallygreaterthantenpicosecondsRMS
induration,butshorterpulselengths,ontheorderof
1ps,canbereachedat lowercharge-per-bunchwith
latticetuning(a“low-alpha”lattice),orathighercharge-
per-bunch but in limited sections of the ring with
RFdeflectionsystems(“crab”cavityschemes).X-ray
pulsedurationsof~0.1–1pshavealsobeenachievedin
storageringsby“laser-slicing”techniquesatrepetition
ratesintothetensofkHz,butwithfluxperpulselimited
bythefractionofthebunchchargeinvolvedinthepro-
cess.FutureringsmayincorporatesoftX-rayFELsina
“partiallasing”mode,orinaswitchedbypass,butwith
limitedgainand/orlimitedrepetitionrate.
5.2.2 Energy Recovery Linacs
ERLs are potential future X-ray synchrotron light
sourcesthatcombinesomeofthequalitiesofstorage
ringswiththoseoflinac-basedlightsources.Ahigh-repe-
tition-rate(uptoGHz)andhigh-current(upto100mAfor
someoperatingmodes)injectorandCWSCRFlinacpro-
videsveryhighbeampower.TheERLconfigurationhas
theadvantageofprovidinganaffordableRFpowersys-
tembyrecoveringmostof theenergyof theelectron
beam.Thisemerging technologypromises toprovide
veryhighaveragebrightnesswithhighspatialcoherence
(~50%ormore)bypreservingthevery lowemittance
(about~0.1µmnormalizedRMStransverseemittance)
andlowrelativeenergyspread(about10-4)achievable
fromafull-energy,high-currentsuperconductinglinac.A
singleturnaroundaring-liketransportlatticeaccommo-
dating several sequential insertion devices produces
spontaneousradiationwithflux-per-pulsesimilartothat
ofstoragerings.Followingtheinsertiondevices,theelec-
tronbeamwouldthenbereturnedtothelinacwhereitis
deceleratedtorecovertheenergyinthesuperconducting
RFstructure.
Energyrecovery linacsoffer thepotential toreach
high spectral brightness (exceeding 1022 photons/s/
mm2/mrad2/0.1%bandwidth)withhighspatialcoher-
ence,andcontrolofpulsedurationdowntotheorderof
1psinahigh-current(~100mA),high-brightnessmode,
anddown to theorderof tensof femtoseconds ina
lower repetition-rate (~1MHz)mode. Fluxperpulse
wouldbesimilartostoragerings,upto~107photons
perpulsedependingonmodeofoperation.Bandwidth
islimitedbythebeamenergyspreadinlonginsertion
devices,andtheX-raypulsesgeneratedbyspontaneous
emission in insertion devices have limited temporal
coherence.ERLshavedemonstratedenergyrecoveryof
over1MW,aswellastwo-passrecirculation,however
withbeamcurrentandwithbeamenergymuchlower
thandesiredandwithtransverseemittancesonetotwo
orders-of-magnitudegreaterthanrequiredforanultra-
brightX-raysource.
AnERLmayincludeanFEL,asinthecurrentstate-of-
the-artJLABIR/UVDemoFEL.1Inthiscase,thehigh-rep-
etition-rate of the JLAB ERL enables an oscillator
configuration,inwhichthelaserspulseiscontainedinan
115
5 . PROPOSED FACILITYNGLS: A TRANSFORMATIVE TOOL FOR X-RAY SCIENCE
5.3 NGLS:ATransformativeTool forX-RayScience
Over the last decade, theoretical innovations and
experimentaladvanceshaveledtoarenaissanceinthe
physicsofX-rayfreeelectronlasers.InGermany,FLASH
usingSCRFlinacs,andatSLAC,theLCLS,usingroom
temperatureRFtechnology,operateroutinelyandreli-
ably,andhaveconclusivelydemonstratedthetechnology
requiredtoproduceanddeliverhigh-brightnessbeams
essentialforX-rayFELs.These,andotherfacilitiesinclud-
ingtheSCSSFELatSPring8inJapan,theFERMI@elettra
FELinTriesteandtheSPARCfacilityinFrascati,arecur-
rently developing seeded FEL capabilities. Looking a
decadeahead,thenextgenerationofX-rayFELsmust
buildontheoutstandingsuccessesofthesepioneering
machines.Inthissectionweprovideanoverviewofthe
NGLSdesignfeaturesandlayout,FELperformance,and
acceleratorparameters.Amoredetaileddescriptionof
thetechnicalfeaturesfollowsinSection5.4.
5.3.1 Machine Overview and Performance
TheNGLSwilloperateinanovelparameterregime,
providingasuiteofuniquefeaturescomparedtoexisting
orplannedX-raylightsources,includingmostnotablya
high-repetition-rate (1MHz),high-brightnesselectron
source,andaSCRFelectronlinacoperatinginCWmode
whichwillprovidebunchesathighaveragebeampower
withuniformbunchspacing.Thesebuncheswillbedis-
tributedviaaspreadersystemtoanarrayofindepen-
dentlyconfigurableFELs,eachoperatingatthreeormore
orders-of-magnitudehigherpulserepetitionratesthan
existingX-rayFELs,andeachwithadjustablephoton
pulsepower,centralwavelength,polarization,andultra-
fast temporal resolution down into the attosecond
regime.Themajorcomponentsofthefacilityareshown
inFigure76.Ourdesignisbasedonmeetingthemost
criticaloftheanticipatedscienceneedsandprovidinga
largeusercapacitywhilerealizingthephysicsandengi-
neeringconstraintsoftheacceleratorandFELs,allthe
whilecognizantofthefacilityandoperatingcosts.Our
baselinedesignforasetofthreesimultaneouslyopera-
bleX-raybeamlineswillservealargenumberofexperi-
mentsperyear,withthecapabilityofprovidingupto
Inconclusion,currentstorageringsdonot,andfuture
storage rings will not, simultaneously deliver the
requiredhighaveragepower,coherent,ultrafast,X-ray
pulsesthataredemandedbythesciencecasedescribed
inSection3.Reference3summarizesthepotentialof
futurering-basedsources.
5.2.4 HHG Laser Systems
Wavelengthsinthehardultraviolet,orpossiblyin
thefuturethesoftX-rayspectralregion,areattainable
intheveryhighharmonicsproducedwhenanintense
infraredlaserpulseisfocusedintoagas.High-harmonic
generationcanbeproducedwithtemporalandspatial
coherenceproperties similar to thoseof thedriving
laserfield.Theyhaveahighdegreeofpolarization,and
sub-femtosecond pulse duration. Such sources have
beengeneratedusingcommercialdrivelasersatthesev-
eral-wattlevel,withrepetitionratesrangingfrom10Hzto
10kHz.Thecut-offoftheharmonicspectrumextendsto
shorter wavelengths as the drive laser intensity is
increased,uptoasaturationintensitywhereharmonic
generationdecreases.IncurrentHHGsystems,theout-
putfluxisroughlyconstantbetween200-500eV,with
about105photonsperpulseinafractionalbandwidth
ofΔλ/λ ≈10-2.Byusinggasspecieswithahigherioniza-
tionpotentials,andahigh-power,longer-wavelength
drivelaser,togetherwithphase-matchingtechniquesin
theharmonic-generationmedium,thespectralcut-off
ofHHGmaybeextendedupto~1keVwithaconversion
efficiencyoftheorderof10-5.Aper-pulseenergyofup
to20nJisprojectedforadvancedlasersoperatingat
100kHzinthefuture.
FutureHHGsourceswillrequiresignificantdevelop-
mentsinlasertechnologytoprovidethehighaverage
power, coherent, ultrafast, X-ray pulses that are
requiredbythesciencecasedescribedinSection3,
andareunlikelytoreachtheaveragepowerlevelsof
100Wobtainablewithhighrepetition-rateFELs.HHG
sourceswill,however,havedirectapplicationsasseed
sourcesforEUV/XUVorsoftX-rayFELs.TheFELspro-
videseveralordersofmagnitudegainactingastun-
ablenarrow-bandX-rayamplifiersfortheHHGsource.
Reference4summarizesthecapabilitiesofpresentand
futureHHGsystems.
116
5 . PROPOSED FACILITYNGLS: A TRANSFORMATIVE TOOL FOR X-RAY SCIENCE
secondprecision.Oneof the twoseededFELswillbe
capableofproducing“two-color”X-raypulses,whilethe
otherseededFELwillprovidebetterenergyresolution
withlongerpulsesandhightemporalcoherence.Thethird
FELwillbeanon-seededSASEdevicecapableofoperat-
ingatthefullrepetitionrateofthelinac,therebyproviding
veryhighaveragepowerX-raybeams.Atapproximately
constantaverageelectronbeampower, theNGLScan
operateatahigherpulserepetitionrateusingbunchesof
lowercharge, shorterduration,buthigherbrightness.
These bunches might enable lasing at shorter wave-
lengths,orpossiblytheoperationofaSASEbeamlinein
so-called“single-spike”configuration,witheachshort
electronpulsenaturallyradiatingintoatmostaveryfew
longitudinalmodes.Table2summarizesmajorFELperfor-
manceparametersforthebaselinemachine,assuminga
pointdesignof300pCbunchcharge(andwhichmayvary
fromafewpCtopotentially1nC).Figure78showsthe
nominalnumberofphotonsperpulseproducedineach
ofthethreebaselineFELs,asafunctionofwavelength
tuning.Furtherdetailsof theFELdesignaregiven in
Section5.4.5.
AsdescribedinSection5.4.2,buncheswiththerequired
highbrightnesswillbegeneratedatthedesiredhighrepe-
titionratebyastate-of-the-artVHFelectronphoto-gun,and
willundergoemittancecompensationandcompression
~100Wofaveragepowertoeachofsixend-stations(two
perFEL),withtunabilityspanningtheimportantabsorp-
tionedgesofcarbon,oxygen,nitrogenandtheL-edgesof
thefirst-rowtransitionmetals(i.e.,to1.2keVinthefunda-
mental,andultimatelyto10keVinthe3rdharmonic).
Whilethefirst-generation,low-repetition-rateX-rayFELs
provideorders-of-magnitudeimprovement,primarilyin
peakpowerandtemporalresolution,comparedtothird-
generationsynchrotronsources,peakpowerisnotasub-
stituteforthelevelofaveragepowerand/orcoherent
powerthatwillbeprovidedbyNGLS.
TheprimaryspectralrangeoftheNGLSbaselinedesign
willextendfrom280eVto1.2keVatthefundamentalof
theundulatoremission(usingundulatorswithdifferent
periods)anduptoapproximately3keVatmuchreduced
intensitywiththegenerationofharmonics.Lowerphoton
energiesmightbereachedbyextractingsomeelectron
bunchesat lowerenergy.Fluxmaybecontrolledfrom
about108toabout1012photonsperpulseinthefunda-
mental,dependingonthedesiredwavelength,pulsedura-
tion,andrepetitionrate.Laserseedingwillbeimplemented
toproducepulseswithdurationasshortas250attosec-
onds,withtemporalcoherenceapproachingfundamental
transformlimits,withthepossibilityofsomecontrolover
chirporlongitudinalpulse-shape,andwithsynchroniza-
tionoftheX-raypulsestoend-stationlaserswithfemto-
Table2 Baseline performance parameters for the three FEL designs. Details are given for example design points of pulse duration and wavelength, and for the baseline 300 pC bunch charge. NGLS will be capable of a broad range of operating configurations, potentially extending the range of pulse lengths, photons per pulse, and repetition rate.
Beamline 1 Beamline 2 Beamline 3
Type Seeded,time-bandwidth-limited 2–colorseeded SASE
Feature Shortcoherentpulses 2-colorX-raypump/probewithadjust-abledelayandattosecondpulses
Highaveragefluxandbrightness
Pulselength(fs,FWHM) 5–150 0.25–25 ~5–250
Wavelengthrange(fundamental,nm)
1.2–4.5(1.0–0.28keV)
1.2–4.5(1.0–0.28keV)
1.0–3.3(1.2–0.38keV)
Maximumrepetitionrate(kHz) 100 100 1,000
Totalphotons/pulse ~1011(150fs,1.2nm)~1012(150fs,4.5nm)
~108(sub-fs) ~1011(250fs,1nm)~1012(250fs,3.3nm)
Photonsper6Dcoherencevolume ~1011 ~108 ~1010
Peakpower(GW) ~0.1(1.2nm)–1(4.5nm) ~0.05(1.2nm)–0.1(4.5nm) ~0.1(1nm)–1(3.3nm)
Averagepower(W) ~1(150fs,1.2nm)–10(150fs,4.5nm)
~0.001(sub-fs)–0.1(fs) ~0.1(5fs)–100(250fs,3.3nm)
Powerin3rdharmonicrelativetofundamental(%)
~0.1(1.2nm)–1(4.5nm)
~1 ~0.1(1nm)–1(3.3nm)
Relativebandwidth(%,FWHM) ~0.005(150fs,1.2nm)–0.02(150fs,4.5nm)
≥1.4(sub-fs) ~0.2(1nm)–0.5(3.3nm)
Polarization Variable,linear/circular Variable,linear/circular Variable,linear/circular
117
5 . PROPOSED FACILITYNGLS: A TRANSFORMATIVE TOOL FOR X-RAY SCIENCE
Veryshort-period,superconductingundulatorswould
allowevenmoreoptions.Section5.4.5.3discussesundu-
latordesign.
Choicesforbeamenergyandpulserepetitionrates
necessitate theadoptionofSCRF technology for the
linac:Sections5.4.3.5and5.4.3.6outlineourcryomod-
ule and RF systems designs, based on the 1.3 GHz
TESLA-typemulticellcavities.Ourchoiceofanacceler-
atinggradientofapproximately14MV/misconserva-
tiveintermsofpresent-daycavitycapabilities;however
itiswithinabroadoptimumofacceleratinggradients
whenfullconstructionandoperatingcostsareconsid-
ered.Furtherstudieswilldetermineanoptimalsetof
operatingparametersfortheNGLSperformance,bal-
ancingriskamongtheinjector,linac,andFEL.Besides
offering the desired high pulse repetition rates, CW
operationoftheSCRFlinachasanothersignificantoper-
ationalbenefitinthatitallowsforautomatedhigh-fre-
quency feedback control to ensure quality and
uniformityoftheelectronbunches,withjitterinX-ray
pulseparametersperhapstentimessmallerthanthat
currentlyachieved.
Considerationofthetrade-offsaffectingtheelectron
brightnesshaveledustoselectforthehigh-qualitycore
ofthebeamthefollowingcharacteristicswhenentering
theFELundulators:0.6μmorsmallernormalizedslice
transverse emittance, 50–60 keV uncorrelated RMS
energyspread,and500Aorhighercurrent.Thelower
limit to the length of the high-quality beam core is
determinedbythetwo-colorFELbeamlineanddesired
levelofradiationoutputfromtheothertwobeamlines.
Inparticulartherequirementtohaveupto150fsdelay
betweenthetwoseedinglaserpulsesandasafetymar-
ginagainsttime-jitterestimatedtobe±50fsimpliesa
needforatleasta250fsdurationfortheusablebeam
core.Aconservativeallowanceforuptohalfofthetotal
bunchchargetoresidewithintheunusableportionof
thebeamthenimpliesatotalbunchchargeof250pCor
larger.Thebaselinemachinedesigndiscussedherepre-
supposes300pCbunches,consistentwithdeliveryby
theinjectorofbuncheswithRMSnormalizedtransverse
emittanceof0.6μmorsmaller.Table3liststhebaseline
electronbeamparametersforthehigh-qualitybunch
core;additionaldetailsaregiveninSection5.4.3.Not
includedinTable3areparametersforlow-chargeSASE
operation,althoughduetoitssimplicitythismaybethe
appropriateconfigurationforcommissioningandinitial
byballisticandvelocitybunchingthroughtheinjector.
Furthercompressionwilloccurthroughamagneticchi-
caneinthelinacbeforeaccelerationtothefinalbeamener-
gy.The machine is designed for an average current
capabilityupto1mA,beyondourinitialparametersof
300pCand1MHzbutconsistentwithawiderangeof
bunchchargeandtimestructures.Ourbaselinedesignhas
beendevelopedassumingabunchchargeof300pC,and
allowsflexibilityto increaseversatility inperformance.
Higherchargeoperationisanticipatedfor longerpulse
durationswhichweexpectmayreach500fs,orforhigher
peakcurrenttoimproveefficiencyofphotonproduction.
Furtherstudieswillberequiredtodelimittheexactbound-
ariesofthebeamparameter-spaceaccessiblebytheNGLS.
Themaximumelectronbeamenergyof1.8GeVhas
beenchoseninourbaselinedesignsoastobeableto
produce1.2keV(1nm)photonswithreadilyavailable
undulator technology (periodsofabout18mm),but
withaminimalacceleratorfootprintandcost.Beamlines
utilizingdifferentundulatorparametersandtechnolo-
giescouldachievedifferentperformanceorcostgoals.
Forexample,anundulatorwitha26mmperiodwould
coverwavelengthsfrom13.4nm(93eV)to1.8nm(688
eV),andanAPPLE-typeundulatorwithperiodof38mm
andamagneticgapof5.5mm(providingabeamclear-
anceof4mm),wouldcoverwavelengthsfrom12.5nm
(99eV)to2.6nm(476eV)andwitharbitrarypolarization.
107
108
109
1010
1011
1012
1013
1 1.5 4 4.53.52Wavelength (nm)2.5 3
Phot
ons
per p
ulse
Beamline 1Beamline 2Beamline 3
150 fs pulses250 as pulses250 fs pulses
Figure78 Projected baseline output at the NGLS: Beamline 1 is a seeded FEL shown here for 150 fs pulse duration; Beamline 2 is a 2-color attosecond beamline, here with 250 as pulses; Beamline 3 is a SASE FEL here with 250 fs pulses. Section 5.4.5 describes FEL design and performance.
118
5 . PROPOSED FACILITYNGLS: A TRANSFORMATIVE TOOL FOR X-RAY SCIENCE
5.3.2 Layout, Conventional Facilities, and Utilities
TheinitialNGLSmachinewillconsistprimarilyofa
straightsection(forelectronaccelerationandtransport)
ofapproximately450m,whichwill then fanoutover
180 m into multiple beamlines and end-stations, as
showninFigure79.
Themajorcivilconstructionstructuresarethelinac
vaultandklystrongallery,spreaderhall,FELvault,and
experimentalhall.Additionalspacewillberequiredforthe
cryogenicsplantincludingassociatedgasstorage,acryo-
moduleacceptancetestfacility,andmachinemaintenance
activities.Conventionalfacilities,includingcoolingtowers,
low-conductivitywatersystems,chilledwatersystems,
electricalswitchingandtransformerstations,alsoneedto
be housed.These construction elements have been
includedinthecostestimategiveninSection8.1.
Anexceptionallystablefoundationwillbeneededto
supporttheentireNGLSmachine.Long-termsettlement
andvibrationmustbeminimizedforefficientmachine
operationandoptimumperformance.
Roughlytenmetersofcombinedconcreteandearth
shieldingwillenclosetheNGLS.Beamdumps,locatedat
theendofthespreaderandattheendofeachFEL,willbe
positioned below floor level and inclined downward.
Concretevaultssurroundingthebeamdumpswillfurther
isolatetheseunitsfromthemainportionofthemachine
andfromthesoilinwhichtheyareburied.
Thewidthandheightoftheinjectorandlinacenclo-
surewillbesufficienttohousethebeamlinecomponents,
supportequipmentandutilitieswhilemaintainingawalk-
wayforinstallationandremovalofafullcryomodule.The
shieldingenclosureinthespreaderregionwillhavean
increasedwidthnecessitatedbytheshallowinitialangle
betweenthebranchlinesandmainbeamaxis.Thiswide
hallwilltransitiontoindividualbranchenclosuresforthe
FELs downstream of the final bend magnet on each
branch.TheFELvaultsextendanadditional~120meters
totheshieldingend-wallandthebeginningoftheexperi-
mentalhall.
SpacingbetweenFELsistobeabout6meters,ade-
quatefortwoormorephotonbranchlines.X-raybeam-
lineswillextendabout50metersfromthefirstoptic,
housedwithintheshieldwall,andreachend-stations
nearthefarwalloftheexperimentalhall.Anapproxi-
operationoftheNGLS.Inthisconfiguration,electron
bunchesofapproximately10pCchargeand10fsbunch
lengthwouldbedeliveredtotheSASEFEL,producing
~108photonsperpulseat1nminpulsesofafewfemto-
seconds duration, and at the full repetition rate of
theinjector.
Distributingtheelectronbeamtoanarrayofbeamlines
throughaspreaderutilizingpulsedkickersoperatingat
100kHzpulserepetitionratedeliversaregularstreamof
bunchessimultaneouslytoeachseededFEL(withtherate
ultimatelylimitedbyseedlaserpowerandspreaderper-
formance),andbunchesforthefinaldownstreamSASE
FEL(notusingaseedlaser)atuptothefullrepetitionrate
of the injector. Section 5.4.4 describes the technical
detailsofthespreader. Table3 Electron beam parameters for baseline operation with 300 pC bunches. With the flexibility offered by a photocathode gun and CW superconducting linac, NGLS will offer a range of other modes of operation, delivering beams required for specific FEL configurations and experimental needs.
Parameter
Bunchcharge(pC) 300
Repetition rate (MHz)
Outoflinac 1
IntoFEL 0.1-1
Average current (mA) 0 .3
Bunchlength(fs)
Outofinjector(FWHM) ~5000
IntoFEL(inusablebunchcore) 250
Peak current (A)
Outofinjector >40
IntoFEL(inusablebunchcore) >500
Emittance (slice, normalized, µm)
Outofinjector <0.6
IntoFEL 0.6
Energy spread (slice, rms, keV)
Outofinjector <4
IntoFEL(inusablebunchcore) 50
WeareengagedinR&Dtofurtherreducetechnical
risk,allowincreaseddefinitionofthemachineconfigura-
tionandperformancedeliverables,andreliablyunder-
standcostsandbenefits:Section5.4.1describestechnical
andphysicschallenges,andSection8.3summarizesour
riskmanagementandR&Dplans.Obviously,muchofthe
physicsofseededFELoperationatX-raywavelengthsis
stillrapidlydevelopingacrossanumberofU.S.andinter-
nationallaboratories;riskmanagementwillinvolvecoor-
dinatedresearchovermanyinstitutions.
119
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
designsformajoracceleratorsystemsoftheNGLS.We
startwith theelectronsourceand injector,andmove
downstreamhighlightingdesignchoicesandfeaturesof
theacceleratorsystemsandFELbeamlines.
5.4.1 Overview of FEL Physics and Technology Challenges
LasingatX-raywavelengthsrequiresveryhighbright-
nesselectronbeams.Topreservebrightness,wenaturally
endeavortobendtheelectronsaslittleaspossibleasthe
bunchesaregenerated,manipulated,accelerated,and
transportedtotheradiatingsections.Thereforeasingle-
passlinac-baseddesignisanaturalchoice.
AssumingtypicalFELconversionefficienciesofthe
electronbeampowertoX-raypowerontheorderof10-4,
NGLSwillachieveanoverallfacilitypowerefficiencyfor
theproductionofradiationontheorderof10-5.Bycom-
parison,thisisseveralorders-of-magnitudegreaterin
efficiencythancanbeachievedtoproducefew-nm-wave-
lengthHHGradiationdrivenbyopticallasers.
Thetargetof1MHzorgreaterbunchrepetitionrate
enables,withrealisticoverallpowerrequirementsand
capitalcosts,anarrayofmultipleX-raybeamlineswith
flexibleandcutting-edgeperformance.Fromanaccelera-
torperspective,onceoneacceptstheutilityofbunchrates
inexcessofabout1kHzorperhaps10kHz,thetechnology
ofchoiceclearlyshiftsfromwarmtosuperconductingRF
linacs.Atthesehighrepetitionrates,CWoperationofthe
SCRF linac enjoys significant advantages over pulsed
operationwithoutagreatpenaltyinpowerrequirements.
Forexample,CWoperationnaturallyallowsforhigh-band-
matelyten-meterwalkwayattheendofthephotonbeam-
linewillprovideaccessfortheinstallationandremovalof
experimentalequipment.Electronicracks,utilitiesand
supportequipmentwillbelocatedinasecondfloorabove
theexperimentalfloor,leavingthegroundfloorclearof
heatandnoisesources.Lasersystemsforthephotocath-
odesource,FELseeding,andendstationexperiments
willbehousedinseparate,temperature-controlledrooms
outsideoftheradiation-shieldedenclosure.
Themostcostlyancillarysystemwillbethetwo-kelvin
liquid helium refrigeration system for cooling of the
superconductingRFstructures.Thecompressorsforthe
refrigeratorwillbehousedonaseparatefoundation,suf-
ficiently removed to limit transmission of vibration
throughthegroundtothemachinebutnotsofarasto
incur unreasonably high cryogenic transmission line
costs.
TheRFpowersupplysystemwillbethelargestpower
drawat~7.2MW,followedbythecryogenicssystems
witha3MWpowerdraw.Magnets,vacuumsystem,
experimentalequipmentandancillarysupportsystems
willcontributeapproximately2.5MWtotheload.Intotal
theutilitypowerloadwillbeapproximately13MW.The
installedcapacitywillbegreater.
5.4 DesignConsiderations andChallenges
Inthefollowingsectionwefirstoutlinethechallenges
ofbuildinganFELthatexceedsthecurrentstate-of-the-
art,andwethendescribeinsomedetailthepre-conceptual
GUNLINAC 2 SPREADER
FEL-TWO COLOR
PHOTONBEAMLINES
1st M
IRROR
SHIELD W
ALL
BEAM DUMP
FEL-SASEFEL-SEEDED
DUMP MAGNET
BEAM DUMP
INJECTO
R
LINAC 0
LASER
HEATE
R
LINAC 1
LINEA
RIZER
BUNCH COMPRESSOR
31
58
107
137
328
366
463
633
120
3
48.5
Figure79 Layout of the NGLS, with dimensions in meters.
120
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
edtheelectronbeambrightnessrequiredforX-rayFEL
operation,andourdesignincludesstate-of-the-artbeam
brightness, theNGLSrequiresthreetofourordersof
magnitudegreater repetition rate than theLCLS.The
NGLSinjectorrequiresdifferenttechnologythanLCLSfor
theelectrongunacceleratingcavityandthephotocath-
ode/lasersystems,aswellasdifferentbeambunchcom-
pressionandemittancecompensationschemes.
ThesaturatedFELoutputpowerisproportionaltothe
electronbeampower.FortheNGLS,theenergyperpulse
mayreachhundredsofmicrojoules,andultimatelymilli-
joules forsomebeamlineswhenoperatingwithhigh-
chargebunches.Ourdualgoalsforlongpulses(250fs
FWHM in the baseline design, and ultimately
longer—whichnaturallyrequireshigherchargeandthere-
forehigher-emittancebunches)togetherwitharelatively
lowbeamenergy(forlowercost),requirecarefuloptimiza-
tionforarobustfacility.AsSCRFtechnologydevelops,the
linacmayallowforincreasedbeamenergywithoutsignifi-
cantlygreaterrisk,therebyreducingtherequirementon
beambrightness.Physicsandtechnologychallengesofan
X-rayFELarediscussedmoregenerallyinReference5.
Thecurrentpreferenceforseedingschemesistouse
echo-enabledharmonicgeneration(EEHG),whichcanbe
implementedat100kHzusingmodestdevelopmentsof
currentlyavailablelasers.Thesecondchoiceistorelyon
HHGseedingatwavelengthsinthe~30nmrange,which
canalsobeeffectedbycurrentlyavailablelasertechnology
operatingat10kHz,andsourcesareanticipatedtoreach
100kHzrepetitionratesatthetimeofcommissioning.
High-gainharmonicgeneration(HGHG)isanalternative
option,andsystemswithpowersuitablefor10–100kHz
operationalreadyexist.Severalfacilitiesworldwideare
developingandevaluatingFELseedingtechniques,and
wehaveincludedseedingexperimentsandtechnology
developmentsintheNGLSR&Dneeds.
Othertechniqueshavebeenproposedtoincreasethe
coherenceorbrightnessoftheFELoutputwithoutseed-
ingtheFELusingexternallasers.Eliminatingseedlasers
has thepotential forenablingmuchhigher repetition
rateswhilestillofferingsomecontroloverthetimingand
durationoftheradiationpulse,butatthecostoflosing
theshot-to-shotconsistency,tightlycontrolledsynchroni-
zation,andtheexquisitecontroloverpulsesofwhich
externalseedingviaanopticallasersystemsiscapable.
OnemethodavoidingseedinglasersisFELself-seeding,
width,automatedfeedbackandcontrolsystemsthatcan
helpensureveryhighstabilityandreliabilityofelectron
bunchpropertiesandconcomitantphotonattributes.The
choiceofCWSCRFtechnologybeyond1–10kHzrepetition
rateopensupallthescientificbenefitsofevenhigherpulse
repetitionrateswithoutsignificantlyfurtherimpactingthe
acceleratorconstructioncosts.Thefinalmachineparame-
terswillbechosentoachieverelativelylowcapitalcostper
experimentallyusablephoton,withpulsesofshortlength
andhighspatialandtemporalcoherence,andmoderately
highbutvariablephotonfluxprovidingintensitiesbelow
thedamageordisruptionthresholdsrequiredformany
sensitiveexperiments.
Ourchoiceofabout14MeV/macceleratinggradient
favorsreducedtechnicalriskinthelinac.Weexpectto
continuetodevelopplansforcryomodulesbasedoncost
andperformanceoptimizationsofthecurrent ILCand
XFEL-baseddesigns—weseethisasoneofseveralareas
whereapartnerlaboratorywithrelevantexperienceand
expertisecouldassumeavaluablerole.Thelinacdesign
isnotyetoptimized,andwedeferfinaljudgmentonthe
exactchoiceofgradienttotheengineeringdesignphase
aftercompletionofthoroughparametriccostandperfor-
manceoptimizationoftheentireFELsystem.
TheFELoperationrequiresthat:εn/γ≤λ/4π,whereεn
theelectronbunch’snormalizedtransverseRMSemit-
tance,γistheLorentzfactor,λistheresonantX-raywave-
lengthgivenbyλ = λu(1+K2/2)/(2γ2),λuistheundulator
period,andKistheundulatorparameter.Additionally,
theenergyspreadmustbesufficientlysmallthatparticles
donotlongitudinallyde-phaseoveranFELgainlength.
Thegainisafunctionofthepeakcurrent,whichscales
proportionallytotheenergyspreadstartingfromagiven
initialbunch.Thus,compressingabunchtoincreasethe
peakcurrentandthusthegainwillunavoidablyincrease
theenergyspread,andbeyondsomepointfurthercom-
pression will become ineffective; conversely, a long
bunchwillhavealowgainowingtolowpeakcurrent.
Thesix-dimensionalelectronbrightness,definedtobe
proportionaltothedensityofelectronsintheirsix-dimen-
sionalphasespace,i.e.,B~Ne/(εnxεnyεnz),istheprimary
beamparameterthatshouldbemaximizedtooptimize
FELperformance.Thisquantityisinvariantalongtheelec-
tronbeamlineunderidealcircumstances,butinpractice
its(near)-conservationcanonlyoccurwithcarefullycon-
sideredmachinedesign.WhiletheLCLShasdemonstrat-
121
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
generatingtheelectronbunchesoftherequiredqualityat
highrepetitionrateusingpresentlyavailablelasertech-
nology.Thefollowingsectionsprovideanoverviewofthe
injectorandofitssimulatedperformance,followedbya
descriptionofitsmainhardwarecomponents.
5 .4 .2 .1 Injector Overview
Aconceptualdesignoftheinjectorlayoutisshown
inFigure80,andaCADrepresentationisprovidedin
Figure81.Theinjectorchainbeginswithaphotocathode
installedina187MHzRFelectrongunoperatinginCW
mode,andadrivelaser.A“bucking”solenoidintegrated
intotheguncontrolsthemagneticfieldatthecathode
surface.Thenextelementsareasolenoidfollowedbya
bunchercavityandthenbyasecondsolenoid.Theseele-
mentsinitiateemittancecompensation8,9whilesimulta-
neously performing“ballistic” bunch compression.10
Thebuncher isanormal-conducting,Cornell-designed
cavity11operatinginCWmodeat1.3GHz.Thenextele-
mentalongthelineisacryostatcontainingasingle1.3
GHz,CW,TESLA-like,superconducting9-cellcavity.
This superconducting cavity accelerates the beam
from750keVatthegunexitandperformsvelocitybunch-
ing12byde-phasingtheRFwithrespecttothemaximum
acceleration phase. Downstream from this cryostat,
anotherroom-temperaturesolenoidcontinuestheemit-
tancecompensationprocessandallowsforthecontrolof
thetransversebeamsizeintheremainingsectionsofthe
injector.Thelastelementintheinjectorisasecondcryo-
statcontainingfive1.3GHzCWTESLA-likesupercon-
ducting9-cellcavities.Withtheexceptionofthefirst
cavityinthislastcryostat,whichisde-phasedforcon-
tinuingthevelocitybunching,alltheothersarephased
formaximumacceleration.Theenergyattheexitofthe
injectorisdesignedtobeabout70MeV.
Figure81showsaCADmodeloftheassembledbeam-
lineincludingbeamdiagnosticsystems,suchasbeam
positionmonitors,currentmonitors, transverseemit-
tancemeasurementsystems,andbeamprofilemonitors,
aswellassteeringmagnetsfororbitcorrections.Inthe
mainlinac(notshowninthefigure,seeSection5.4.3.1)
downstreamfromtheinjector,atransversedeflecting
cavitysystemjointlywithaspectrometersystemwill
allowforbunchlengthmeasurements,“slice”emittance
measurements,andfullcharacterizationofthelongitudi-
nalphasespace.
whereundulatorX-rayoutputgeneratedfromthebeam
itself ispassed throughamonochromator toensure
coherenceandthenmadetooverlapthesameelectron
bunch,suitablydelayed.Whileself-seedingmethods
may have important applications, especially in hard
X-raymachines,forsoftX-raystheenormousadvantag-
esofexternalseedingusingshort-pulselasers,asinthe
EEHG, HHG and HGHG schemes, are major perfor-
mance-enhancingcapabilitiesthatareexpectedtoplay
criticalrolesinfutureX-rayFELfacilities.Atrepetition
ratesofafewMHzorhigher,oscillatorsmightbeusedin
placeofexternal lasersforseeding,openingupnew
regimesofoperation.However,thispotentiallybreak-
through technique is in its infancy at the moment.
AnotherpossibilityistogenerateshortX-raypulsesof
highcoherencewhileavoidingseedingaltogether,by
producingFELradiationin“single-spike”SASEmode,
using low-charge, low-emittance electron bunches
whoselengthisontheorderofafewSASEcooperation
lengths.Thismodeofoperationoffersalmostfulltem-
poralandtransversecoherenceaswellasfemtosecond
timingcharacteristics,butsuffersfromlargepulse-to-
pulsevariationsinX-raypower,andlesscapabilityto
synchronize timingprecisely toexternal lasers.Note
thateachalternativetechniquemightrequireadistinct
beamlineconfiguration,andinsomecasessubstantially
different electron beam parameters, but the relative
conceptualsimplicityoftheseideassuggeststhateach
maywarrantfurtherexploration.
5.4.2 Injector
TheelectronbeamqualityandthustheFELperformance
dependfundamentallyontheinjector.Theremainingsys-
tems of the accelerator can at best preserve, but not
improve,the6Dbrightnessoftheelectronbunchfromthe
injector.WhiletheexcellentresultsofLCLSandPITZ/FLASH
injectors6,7havealreadyproventhecapabilityofgenerat-
ingthebrightbeamsasneededforNGLSatrelativelylow
repetitionrates(~100Hz),noneofthepresentguntechnol-
ogieshaveyetdemonstratedacomparableperformanceat
highrepetitionrates(i.e.,tensofkHztoMHzorgreater).
TheinjectorforNGLSisbasedonanovelelectron
photo-gun design presently being pursued at LBNL.
Thisgun,inconjunctionwiththeuseofhighquantum
efficiency(QE)photo-cathodes,ispotentiallycapableof
122
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
fromthecathode,ischosentobetrapezoidal,withaflat
topoftheorderof50psandroughly10%riseandfall
times.Inthelow-energyregiondownstreamfromthe
gun,space-chargerepulsionfurtherexpandsthebeam
longitudinally, and, in order to achieve the required
~250fsbunchcorelengthattheFELundulatorentrance,
anappropriatecompression isrequired.Bunchcom-
pressionstartsintheinjector,wherethebunchlengthis
reducedtoatypicalFWHMvalueof~5ps,anditissub-
sequentlycompletedfurtherdownstreaminthelinac.
Atdifferentbunchcharge,bunchlengthsandcompres-
sionfactorscandifferduetodifferentspacechargecon-
ditions,wakefieldandCSReffects,etc.,buttheoverall
pictureissimilar.
IntheNGLSinjector,beamcompressionisrealizedwith
a“conventional” ballistic buncher stage, followed by
velocitybunchingintheacceleratingcavities.Theballistic
Table3containsthebaselineinjectorbeamrequire-
ments.Inaddition,preliminarysimulationsofdifferent
modesofoperationwithbunchchargefrom~10pCto
~1nCindicatetheabilityoftheinjectortosuccessfully
operateoverawiderangeofconditions.
InadditiontotheparametersindicatedinTable3,the
controlofenergy-timecorrelations(chirp)inthelongitu-
dinalphasespaceattheinjectorexitwillbecriticalto
effectiveFELperformance.Thechirpcanbepartiallycom-
pensatedbydephasingtheRFinspecificlinacsections,
andtosecond-orderbyusingahigher-harmoniccavity
linearizerupstreamfromthelinacmagneticcompressor.
Higher-ordercorrelationtermsmustbeminimizedtoallow
forsmoothcompressioninthelinacbunchcompressor.
Inordertocontrolspace-chargeemittancedilutionfor
thebaselineNGLSinjectorconfiguration,theshapeof
thelaserpulseand,hence,oftheelectronbunchemitted
Gun
~15 m
Bunc
her
Room temperature solenoid magnet Room temperature RF cavity Superconducting RF cavity and cryomodule
Exit of Gun0.75 MeV
Exit of First Cavity
~10 MeVExit of Injector
70 MeV
Figure80 Schematic layout showing the main components of the NGLS injector. RF systems operate in CW mode, and repetition rate is 1 MHz (upgradeable to ≥100 MHz at low bunch charge).
Figure81 CAD view of the NGLS injector, showing assembled systems including gun, diagnostics, buncher cavity, solenoids, and cryomodules. The injector delivers stable, high-brightness bunches at 1 MHz initially (upgradeable), for further acceleration, compression, and transport to the FELs.
123
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
5 .4 .2 .2 Photocathode Materials
ThephotoinjectorforNGLSisdesignedtooperateat
1MHzrepetitionrateandupto1nCpulsecharge,orpossibly
athigherrepetitionratebutcorrespondinglylowercharge.
Theoptimalwaytoachievethisdesigngoalistousemuch
higherQEcathodematerials.ConsideringthattheLCLS
coppercathodehasaQEoftypically2×10-5,andrequiresa
largelasersystem,weneedtooperateat~104timeshigher
repetitionrate,sowerequireacathodematerialwithan
overallefficiencythatishigherbyapproximatelythisfactor
tomaintainreasonabledrivelaserparameters.
Weare thereforeassessingpositive-electron-affinity
semiconductorphotocathodessuchascesiumtelluride
(Cs2Te)asusedatFLASH,anddi-potassiumcesiumantimo-
nide(K2CsSb).BothcathodesofferinitialQEssignificantly
higherthan5%withphotoemissionintheUVforCs2Teand
inthevisiblefortheK2CsSb.Becauseofitslowerelectron
affinity,thelatterisaparticularlyappealingcandidatefor
theNGLSgun.Ininitialworkinourlaboratory(seeFigure
84),wehaveshownthatthismaterialhasaQEofaround
7%underilluminationwithgreenlightat532nm,givingan
effectiveQE,includingSchottkybarrierloweringduetothe
gun’sacceleratinggradient,of~15%.K2CsSbhashighreac-
tivityandrequiresvacuumoperatingpressuresinthelow
10-11Torr,andinparticularverylowpartialpressuresfor
reactivegassessuchasO2,H2O,andCO2.
buncher,operatedin“zerocrossing”mode10doesnoton
averageacceleratethebeam. Incontrast, thevelocity
bunching,whichinourcaseisperformedbyoperating
thefirsttwosuperconductingcavitiessignificantlyoff-
crest,allowsforasimultaneousaccelerationandcom-
pressionofthebunch.12
Intheballisticbuncher,theenergychirpinducedon
thebeamisnearlylinear,andtheresultingcompression
usuallygeneratesmoresymmetricdistributionsthanin
thecaseofvelocitybunching,wheretheoff-crestopera-
tionandtherelativisticvelocitycompressionintroduce
non-lineareffectsandhenceasymmetricdistributions
withlongertails.Ontheotherhand,theadvantageof
velocitybunchingisthatitacceleratesthebeamasquick-
lyaspossible,allowingforbettercontroloverthetrans-
verseemittancegrowthduetospacecharge.
Theinjectoroptimizationprocessconsistsinfinding
anappropriatetrade-offbetweenthesetwocompression
schemeswhilesimultaneouslycontrollingspace-charge
inducedemittancegrowthbyuseofemittancecompen-
sationtechniques.Longitudinalcompressionincreases
thepeakcurrentandhenceincreasestransversespace
charge,couplinglongitudinalandtransversedynamics.
A largenumberofvariablesaffect the injectorperfor-
mance.ForNGLS,weapproachedtheinjectordesignby
usingamulti-objectivegeneticoptimizationalgorithm,
introducedinthecontextofacceleratorphysicsandpho-
to-injectorsbytheCornellgroup.13TheASTRAtracking
codewasusedforthesimulations.14
Anexampleoftheoptimizationforthebaseline300pC
caseisgiveninFigure82,whereasetofpossiblesolutions
isshowntohighlightthetrade-offbetweenRMSbunch
length(y-axis)andnormalizedtransverseprojectedemit-
tance(x-axis)attheendoftheinjector.Figure83shows
beamphase-spaceforaparticularsolutionthatrepresents
a possible match to the requirements ofTable 3.The
simulationshavebeenperformedassuminganintrinsic
(thermal) emittance for cesium telluride cathodes, as
experimentallydeterminedatPITZ.15Additionally,auni-
form“hard-edge” transversedistribution for the laser
pulseatthecathodewasusedtominimizespace-charge
effects.ForthesolutionsshowninFigure83,themaxi-
mumacceleratinggradientintheTESLA-likecavitieswas
~14MV/m,theCornellbuncherwasusedwithinitsdesign
limits,andthemaximumfieldinthe20cmlongsolenoids
was~0.1T.
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
7
6
5
4
3
2
1
0
RMS
Bunc
h Le
ngth
(mm
)
Normalized Transverse Emittance (mm-mrad)
Figure82 Results of a multi-objective optimization study of injector configurations showing trade-off between normalized transverse projected emittance and RMS bunch length at the exit of the injec-tor for a 300 pC bunch.
124
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
emitsinunderUVillumination,sothattheradiationfrom
theIRlasermustbefrequencyquadrupled,withadditional
lossesinefficiency,andhenceamorepowerfulIRdrive
laserisrequiredthanisthecaseforK2CsSbwheretheIRis
onlyfrequencydoubled(seeSection5.4.2.3).
Withthisdualapproach,wearedevelopingtheideal
materialaswellasalowerriskalternativeincaseofunan-
ticipatedproblemswiththeantimonidecathodes.LBNL
hasaphotocathodelaboratorydedicatedtothiswork.We
cangrowthesematerialsbyMBEtechniquesaswellas
characterizetheirpropertiesusingwavelength-dependent
yield,angle-resolvedphotoelectronspectroscopy,PEEM
andmanyotherstandardtoolsformaterialsdevelopment
andanalysis.Wearealsousingmaterials-sciencebeam-
linesatALSandatNSLSincollaborationwithBNLtomea-
AlthoughK2CsSbhasbeentestedbeforeinphotoin-
jectors,16weareengagedinanR&Dprogramtoassess
performanceanddevelopmaterialsforoperationinour
photocathodegun,includingunderstandingoflifetime
under high repetition-rate laser shock loading and
vacuumconditionsinthegun,andcharacterizingtrans-
versemomentumspectra,surfaceroughness,andmany
otherissues.
InparallelandincollaborationwithINFN-MilanoLASA,
wearedevelopinganalternativepathofferedbytheCs2Te
cathode technology.This material does not require
vacuumpressurestobeaslowasK2CsSbdoes,andhas
beenextensivelyusedinFLASHathighrepetitionratesin
burstmode.WewillextendthisworktoexplorethefullCW
operatingmodeoftheNGLS.Thiskindofcathodephoto-
Emitt
ance
(mm
-mra
d)
-12 -10 -8 -6 -4 -2 0 2 4 6
100% projectednormalized emittance:
0.66 μm
t–t0 (ps)-12 -10 -8 -6 -4 -2 0 2 4 6
t–t0 (ps)
RMS
Ener
gy S
prea
d (k
eV)
-12 -10 -8 -6 -4 -2 0 2 4 6
Pz0 = 70 MeV/c
t–t0 (ps)
-12 -10 -8 -6 -4 -2 0 2 4 6
Current Profile
t–t0 (ps)
I (A)
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
-0.8
60
50
40
30
20
10
0
P z _ P z
0 (M
eV/c
)
6
5
4
3
2
1
0
Normalized Slice Transfer Emittance Slice RMS Energy Spread
Longitudinal Phase Space
Figure83 Example of phase-space parameters at the exit of the injector for a single optimization point in Figure 82, and for the baseline beam parameters of Table 3. The bunch head is at t-t0 > 0. The bunch has the characteristics to allow the linac systems to deliver beam of required pararameters to the FELs.
125
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
theinfraredregion,andlaserswitha1064nmcentral
wavelengtharereadilyavailable.Startingfromthiswave-
length,frequencydoublingandquadruplingcangener-
atethe532nmandthe266nmpulsesrequiredbythe
K2CsSbandCs2Tecathodes,respectively.Frequencyup-
conversionefficiencyvaluesare30%orbetterforsecond-
harmonic generation and ~5% for fourth-harmonic
generation.Therequirementsforper-pulseenergyatthe
cathode,given inTable4,havebeencalculated fora
chargeperbunchofupto1nCandaQEof1%.InitialQE
valuesforthesecathodematerialscanbeasmuchasone
order-of-magnitudehigher,but,becauseoffinitelifetime,
theQEprogressivelydecreasesduringoperation.Inour
assumptions, such degradation will be compensated
byincreasingthelaserenergyuntilthelowerQEbound
of 1% is reached, at which point the cathode may
bereplaced.
Thetotalper-pulseenergybudgetmustalsotakeinto
account losses in thebeamshapingoptics, transport
opticstothecathode,theUHVvacuumwindowreflectiv-
ity, and beam sampling for diagnostic purposes.
Accountingfortheselosses,anoverall15%energyeffi-
ciencyforthe532nmlaserisestimatedfromtheIRampli-
fieroutputtothecathodeplane,whichimpliesarequired
outputIRenergyfromthelaseramplifierofabout1μJ
perpulse(or1Wonaverageat1MHz).Inthecaseof
266nm,theoverallenergyefficiencygoesdownto~2%,
duetothephotonenergyquadrupling.ThisleadstoanIR
outputpowerrequirementofupto10μJperpulseafter
surethegrowthandpropertiesofthesematerials,using
X-rayprobes.
5 .4 .2 .3 Photocathode Laser
Thechoiceofthephotocathodelasersystemiscriti-
calinthedesignofamachinedevotedtosupportauser
facility. In addition to the technical specifications in
termsofpower,energyandpulseduration,otherquan-
titiessuchasrobustreliability,stability,andreproduc-
ibility,areimportanttechnicalcharacteristicsthatmust
beaddressed.
FortheNGLS,fiberlasersrepresentagoodmatchfor
thefollowingreasons:
• Theyhavebeen in industrialproductionforsome
time,andhighlyengineeredlaserlayoutswithhigh
reliabilityarenowcommerciallyavailable17
• Fiberlasershaveoptimumperformanceintermsof
timingandenergyjitter
• Theycanbeefficientlypumpedbydiodelasers
• Theyhavethecapabilityofdeliveringhighaverage
power—fiberlaserswithoutputpowerofupto~10W
ataMHzrepetitionrateareavailable
• TheuseofanactivemediumsuchasYb3+assuresa
gainbandwidthlargeenoughtosupportsub-picosec-
ond pulse durations and rise-times as may be
demandedinsomeoperatingmodesofNGLS
InTable4thelaserparametersrequiredatthecathode
planefortheNGLSareshownforthetwocathodematerials
alsounderdevelopmentanddescribedinSection5.4.2.2.
Dependingonthechoiceofphotocathodematerial,
thelaserwillberequiredtooperateatdifferentwave-
lengths.Mostofthecommercialfiberlasersoperatein
25
20
15
10
5
0200 300 400 500 600 700
Wavelength (nm)
Quan
tum
effi
cien
cy (%
)
Figure84 Quantum efficiency of K2CsSb measured in the photo-cathode laboratory at LBNL, potentially offering a very efficient photocathode design.
Table4 Laser requirements, at the cathode, for the two photo-cathodes under development to provide high brightness bunches at high repetition rate and with conventional laser systems.
LaserParameters Valueatthecathodeplane(K2CsSb)
Valueatthecathodeplane(Cs2Te)
Wavelength(nm) 532 266
Energyperpulse(nJ) upto100 upto200
Transversedistribution Quasi-uniformhard-edge
RMStransversesize*(mm) from~0.1to~1
Longitudinaldistribution Trapezoidalwith~10%riseandfalltimes
Flat-topwidth*(ps) From~1to~60
Repetitionrate(MHz) upto1(higherchargeperbunch)[Goal of 100+ at low charge per bunch, with future developments]
*Dependingonthechargeperbunch.ThechargemaybeintherangefromafewpCuptoonenC.
126
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
5 .4 .2 .4 Photocathode Gun
TheRFphoto-gunproposedfortheNGLSisbeing
developedatLBNLintheAPEXR&Dproject.Thegunis
basedonreliableandmaturemechanicalandRFtechnol-
ogies21,22,23—characteristicsthatareimportanttopro-
videthenecessaryreliabilityfortheoperationofauser
facility.Theprincipalcomponentofthegunisanormal-
conducting,copperRFcavityresonatingat~187MHz
(intheVHFband).Figure85showsacrosssectionofthe
VHFcavityidentifyingthemaincomponents,whileTable5
listsitsbaselinedesignparameters.
As described in Section 5.4.2.2, semiconductor
cathodescanoffertherequiredQEbutaretypicallyvery
sensitivetoionback-bombardmentdamageandcontam-
ination.Extremelylowvacuumpressures,inthe10-11torr
range,arehencenecessaryinordertomaximizethelife-
timeofthecathodes.
Two major goals are targeted in the gun design:
CWoperationcapabilitytoallowoperationathighrepeti-
tionrateofMHzorgreater,and lowvacuumpressure
underoperatingconditions.Becauseofthelowresonant
frequency,thecavitydimensionsarelargeandhencethe
powerdensityonthecavityinnerwalls,producedbyRF
currents,issufficientlysmalltobecontrolledbyconven-
tionalwater-coolingtechniques.Thus,thecavitycanwith-
standtheheatloadwhenoperatinginCWmodeatthe
requiredgradients.Thisdesigncanrealizeasignificantly
highergradientandbeamenergythancanbeachievedin
DCguns.Furthermore,thelongRFwavelengthallowsfor
thelargehigh-conductancevacuumports(thenumerous
finalamplification,or~10Wofaveragepowerat1MHz
operation.CommercialYbfiber lasersdelivering2μJ
perpulseat1MHz,andupto10μJperpulsewithsub-ps
pulsedurationarecurrentlyavailable.
Uniform,“hard-edge”transversedistributionscanbe
achievedbymeansofpulse-conditioningopticalsys-
tems.Anexampleofapracticalandflexiblelayoutcon-
sistsofatelescopesystemfollowedbyanaperture.The
telescopeexpandsthebeamtransversely,overfillingthe
aperture,therebyselectingthepulse’scentralquasi-uni-
formregion.Relayopticsthenimagetheapertureonthe
cathodeplane,withthedesiredmagnification,andwith
thebeneficialeffectofimprovingthelaserpointingsta-
bility.Thisisaneffectivebutinefficientscheme,withfor
exampleanorder-of-magnitudelossintransmissionfor
a10%flattopintensitydistribution.Alternativeschemes
basedonasphericlensescanalsobeused.18Thesesys-
temsbenefit fromahigherefficiency,but requirean
accurateandstablealignmentandrelyontheGaussian-
liketransversebeamprofilesaffordedbyfiberlasersin
ordertooperatecorrectly.Bothschemesareundercon-
siderationfortheNGLS.
Therequiredtemporallasershapingcanbeachieved
byvariousschemes.Amongthese,pulse-stackingusing
birefringentcrystalshavebeendemonstratedtobeboth
reliableandefficient.19Insuchsystems,asinglepulseis
splitintoapairofpulseswithorthogonalpolarizationby
abirefringentcrystal; thispairofpulsesthenpasses
throughanotherbirefringentcrystal,witheachpulse
formingapairofdaughterpulses,forming2npulsesfor
ncrystals;thesepulsereplicascanbearrangedtopar-
tiallyoverlapintime,formingaquasi-flat-topbeam.The
riseandfalltimesoftheshapedpulsedependonthe
originalpulselength.Theappealingcharacteristicsof
thisschemearesimplicityandefficiency (more than
90%whenusinganti-reflectioncoatingsonthecrystals).
Thedrawbackisalackofflexibilityinpulsetimedura-
tion.Otherpulse-shapingschemescanbeusedifthe
pulselengthneedstobecontinuouslytuned.Forexam-
ple,shapersbasedongratingpairsaregoodcandidates
insuchacase,20allowinganextendedrangeofcontinu-
oustuningofthefinalpulselength.Ontheotherhand,
suchflexibilityisprovidedattheexpenseofsystemeffi-
ciency,whichtypicallydropsto~50%.FortheNGLS,we
envisionthatultimatelyacombinationofsystemswill
beusedtoaccommodatethedifferentmodesofopera-
tionofthefacility.
Table5 The main parameters for the CW photocathode gun built and undergoing tests at LBNL. The nominal bunch rate is 1 MHz, with bunches up to 1 nC, upgradable to ≥100 MHz with significantly lower bunch charge.
Totallength(m) 0.35
Acceleratinggap(mm) 40
Q0(idealcopperconductor) 30900
Electricfieldatthecathode(MV/m) 19.5
Storedenergy(J) 2.3
Maximumwallpowerdensity(at0.75MVgapvoltage)(W/cm2) 25.0
Cavityinternaldiameter(m) 0.694
Cavityresonantfrequency(MHz) ~187
Gapvoltage(MV) 0.75
Peaksurfaceelectricfield(MV/m) 24.1
RFpowerfor0.75MVatQ0(kW) 87.5
Operationpressure(Torr) ~10-11
127
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
The resonant copper structure is surrounded by a
stainlesssteelshellthatensurestherequiredmechanical
rigidity,andprovidesthepumpingplenumandvacuum
sealing.Toavoidmechanicalmovingpartsinsidethecav-
ity,thefrequencytuningisachievedbyaremotelyoper-
atedmechanicalsystemthatslightlydeformsthecavity
wallatthebeamexitplane.TheRFpowerissupplied
through two magnetic loop couplers diametrically
opposedonthecathodebackwall.The~187MHzfrequen-
cychoiceiscompatiblewithboth1.3and1.5GHz,thefre-
quenciesofthetwodominantSCRFlinactechnologies
(theILCandXFELaredesignedwith1.3GHzstructures
operatinginpulsedmode,theTJNAFCEBAFandits12GeV
upgradecavitiesandtheJLABFELcavitiesare1.5GHz
structuresandoperateinCWmode.)
TheVHFcavityfabricationhasbeencompleted,and
thefirstRFtestsatlowpowerhavebeensuccessfullyper-
formed; the firstvacuumtestshavealsosuccessfully
completed.Figure11showsthecompletedVHFcavity
duringcalibrationoftheRFcouplers.
5.4.3 Linac
5 .4 .3 .1 Overview of the Linac
Thelinacisthecentralpartofthebeam-deliverysys-
tem,performingthebasicfunctionsofacceleratingand
manipulatingtheelectronbeamasrequiredforlasing.
Designedtoacceptelectronbunchesatabout70MeV
energyfromtheinjector,itprovidesaccelerationupto
1.8GeVbeforedirecting thebeamto thespreader for
distributionintotheseparateFELundulatorlines.
Theproposedlayout,basedonthepreliminarychoice
of TESLA-like superconducting cavity technology,
includescomponentsthathavebecomeconventionalin
existingorproposed4th-generationlightsources25,26,27,28
(seeFigure76).Thelinacconsistsofsixmainsections.
Thefirstsection,Linac0, interfacesthelinacwiththe
injector,providesabout90MeVacceleration,andaccom-
modatesthediagnosticsstationsneededtomonitorthe
beam phase space (see Section 5.4.2.1) before its
entranceintothe“laserheater.”
Thelaserheaterisintendedforcontrolofthebeam’s
uncorrelatedenergyspreadandforstabilizationofthe
beamdynamics.Thebeamisthenfurtheracceleratedin
Linac1(with225MeVenergygain),conditionedbypas-
sagethrougha3.9GHzthird-harmonicRFstructure,com-
pressedthroughasingle-chicanebunchcompressorat
slotsvisiblealongthe“equator”ofthecavityinFigure85)
thatarenecessaryforachievingthedesiredvacuumpres-
surewhencoupledtoapumpingplenumsurroundingthe
cavityequator,andwithnegligibleRFfielddistortion.The
useofa largenumberofnon-evaporablegetter (NEG)
modules as the main pumping system will efficiently
removethosemolecules(H2O,O2,CO2,etc.)thatarepar-
ticularlydamagingtothecathodeandreducelifetime.
Avacuumload-locksystem,basedonadesignusedat
FLASH24willallowforthereplacementofandtheinsitu
conditioningofphotocathodes.
IncontrasttoSCRFguns,whichareaffectedbyfield
exclusionandmagneticfieldquenchinglimits,thenor-
mal-conducting structure allows for straightforward
applicationofmagnetic fields, required foremittance
compensationandexchangetechniques.
NEG modules
Tuner plate
Cathode
Beam exitportSolenoid
RF Couplers
Cathodeinjection/extraction
channel
Figure86 Assembly of the coaxial power input lines on the CW VHF cavity.
Figure85 A cross-section through the diameter of the CW VHF cavity, showing the main components. The cavity design allows for ≥1 MHz bunch rate, with a relatively high gradient at the cath-ode, and low vacuum pressure.
128
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
emittance,andenergyspread)thatmatchthosefromthe
injector(seeSection5.4.2).Furtherstudieswillincludefull
start-to-endbeamdynamicssimulationsanddesignopti-
mizationoftheintegratedinjectorandlinacsystems.
5 .4 .3 .2 Collective Effects
Thebeamdynamicsthroughthelinacareprimarily
determinedbytheelectrons’interactionwiththeexter-
nallyappliedelectromagneticfields,withthebeamself-
fieldsplayinganon-negligibleandmostlydisruptiverole.
Indeed,considerationofcollectiveeffects,andmeansto
mitigatetheirimpactonthebeam,isanimportantaspect
ofasuccessfullinacdesign.Thesignificantsourcesofcol-
lectiveeffectsincludeRFwakefields,space-chargeforces,
andcoherentsynchrotronradiation(CSR).
RFwakefields impactthesingle-bunchlongitudinal
phasespacebydirectlyinfluencingthebeam’slinearand
nonlinearenergychirpandby indirectlyaffecting the
bunchcurrentprofileasthebeamtravelsthroughthe
magneticcompressor.TheRFwakefieldcontributionto
thelinearchirpisgenerallybenignand,infact,itturns
outtobebeneficialasitassistswiththeremovalofthe
beam energy chirp beyond the bunch compressor.
QuadraticcontributionsbytheRFwakestotheenergy
chirpbeforethebunchcompressorareusuallymodestand
can,inprinciple,beoffsetbyappropriatetuningofthelin-
earizer,whilethethird-ordercontributionscanbelarge
andarenoteasilycompensated.Tosomeextenttheirpres-
encecanalsobebeneficial;forexample,theymaycause
thepeakedbunchprofilesnaturallyemergingfromthe
injectortobecomeflatterasthebeamexitsthemagnetic
compressor,whichisnotundesirable.However,strong
cubicandhigher-ordercomponentsintheenergychirp
haveatendencytocausefoldingofthebeamdistribution
inphasespaceandleadtocurrentspikesattheedgesof
thebunches.Thiscanlimitthemaximumachievablecom-
pressionorcompromisethepreservationofbeamquality.
about350MeVenergy,andthenacceleratedtothefinal
energybyLinac2,thelastlinacsection.Giventhe30–50
Arangeforthebeampeakcurrentoutoftheinjector,a
10–17compressionfactorisrequiredinthelinac.Figure
87showsthebeamenergyatdifferentsectionsofthe
machine.Thebetatronanddispersionfunctionsthrough
thespreaderareplottedinFigure88.Thelatticedesign
includesprovisionsforbeamcollimatorsplacedinthe
bunchcompressorandatvariouslocationsalongLinac2
incorrespondencetothelocalmaximaofthebetatron
functions,andadditionalshieldingwillbeemployedin
theseareas.
Thelatticeisavariantofourearlierconceptofa2.4GeV
linacdriverdiscussed inReference29,andhasbeen
modifiedtodeliver1.8GeVenergybeams.Thestudieswe
havecarriedouttodatetosimulatethemachineperfor-
mancehaveusedidealized(i.e.Gaussian)electronbunch-
es at injection with basic properties (peak current,
Exit of Injector70 MeV
Laser Heater160 MeV
Exit of Linac 0160 MeV
Exit of Linac 1385 MeV
Exit of Harmonic Linearizer350 MeV
Bunch Compressor350 MeV
Exit of Linac 21.8 GeV
Figure87 Schematic layout showing beam energy at major sub-systems along the linac for the NGLS baseline design.
Dz
300.
150.
200.
150.
100.
50.
0.0
Beta
tron
func
tions
(m)
Disp
ersi
on fu
nctio
n (m
)
0.0 100. 200. 300. 400. 500.
0.6
0.4
0.2
0.0
-0.2
-0.4
-0.6
s (m)
Figure88 Lattice functions from Linac 0 through the beam spread-er. The expansion of the betatron functions along Linac 2 allows for some compensation of the geometric emittance reduction due to acceleration, as a means to ease collimation. Collimators may be located at the maxima of the betatron functions in the linac.
129
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
Thechoiceofbeamenergyatwhichthemagneticcom-
pressionoccursaimsatreducingtheimpactofCSRonthe
horizontalemittance(whichscalesinverselywiththebeam
energy),butfacesothertrade-offs.Alowerbeamenergy
wouldbefavored,amongotherreasons,byconsideration
ofthemicrobunchinginstability(alowerenergywould
increaselongitudinalphase-spacemixingandreducethe
impactofLSCforces,asalower-energychicanewould
reducethebeamtime-of-flightbetweentheinjectorand
thechicane).Thevalueadoptedforthisproposal,350MeV,
appearstostrikeanadequatebalancebetweenthesecom-
petingrequirements.Furthercontainmentofthecollective
effects can be accomplished by careful lattice design
aimedatminimizingthedispersioninvariantfunctionin
theendregionofthechicane.Theadopteddesignisacon-
ventionalfour-bend,C-shaped,12.64mlongchicanewith
nominalR56=–0.135m.Thelayoutincludestwosmalltrim
quadrupolesfollowingthefirstandprecedingthefourth
dipoleandacollimatorplacedbetweenthesecondand
thirddipoles.
Adegreeofcontroloverthemicrobunchinginstability
isofferedbytheuseofalaserheater.38Thelaserheateris
essentiallyaninverseFELconsistingofawigglerinserted
inasmall,dedicatedchicane.Aconventionallaserpulse
interactswiththebeaminthewigglerandinducesan
energymodulation(atawavelengthequaltothatofthe
laser),whichbytheexitofthechicaneiseffectivelycon-
vertedintoanuncorrelatedenergyspread.Asthedevelop-
mentofmicrobunchingissensitivetotheuncorrelated
energyspread,propertuningofthelaserpulsepower
allowsforaneffectivecontroloftheinstability.Thepro-
posedlaserheaterlocatedataboutthepointof160MeV
beamenergyissimilartotheLCLSdesign39andisbased
ona800nmlaseranda9-period,3cmwavelengthwig-
gler.Alaserpulse,sufficientlylongtoaccommodatethe
electronbunchandcarryingafewμJ’sofenergy,willsuf-
ficetoinducethefew-keVenergyspreadthatourstudies
indicateareneededtostabilizethebeam.29Commercial
laserswithsuchcharacteristicsarereadilyavailable.
Anessentialcomponentofthemachinelayoutisahigh-
erharmonicRFstructure40,41neededtolinearizethelongi-
tudinalphasespacebycorrectingthequadraticterminthe
beamenergychirpbeforethebeamentersthebunchcom-
pressor.Theseenergy/positioncorrelationsarecausedby
theRFwaveformintheacceleratingstructures,thenonlin-
eartermsinthemomentumcompactioninthebunchcom-
pressionchicaneand,possibly,acontributionfromtheRF
Transversespace-chargeeffectsaremostlyconfined
tolowbeamenergyandaregenerallynotofconcernin
thelinac,havingbeensuccessfullydealtwithintheinjec-
tor. Longitudinal space-charge (LSC) effects are also
strongeratlowerenergy,butcontinuetohaveanimpact
onbeamdynamicsathigherbeamenergy,particularlyon
shortlength-scales,andarethemaindriveroftheso-
called“microbunching instability”.30,31This instability
developsfromenergymodulationsalongabunchcaused
byLSCorothercollectiveeffectsandbytheunavoidable
smallchargedensityfluctuations(i.e.,shotnoise),pres-
entinthebeam.Asthebeamtravelsthroughthedisper-
siveregioninthebunchcompressorandthesubsequent
acceleratingand transport sections, theamplitudeof
thesedensityandenergyfluctuationscangrowandspoil
thebeamquality,unlesssuppressedbythelaserheater,
tobedescribedbelow.
Coherentsynchrotronradiationemittedinthebending
magnetsbyalongitudinallysmoothbeamcausesener-
gy/position correlations along a bunch analogous to
thosegeneratedbyRFwakefields,32,33andhorizontal
emittance growth via longitudinal/ transverse motion
coupling.Inaddition,CSRcanaggravatethepresenceof
smallcharge-densityfluctuationsandfurtherenhance
themicrobunchingphenomenoncausedbyLSC.34,35
5 .4 .3 .3 Bunch Compressor, Laser Heater, and
RF Cavity Linearizer
Considerationofthemicrobunchinginstabilityisthe
mainmotivationforourpreferencetohaveasingle-chi-
canebunchcompressorinthelattice.Themicrobunching
instabilitycancauseanincreaseinuncorrelatedenergy
spreadbeyondaleveltolerableforefficientapplicationof
laserseeding,andpreviousstudies36,37haveshownthat
the instability is substantially amplified by passage
throughmultiplechicanes.Themagnitudeoftheamplifi-
cation,however,isalsocriticallydependentonthebeam
current,andourlatticedesignchoiceswillberevisited,to
furtherweighthebenefitsofamultiple-chicanecompres-
sor(suchasreducedsensitivitytobeamtimingjitterand
morecontroloverthebeamenergychirp)againstthe
consequences of the microbunching instability.
Moreover,additionalbunchcompressorsmaybeneces-
saryiffuturedesignoptimizationstudiesindicateaneed
to modify the balance in favor of more compression
occurring in the linacversus thatperformedat lower
energyintheinjector.
130
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
TheELEGANTsimulationsincludedthelongitudinal
RFwakefields,employingtheavailablemodelsforthe
TESLA-likecavities,46,47,29CSRasimplementedbythe1D
modeldescribed inReference48butnot longitudinal
spacechargeeffects(toavoidamplificationofartificial
instabilitiescausedbythelimitednumberofmacroparti-
cles),whereastheIMPACTsimulationsincludedfulllon-
gitudinalandtransversespace-chargemodelingaswell.
TheIMPACTsimulationswerecarriedoutwithonebillion
macroparticles(onlyaboutafactoroftwosmallerthan
theactualelectronbunchpopulation).
wakefields.Athirdharmonic(3.9GHz),RFstructuredevel-
opedfromonerecentlyinstalledatFLASH,42with5MeV
maximalenergypercavitybutwith7orperhaps9cavities
insteadofthe4cavitiesoftheFLASHlinearizer,representsa
natural choice. Beam dynamics simulations point to a
requirementforthelinearizervoltageontheorderof35MV.
5 .4 .3 .4 Simulated Beam Dynamics and
Expected Performance
Theperformanceof theproposed linacdesignhas
beeninvestigatedwithmacroparticlesimulations.These
numericalstudiesfocussedonthreeimportantaspectsof
beamdynamics:theevolutionofthelong-scalefeatures
ofthelongitudinalphasespace,CSR-inducedemittance
growth,andthemicrobunchinginstability.Weevaluated
thefirsttwoeffectswiththecodeELEGANT 43usinga
relativelysmall(butforthispurposeadequate)number
of macroparticles (2×105), whereas we employed the
IMPACTcode’s44capabilitiesforhighresolution,billion-
macroparticlesimulationstoaddressthemicrobunching
instability,45whichisnotoriouslysensitivetospurious
noiseinducedbyasmallpopulationofmacroparticles.
ResultsofthesesimulationsareshowninFigure89and
Figure90.Inbothcasesweassumedatthelinacinjection
a300pCbeamwithGaussiandensitytruncatedatabout
3σinthefull6Dphasespace(1.8σinzintheIMPACTsim-
ulations),40Apeakcurrent,and0.6μmnormalizedtrans-
verseRMSemittance(i.e.abeammatchingthebasic
propertiesoftheinjectorbeamasrevealedbytheASTRA
simulationspresentedinSection5.4.2.1).Thelinacwas
tunedtogenerateabouta15-foldcompressiontoachieve
apeakcurrentbetween550–600Aatextraction.
1804
1802
1800
1798
1796
1200
1000
800
600
400
200
0
E (M
eV)
Curr
ent (
A)
z (mm) z (mm)-0.05 0.00 0.05 0.10 -0.05 0.00 0.05 0.10
dE/E
(%)
z (mm)
–0.18
–0.08 –0.06 –0.04 –0.02 0 0.02 0.04 0.06 0.08 0.1
–0.16
–0.14
–0.12
–0.1
–0.08
–0.06
Figure89 Density plot of the beam longitudinal phase space (left) and current profile (right) at exit of the spreader starting from a 40 A peak current Gaussian bunch at injection to the main linac. Note a small residual energy chirp (left) and relatively flat charge density (right) in the beam core. The bunch head is at z<0 (ELEGANT simulation).
Figure90 Density plot of the beam longitudinal phase space at exit of the linac showing evidence of modest microbunch-ing instability growth. The slice rms energy spread averaged over the 300 fs (or 100 μm) long beam core is about 62 keV, having started with a 4 keV uncorrelated RMS energy spread beam at injection (high-resolution IMPACT simulation using a billion macroparticles).
131
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
structuresmustoperateatatemperatureof~2K,there-
forerequiringalargecryogenicsystem.
TheoriginalTESLAmodulesdevelopedatDESYwere
intendedtorunatalowdutycycle(1%),andhencethe
cryostatdesignisnotsuitableforCWoperation.However,
there already exist successful examples of cryostat
designs51supportingTESLA-likecavitiesthathavebeen
modifiedandconvertedtoallowforCWoperationata
reducedaccelerationgradient,and theTJNAF12GeV
upgrademodulesweredesignedtoruninCWmodeat
highgradientof~20MV/m;seeFigure91.
FortheNGLSlinacwehaveassumedanacceleration
gradientofabout~14MV/m,asanestimatedoptimaltrad-
eoffbetweencapitalcostsscalingwiththenumberofcryo-
modulesandsizeofcryogenicssystems,andoperating
costs.Furtherstudieswillrefinethisvalue,andwenote
thatwithahigherQ-valuewemayuseahigheraccelerat-
inggradientforafixedcryosystemcapacity,increasingthe
electronbeamenergyandtherebythemachineperfor-
mance,withoutsignificantincreaseincosts.
Thecryomoduleconfigurationmustprovideadequate
operationalflexibilityandeasyaccessforfuturemainte-
nanceandrepair.Itshouldalsohaveamanageablesizethat
candelivertherequiredacceleratingvoltage,acceptable
higher-order-mode(HOM)damping,capabilitytoabsorb
beam-inducedpower,andnecessarycryogeniccooling.
Thefinalcryomoduledesignwillberefinedtoreliablymeet
therequirementsfortheNGLS.ExperienceatTJNAFwith
both the12GeVupgradecryomoduledesign,and the
operationoftheIRFELinwhichsignificantlygreaterthan
1mAaveragebeamcurrenthasbeencirculatedsuccess-
fully(albeitatlowerenergy),indicatethatoptimalengineer-
ingsolutionscanbefoundfortheNGLSparameters.
ThecryomodulesandSCRFcavitiesattherelevantfre-
quencyfortheS-band(3.9GHz)harmoniccavitylinearizer
BoththeELEGANTandIMPACTsimulationsshowthat
theinitialGaussianbunchtransformsalongthelinacinto
abunchwitharelativelyflatenergydistributioninthe
coreasaresultofthelongitudinalRFwakefieldsgenerat-
edintheRFstructuresbeforethebunchcompressor.The
beamenergychirpbeyondthebunchcompressorispar-
tiallyoffsetby theRFwakes inLinac2,butcomplete
removalofthechirprequiresoperatingLinac2offcrest
byabout25degrees.TheELEGANTsimulationsshowa
CSRinducedprojectedemittancegrowthto0.73µmover
theentirebunchandto0.65µmovertheusefulcoreof
the beam (with the slice emittance in the beam core
remainingaboutunchangedat0.6µm).
IMPACTruns,takingintofullaccountLSCeffects,were
repeatedforvariousvaluesoftheuncorrelatedenergy
spreadoftheinputbunch,meanttomodeldifferentset-
tingsofthelaserheater.Wefoundthatabeamwithan
initial4keVuncorrelatedRMSenergyspreadisscarcely
affectedbythemicrobunchinginstability(seeFigure90),
andthattheresultingenergyspreadattheendofthe
linacremainsclosetothevalueexpectedfromidealcom-
pression(i.e.,60keV).
5 .4 .3 .5 CW SCRF Cryomodules
Thehighenergybeamrequiredforlasinginthesoft
X-rayrangecanonlybeattainedattheintendedhighrep-
etitionrateof1MHzbymeansofSCRFcavitiesoperating
intheCWmode.SuperconductingRFlinacstructures
havebeendevelopedovermanydecades,withsignifi-
cantprogresshavingbeenmadeonL-Band(1.3-1.5GHz)
operation.Thebaselineoptionadoptedforthisproposal
assumesthe1.3GHzTESLA-typecryomoduleswith9-cell
cavitystructures,49,50with the1.5GHzTJNAF12GeV
upgrademoduleswith7-cellcavitystructuresobviously
representingaviablealternative.Thelinacaccelerating
Figure91 TJNAF concept for a CW SCRF cryomodule for the 12 GeV upgrade of CEBAF. SCRF cryomodules operating in CW mode with accelerating gradient exceeding the NGLS specifications have already been developed at TJNAF.
132
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
The RF system will be distributed along the path of the
linac and in close proximity to it in order to limit the length
of the RF transmission lines. Each RF station will be
equipped with an RF power distribution system including
circulators to protect amplifiers from reflected power, and
a digital RF control system (see Section 5.4.3.7). The RF
controller design derives directly from experience both
with controlling the SNS superconducting linac53 as well
as with the FERMI@elettra FEL.54 This system will be close-
ly integrated with the RF timing and pulse distribution sys-
tem such as the one LBNL delivered to the LCLS.55, 56, 57
The RF system for the S-band linearizer will follow the
same approach as that of the main linac. A 3.9 GHz CW klys-
tron is commercially available. The RF controller for the lin-
earizer will be analogous to the controller for the main linac
for a configuration in which all clocks and reference pulses
are commonly derived and fully synchronized.
5.4.3.7 RFControl
A digital RF controller will constitute the signal coordi-
nation hub of each RF station. The information derived
from cavity RF vector measurements will be used to con-
trol the RF power source, correct for cable drift, detect
faults, operate the tuner(s), and set the cryogenic balanc-
ing heater; the controller’s digital nature also will allow
the recording of all key variables to aid in first-fault identi-
fication. All these functions will run autonomously under
the global control system and in coordination with the
logic embedded in the RF power system. At the lowest
level, a digital self-excited-loop, as pioneered by Jefferson
Lab, is the appropriate model for turning on and stabiliz-
ing a narrow-band CW superconducting cavity. The RF
controller is a key element in managing cavity frequency
perturbations caused in particular by mechanical vibra-
tion (i.e., microphonics).
Drift and jitter of the phase and amplitude of the accel-
erating sections of the linac would cause undesired fluc-
tuations of the electron beam energy, correlated energy
spread, peak current, slice emittance, and arrival-time of
the beam at the undulator. This could include drift and jit-
ter of an individual RF station as well as drift of the rela-
tive phase and amplitude of different linac sections. The
control of the vector-sum of the accelerating fields will be
accomplished by a combination of local and global feed-
back and feed-forward.
The requirements of the RF control system are derived
from the desired beam parameters such as bunch-
to-bunch energy spread, the bunch compression in the
have already been developed at Fermilab.52 The design will
be developed to accommodate CW operation.
LBNL plans to engage partner laboratories and institu-
tions in providing the required SCRF expertise for NGLS.
Several DOE Laboratories and NSF institutions have the
required experience and infrastructure.
5.4.3.6 RFPowerSystems
The RF power system must allow for delivery of beam of
up to 1 mA at 1.8 GeV energy (i.e. a beam power of 1.8 MW),
with sufficient margin for stability. We assume for the base-
line design an unloaded Q0 for the cavities of 1x1010 and
further design studies will refine the performance
capabilities,with potential improvements to be gained with
higher Q0. Table 6 shows major RF system parameters.
AQ-value of 1.4x1010 would allow a 20% increase in gradient
(see Section 5.4.3.6 and Table 6), and improve performance
of the FELs both in the power output in the fundamental and
in the harmonics, as well as in the photon energy reach.
Each multicell cavity in the linac will be powered by a ded-
icated RF power source, of approximately 21 kW output
power. Individual sources offer robustness and beam avail-
ability, and optimal control of the high-Q superconducting
cavities. Both suitable CW klystrons and Inductive Output
Tubes (IOTs) are commercially available. The latter offer high-
er efficiency and lower operating cost, although at a lower
gain. One of the main advantages of IOTs is their low group
delay, which allows for the design of more effective feedback
control loops around the power source for gain, phase, and
amplitude control. In either case, we plan to develop a solu-
tion in close collaboration with industry and, whenever pos-
sible, with multiple sources to ensure competition as well as
to reduce risk. A 21 kW IOT requires a 40–50 kW DC supply,
and ~80 kW installed capacity, drawing ~53 kW wall-plug
power for the conditions in Table 6.
Table 6 Parameters for the NGLS linac RF structures. Mainlinac Linearizer
RF frequency (GHz) 1.3 3.9
Qo 1×1010 5×109
Qext 1x107 3.3×106
R/Q (Ω) 1036 750
Ibeam (mA) 1 1
Cavity voltage (MV) 14 5
Cavity gradient (MV/m) 13.5 14
Beam phase (degrees) variable –180
Cavity type 9 cell 9 cell
Number of cavities 144 7
RF tube power rating per cavity (kW) 21 3.1
Total installed RF power capability (MW) 3 0.02
133
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
on the distribution system. Practical limits on heat
exchangersizesforthe5Kto2Kstageareconsistentwith
incorporatingsuchaunitintoeachcryomodule.
Returngaspipesizeswithinthecryomodulesmustbe
sufficienttocontroltwo-phaseflowvelocitiesand,incon-
junctionwiththegasreturnpiping,mustlimitthepressure
droptoafewmbar.Cryomodulepipingsystemsmustalso
accommodatetherapidheliumvaporgenerationassoci-
atedwithaloss-of-vacuumaccident.
Detailedrequirementsforthecryogenicssystemswill
bedevelopedinparallelwiththeoveralllinacdesign.
5.4.4 Beam Spreader
Theelectronbeamspreaderdistributesthebunchesfrom
thelinacintotheindividualFELundulatorlines.Inthepro-
poseddesign,allthebeamlineslieononesideofthelinac
axis.Thebunchesareextractedintoallthebeamlines(except
thelastone)byfastkickersoperatingatupto100kHzrepeti-
tionrate,anddistributedalongaFODOtransportchannel,as
sketchedinFigure92.Thelastbeamlinereceivesbunches
diverted by a conventional electromagnet and is thus
enabledtooperateatthemaximumrepetitionrateallowed
bythemachine,representingtheobviouschoiceforhosting
aSASEFELlineunconstrainedbyseedlaserrequirements.
Thetwo-meterlongstriplinemagnetkickersarelocated
betweenfocusing(F)anddefocusing(D)quadrupoles,with
the downstream defocusing quadrupole adding
0.7mradtotheprimarykick.Theorbitpassesthroughthe
focusingquadrupolewitha15.8mmoff-setfromthemagnet
centerandisforcedtofollowalinealmostparalleltothe
FODOaxisbeforethebeamenterstheseptum.Thedefocus-
ingquadrupoledownstreamoftheseptumisalarge-bore
design,centeredontheFODOaxisandsupplyinganaddi-
tional17mradkick.Pastthisquadrupole,thetwobeamlines
continueinseparatedvacuumchambers.Thenextdown-
streamfocusingquadrupolealongtheFODObeamlineisa
small-boreseptumquadrupole:thelinebranchingoffpasses
injector,andthearrival-timeofthebeamattheundulators.
Thebeamparameterscanbetranslatedintotherequire-
mentsforphaseandamplitudestabilityoftheaccelerating
fieldofindividualcavities.Forexample,RFsystemsinthe
injectorwillrequiretightfieldcontrolontheorderof0.01%
fortheamplitudeand0.01°forthephase.29
Thedegreeofprecisionachievableforthecontrolof
each individualRFstation isultimately limitedbythe
noiseflooroftheRFsignalprocessingelectronics.For
furthercontroloftheacceleratingfields,itisenvisioned
thatthecontrolsateachRFstationwillincludeinputs
from beam measurements derived from diagnostic
stationsalongthelinac,allowingbeam-basedfeedback
controlofthelinac.Diagnosticsincludemeasurementof
the relativebeamenergy,bunch length,charge,and
arrivaltimerelativetothemasterclock.Asuitablelinear
combinationoftheseparameters,alongwithindividual
RFstationcontrol,willallowthebestpossiblestabiliza-
tionofthebeamparameterscriticalforstableoperation
of theFEL.Adiscussionof thepotentialadvantages
offeedbacksystemsonstabilityoftheX-raypulsescan
befoundinReference29.
5 .4 .3 .8 Cryogenic Refrigeration and Distribution
Thecryogenicheatload,scaledfromtheCEBAF12GeV
upgradecryomoduletestresultstoNGLSlinacoperating
conditions(Table6),isabout3kWat2K.Toallowfortran-
sients and non-optimal equipment performance, an
installedcapacityof4.5kWispreferred.Heliumrefrigera-
torsofcomparablescaleareinoperation,forexample,at
TJNAF’s12GeVrefrigerationsystem.Theratioofelectric
powertorefrigerationcapacityforsuchsystemsisabout
1kWinputpowerperWdissipatedat2K,andwethus
requireabout4.5MWinstalledcapacityand3MWtypical
powerdraw.
Distribution of liquid helium in vacuum-insulated
coaxiallinesiswell-establishedtechnology.Distributing
the5Kratherthan2Kliquidcaneasetherequirements
Kicker
F F10800
F F
F
D
F
D
D D DD
KickerKickerSeptum Septum
150 466
Figure92 Schematic layout of a section of the electron beam spreader showing the fast kickers and septa modularly inserted along the FODO channel. Dimensions are given in mm. Note that scales are different in the vertical and horizontal direction.
134
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
thoseofthebeamscontinuingstraightdownstreamofthe
take-offsection(justoutsidethemagnet)—seealsothe
insertpicture.Athinmagneticshieldiswrappedaroundthe
vacuumchamberinthislocationinordertoreducetheresid-
ualfieldexperiencedbythebeamtravelingoutsidethemag-
net.Calculationsshowanacceptableresidualfieldonthe
orderof0.05Gorsmaller(withthemagneticfieldinsidethe
magnetbeingabout1.1kG).To-datewehavemadea2D
modeloftheseptum:afull3Dmodelwillberequiredtofully
understandimportantendeffects.
5.4.5 FEL Beamlines
5 .4 .5 .1 Overview of the FELs
TheNGLSdesignincorporatesmultipleFELbeamlines
(assummarizedinTable2andinmoredetailinSection
5.4.5.2below),eachofwhichwilldeliverX-raybeamswith
distinctivephotonattributes,astoenergy,pulseduration,
bandwidth,polarization,photonflux,synchronization,and
pump-probecapabilities.Typicallyasinglebeamlinewill
spanafactorof3-5inphotonenergy,dependingonthe
undulatorparametersandtechnology(seeSection5.4.5.4).
Thethreeproposedbeamlinesare:
• Beamline1:Aseededbeamlineproducingcloseto
transform-limitedpulses
• Beamline2:Aseeded two-colorX-ray,ultrashort-
pulsebeamline
• Beamline3:ASASEbeamline
Thetwoseededbeamlinesarecapableofoperatingat
upto100kHzrepetitionrate,whereastheSASEbeamline
mayoperateat the fullmachine repetition rate.Each
beamlinewillcoverawavelengthrangefromaboutone
bythisquadrupoleata150mmdistancefromtheaxisofthe
FODObeamline.
Thelatticefunctionsthroughalineinthespreaderare
showninFigure88.Thespreaderlatticehastwodistinct
parts,namelythebeamtake-offsectionandtheFELfan-out
distributionsection.Eachpartisbuiltasatriple-bendachro-
mat.Inthebeamtake-offsectionthekicker,septumandoff-
setquadrupolesarefunctionallyequivalenttoonebending
magnet,whileanadditionalpairofbendingmagnetscom-
pletesthefirstachromatsupplyinga60mradangle.The
designforthesecondachromatprovidesfurtherbendingto
directthebeamlinesintotheexperimentalhallasdesired
withananglethatcanbeeasilyarrangedtobebetween
10–140mrad,accordingtotheoveralllayoutofthefacility.
Thedesignoftheachromatshastheflexibilitytoallowfor
tuningofthetransfermatrixelementR56(controllingthepar-
ticletime-of-flight)inordertoenforceisochronicity.Previous
beamdynamicsstudies58showedtheimportanceofthetwo
triple-bendachromatsbeingindependentlyisochronousso
astominimizetheeffectofmicrobunchingonthemicron
length-scale,whichcandevelopasthebeamtravelsthrough
thespreader.Simulationsshownosignificantdeterioration
inbeamqualityinpassingthroughthespreader.
5 .4 .4 .1 Kickers and Septa
The kickers and septa are key components of the
spreader,andaprototypekickerandpulserarebeingbuilt
atLBNLtodemonstrateperformancegoals.
Thekickersarerequiredtosupply3mradkicktothe
1.8GeVbeamoveramagneticlengthoftwometers,with
desiredriseandfalltimesofabout5ns(allowingforfuture
operationwithmuchhigherbunchrates),andapulserepeti-
tionfrequencyupto100kHz.Pulse-to-pulsefluctuations
andripplebetweenpulsesshouldbelessthan±0.01%of
thepulseamplitude.Thedesignassumesavacuum-core,
matched-impedance stripline kicker magnet.A solid-
state,transmission-lineaddertopologywasselectedas
thebaselinechoice,providingbipolar205Apulsesat10.2kV
with a 50 Ω termination. Each adder cell will require
sixMOSFETsinparallel,andtherewillbefifteencellsin
totalforeachpolarity.Anadvantageofthistopologyis
thatitiseasilyupgradablebyaddingmorecells.
InFigure93weshowacrosssectionoftheconceptual
designfortheseptummagnetstogetherwithrepresentative
magneticfluxlines.ThemagnetisaC-typewithasmallcon-
ductorminimizing theseparationbetween theadjacent
orbitsofthebeamsbeingkicked(justinsidethemagnet)and
Single turn coil
15 mm Beampipe
Magnetic shields
R = 3.5 mmHalf-gap 4 mm
Beam spacing
30 mm
Figure93 Cross section of the septum magnet located downstream from the fast kickers. The inset figure shows the 15 mm separation of unperturbed and deflected beams; the larger figure shows the magnetic field lines and shielding, clearly excluding the field from the unperturbed beam pipe. A length scale of 30 mm is indicated.
135
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
latorsarealsorequiredforresonantinteractionswithseed
lasersoperatingbetween200and800nm.
Beamlines1and3bothemploymultiple3mradiating
undulatorsectionsseparatedby1.5mbreaksforfocusing,
diagnostics, and required phase-shifting elements.
Thebreaksaresomewhatlongerthanthe1.3musedin
FERMI@elettra,27andwillincorporatesimilarcomponents.
Beamline2—the2-colorattosecondbeamline—requires
onlytwoindependentradiatingundulatorsections,each
about1minlength,oneforeachoftherequiredX-raypulses.
Theplanarundulatorswillnaturallyproduceradiation
withahighdegreeoflinearpolarization,estimatedtobe
about99%.Variablepolarizationcanbegeneratedusing
orthogonalplanarundulatorsseparatedbyaphase-shifter,
asdescribedinSection5.4.5.3.Theresultingoverlapofthe
radiationpulsesinthefarfieldyieldsadjustablepolarization.
Preliminaryinvestigations59suggestthatevenwithSASE
X-rayFELs,circularpolarizationof80%ormorecanbe
achieved with only moderate sacrifice of peak power.
Polarizationcontrolofattosecondpulsesmayprovemore
challengingandwarrantsfurtherstudy.
Withplanarundulators,eachbeamlinewillsimultane-
ouslyproduceharmonicsofthefundamentalFELradiation.
However,asthecurrentchoiceofelectronenergy(1.8GeV)is
neartheminimumforproductionof1nmradiation(inorder
tominimizecost),aroundthisresonancewavelengththepla-
narwigglerswillhaverelativelysmallK-values,resultingin
lowlevelsofharmoniccontent(approximately0.1%ofthe
fluxesachievableatthefundamental).Modestincreasesin
beamenergywouldgenerateuptoanorder-of-magnitude
moreharmonicenergyinBeamlines1and3. Increased
beamenergytoapproximately2GeVwouldimproveperfor-
manceandmaybeachievablewithoutsignificantimpacton
cost;furtherstudieswilldetermineanoptimalsetofparam-
etersfortheNGLSperformance,balancingriskintheinjec-
tor,linac,andFEL.
Synchronizationtoexternalpumporprobelasersinthe
UV,visible,IR,orTHzbandscanbeachievedbythetiming
andsynchronizationsystemsdescribedinSection5.4.7.This
approachwillbeparticularlyeffectivewhenthedurationof
theseedlaserpulseisshorterthan150fssoastofitwithin
the250fslongcoreoftheelectronbunchwhileallowingfor
±50fsjitterinthetimingofthebunch.If,instead,theseed
laseroverlapstheentireelectronbunch,thetimingofthe
radiationpulsewillbedrivenbytheaveragearrivaltime
oftheelectrons.Inthiscase,theseedlasershouldbeatleast
350fsindurationtoallowfortimingjitter.
toafewnanometers,buttheeventualcompleterangeof
photonenergiesaccessiblebytheoverallfacilitycouldbe
considerablylargerthanthis.
Asummaryofthecriticalelectronbeamparameters
usedinthebaselineFELsimulationsisgiveninTable7.
A“slice”isheredefinedasthedurationofthecoherent
FEL interactionorroughly tencooperation lengths—
about5fsfortheparameterregimeassumed.Theparam-
eterscloselyapproximate thoseobtained in the linac
simulations:seeSection5.4.3.4.
Table7 Electron beam parameters used in FEL simulations.
Parameter Value
Energy(GeV) 1.8
Peakcurrent(A) 500
Slicetransverseemittance(µm) 0.6
Sliceenergyspread(keV) 50
Lengthofcoreofbunchconditionedforlasing(fs) 250
Rangeofenergieswithinthecore(keV) ±250
Inthestraightforwardmodeofoperationpresented
here,theCWlinacwilldeliverelectronbuncheswiththe
samenominalparameters toall FELbeamlines.Most
importantly, theelectronbeamenergy,peakcurrent,
energyspread,andtransverseemittancewillallbeidenti-
caluptosmalllevelsofjitter.Becausethedifferentbeam-
linesaredesignedfordifferingFELschemes(SASE,EEHG,
etc.),overallperformancemightbeoptimizedbypresent-
ingdifferentelectronbunchestoeachbeamline.Thepoten-
tial capabilities todeliverdifferentclassesofbunches
withoutsacrificing totalbeamtimewillbe thoroughly
exploredduringdetaileddesignofthelinacandinjector.
Theradiationwillbeproducedbyin-vacuumhybrid
permanent-magnetundulatorswithamagneticgapof
4mmtoprovidetherequiredbeamclearance.An18.5-mm
periodundulatorwithK-value0.8–2.6willproduceatun-
ingrangefrom3.28nm(377eV)to1nm(1240eV)atthe
baselineoperatingenergyof1.8GeV.Aslightlydifferent
undulatorwithaperiodof20mmandwithK-value0.8–3.0
providesatuningrangefrom4.5nm(276eV)to1.2nm
(1033eV).TheseparametersaresummarizedinTable8.
Thelong-wavelengthlimitisdeterminedbythemaximum
fieldachievablewithintheconstraintsoftheclearance
requiredfortheelectronbeam,whiletheshort-wave-
lengthlimitisdominatedbytherequirementtoyieldsuf-
ficientlyhighphotonfluxwithinareasonablesaturation
length.Finally,long-period(200–400mm)modulatingundu-
136
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
enablefastmachineprotectionsystems.Sufficientspacein
allbeamlinesisallocatedforsuchdiagnostics.
5 .4 .5 .2 Beamline Examples
Thissectionprovidesperformanceestimatesofthethree
beamlinesenvisionedfortheNGLSFacility.Thefirsttwoare
seededFELs,thethirdaSASEFEL.SimulationsofFELper-
formance have been performed using GENESIS65 and
GINGER.66
Beamline1usestheEEHGseedingscheme66withtwo
laser-drivenmodulatorstogeneratehighlyupshiftedpho-
tons.The pulses can have a duration ranging from
5to150fs.Ifsynchronizationtotheelectronbeamrather
thantheseedlaserpulsesisacceptable,theradiationpulse
lengthwillmatchthepulselengthoftheelectronbeamcore
(250fs).Eachpulseishighlycoherent,withmodestpower
fluctuations (12%RMS)andphase fluctuations (0.2 rad
RMS),varyingonacorrelationtime-scaleof~5fs.Figure94
illustratesa50fssampleofthepredictedoutputpulse.
Beamline2,usingavariantoftheEEHGseedingscheme,
producestwosub-femtosecondpulses,eachwith~250as
duration.Thewavelengthofeachpulsecanbeindependent-
lycontrolled,andthetimedelaybetweenthetwopulsescan
becontrolledwithaprecisioncomparabletothedurationof
eachpulse.Atypicalsingle1keVphotonoutputpulseis
showninFigure95.Notethatthefigureillustratesa2fstime-
slice; the pulse itself is sub-femtosecond in duration.
Increasingthelengthoftheundulatorbeyond~1mdoesnot
increasethepeakpower;whiledifficulttoobserveinthefig-
ure,thepulsedurationbeginstoincreasewithincreasing
undulatorlengthafterthispoint.Thebandwidthoftheoutput
pulseisclosetothetransformlimit.
Beamline3isaSASEbeamlineandneedsnoexternalseed
laser.A50fssectionoftheoutputpulseisshowninFigure96.
Thepowerprofileconsistsofmultiplespikesthatareeachon
theoforder5fsinduration,butareincoherentinphasewith
respecttoeachother,andhavenearlyindependentintensity
fluctuations.WhilethepeakpoweroftheSASEFELiscompa-
rabletothatoftheseededFELofBeamline1,theSASEpulse
lacksthehighlongitudinalcoherenceorenhancedsynchroni-
zationcapabilitiesofaseededFEL.However,thisbeamline
hasamorerobustdesignandcanoperateatthefull1MHz
electronbunchrepetitionrate,andpotentiallygreater.
Each of the three beamlines incorporates a distinct
arrangementofundulators,seedlasers,chicanes,magnetic
focusingoptics,anddiagnostics,thedetailsofwhichare
describedonthenextpage.
Thelaserseedingschemesusedintheexamplebeam-
lines are based on echo-enabled harmonic generation
(EEHG)60orvariantsthereof.Thischoicehasbeenmade
basedupontheprojectedcapabilityofEEHGtogenerate
photonenergiesordersofmagnitudeabovethatoftheinput
radiation,allowingconventionalopticallaserstobeusedas
theinputseed.Thisisaccomplishedwithonlymoderate
increases(byafactorof10orless)intheenergyspreadand
peakcurrentoftheelectronbunch,andwithoutrelyingon
“fresh-bunch”techniquesthatlimittheoutputpulsedura-
tiontoafractionofthecoreoftheelectronbunch.EEHGis
alsolesssensitive61tolongitudinalvariationsintheelectron
beamthanotherschemes,whichrelaxessometolerances
ontheelectronbeam.WhileEEHGisanovelconcept,it
showsgreatpromisebasedontheory,simulations,andini-
tialexperimentalstudies.EEHGhasbeentestedwithprom-
isingresultsuptothe4thharmonic,62limitedbydiagnostic
capabilities. Further study and experimentation will be
importantinunderstandingtherequirementsforapplying
EEHGtoharmonicnumberswellabove100;theNGLSR&D
planwilladdressthedemonstrationofseedingtechniques
toshorterwavelengthspriortofinaldesignoftheFELs.
WithintheFELbeamlines,threeadversebeamdynamics
effectsmustbeconsidered:increasedenergyspreaddueto
spontaneoussynchrotronradiationineitherundulators63or
chicanes;64distortionsintheelectronbeamduetowake-
fields;andparticlelosses.Collectiveeffectsyieldslowvaria-
tionsinthepositionofthecentroidofeachslice,which,while
predictedtobesmallerthantheRMSbeamwidthsinposi-
tionandangle,willresultinamodestdegradationoftrans-
verse coherenceover theentiredurationof theoutput
radiationpulse.Eachbeamlinehasbeencarefullydesigned
totakeintoaccountphysicslimitationsandconstraints:the
largeharmonicnumberlimitstheinitialbunchingfactor,
requiringeitheranamplificationsectionor,forBeamline2,
localizedbunchcompression;incoherentsynchrotronradia-
tion (ISR) in the modulating undulators and chicanes
degradesthebeam,requiringlongelementswithlowmag-
neticfields;shot-noise-seededradiationinthemodulators
willpollutethesignal;thelaserspotsizemustbemuchlarg-
erthantheelectronbeamspotsizetominimizeradialvaria-
tionthatotherwisewouldquicklyshiftthemicro-bunching
awayfromthetargetharmonic.Electronbeamandradiation
diagnosticsarecriticalforoptimizingandmaintainingper-
formanceinthepresenceoftheseeffects.Diagnosticsalso
facilitateclosesynchronizationofexperimentalsystemsand
137
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
400
300
200
100
0
30
40
2020
10z (m) Time (fs)
Pow
er (M
W)
0
0
60
40
20
0
1.5
2.0
2.0
1.0
0.5
z (m)Time (fs)
Pow
er (M
W)
0.0
0.0
1.5
1.0
0.5
400
300
200
100
0
60
40
2040
20
z (m)Time (fs)
Pow
er (M
W)
0
0
Figure94 Beamline 1: Growth of X-ray power at 1.2 nm wavelength in a 50 fs section of the seeded pulse, as a function of distance z along the undulator. The final power fluctuations have RMS fluctu-ations of 12%, while the phase fluctua-tions have an RMS deviation of 0.2 radian, which are small compared to SASE fluc-tuations (see Fig. 96), and close to the transform limit.
Figure95 Beamline 2: Growth of X-ray power at 1.2 nm wavelength in a single sub-femtosecond seeded pulse, as a function of distance z along the undulator. The time window is a 2 fs section of the electron beam. Lengthening the undulator past ~1 m does not increase the peak power, but the duration of the pulse increases from 130 as to above 200 as. The pulse is almost transform limited.
Figure96 Beamline 3: Growth of X-ray power at 1.2 nm wavelength in a 50 fs section of the SASE pulse as a function of distance z along the undulator. The power profile is broken up into distinct spikes, with uncorrelated phases among spikes. The pulse has large fluctuations and is significantly further from the trans-form limit than the seeded pulses illus-trated in Figs. 94 and 95.
Beamline 1: Seeded FEL
Beamline1 isaseededFELprovidingpulseswith
highdegreesoflongitudinalandtransversecoherence.
AschematicdiagramoftheFELlayoutisshowninFigure97;
thetuningrangeisfrom1.2nmto4.5nm(1.0to0.28keV).
TheEEHGtechniqueemployed60incorporatestwomod-
ulators,bothusing~200nmlaserseeds,separatedbya
verystrongchicane(withanR56~-15mm).Thischicane
generatesa“striped”phasespacewithwell-separated
energybandsatanygivenlongitudinalpositionwithin
thebunch(seeFigure98).Afinalchicane(R56~-200μm)
transformstheseenergybandsintolongitudinalmicro-
138
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
Matching
10 1 1
X-ray production(8 undulators)
Undulator Chicane Laser
10 10 4.53
34.5
U U U
U
UU U
Energy bands Micobunching
Figure97 Schematic of layout for Beamline 1 utilizing EEHG seeding, showing main components, scale is in meters.
1799.8
1800.0
1800.2
Electron densityMax
0Ener
gy (M
eV)
Phase
1799.2
1799.4
1799.6
1799.8
1800.0
1800.2
1800.4
1800.6
1800.8
Phase
Phase
Electron densityMax
0
Ener
gy (M
eV)
50010001500200025003000
Curr
ent (
Amps
)
Figure98 Phase space density after the first modulating undu-lator and chicane. There is minimal density modulation at this stage. The horizontal axis is phase of the seed laser, and the full scale correlates to a distance of one seed laser wavelength of 200 nm along a section of the bunch.
Figure99 Phase space density plot of one modulation period of the electron beam after the second modulating undulator and chicane yielding maximal compression of the microbunch. The resulting modulation of the peak current at very short scales is shown in the lower half of the figure. The horizontal axis is phase of the seed laser, and the full scale correlates to a distance of one seed laser wavelength of 200 nm along a section of the bunch.
bunching (via phase space rotation, see Figure 99),
yieldinghighharmoniccontentatmultiplewavelengths.
Figure100showstheresultingbunchingspectrum.The
resulting beam is then passed through a final set of
undulatorstunestoaveryhighharmonicoftheoriginal
seedlaserpulses.TheFELbeamlinecanbeoptimizedfor
anyspecificharmonicbyadjustingthestrengthofthe
chicanes.Becauseharmonicsof the laserwavelength
willbecloselyspaced,combiningchangesinthechi-
caneswithasmalltuningrangefortheseedlaserswill
permitcontinuoustunabilityoverthewholerangeacces-
siblebytheundulators.
AsshowninFigure101,Beamline1producespulses
nominallycontainingfrom3×1011photonsat1.2nmto
5×1012photonsat4.5nm.Thesaturationlengthisconsid-
erablyreducedatlongerwavelengths.At1keV,theFEL
outputpowersaturatesat~300MW.
Thechoiceof200nmseedlasers(correspondingto
the4thharmonicofthedrivingIRlaser)wasadoptedto
reducetheharmonicjumprequiredtoreach1.2nm,while
stillutilizingconventionallasertechnology.About8MW
(peakpower)at200nmisrequiredinthefirstmodulator
and30MWpeakinthesecond.Itmaybedesirableto
havethesecond,higher-powerseedlaserextendoverthe
entireelectronbunch, thusuniformly increasing the
energyspreadthroughoutthebeam;thissuggestsaseed
laserpulselengthof700fs,yieldingarequiredenergy
perpulseof21µJat200nm,correspondingto~50W
averagepowerinthedrivingIRlaser.Alternativeschemes
involving longer seed wavelengths would typically
requirehigherpeakpower.Thehigh-powerseedlasers
arefurtherdiscussedinSection5.4.5.5.
ThefirstchicaneisrequiredtogenerateanR56ofupto
-30mmandconsistsoffourmagnets,eachabout1mlong,
139
withhalf-meterlonggapsinbetween.Thesecondchicaneis
significantlyweaker,withR56ofatmost–1mm,andcanuse
magnetswithalengthof0.5m.Themagneticlatticebefore
theradiatorsectionispredominantlycomposedofdoublet
quadrupolesinordertoallowstrongsimultaneousfocusing
inhorizontalandverticalplaneswithintheundulators.
Beamline 2: Two-color X-ray Pump / X-ray Probe
Beamline2isatwo-color,short-pulseFELemployinga
variationofecho-enabledharmonicgenerationtodelivertwo
X-raypulseswithdurationsof250fsFWHMorless(simula-
tionsshowpulsesasshortas130asmaybeobtained)while
stillcontainingoforder108photonsineachpulse.67Each
pulsemaybetunedindependentlyforphotonenergyand
timing.Thetuningrangeisfrom1.2nmto4.5nm.Beamline2
utilizesthesameenergymodulationschemeasBeamline1
butdeploystwomicrobunchingmodulatorsoperatingon
separateportionsoftheelectronbeam,eachofwhichthen
canbeforcedtoradiateindependently.Thefinalmicrobunch-
ingisproducedbyfew-cyclecarrier-envelopephasestabilized
800nmlaserpulsescombinedwithsingle-periodundulators.
AschematicdiagramisshowninFigure102.
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
00 50 100
Harmonic of 200 nm150 200
Harm
onic
bun
chin
g (%
)
2
4
6
8
10
0 0
5
10
15
20
25
30
35
1 1.5 4 4.53.52Wavelength (nm)
Photons per pulseDistance to saturation
2.5 3
Phot
ons
per p
ulse
(1012
)
Dist
ance
to s
atur
atio
n (m
)
1
2
3
6
4
5
Figure100 Electron beam bunching spectrum after EEHG manipulations.
Figure101 Beamline 1: Predicted output and saturation length vs. wavelength for the EEHG-seeded FEL.
Attosecondpulse 1
Attosecondpulse 2
Matching
10 1 112
U
10 10 10
U
Undulator Chicane LaserU
U U U
Energy bands Micobunching 1 Micobunching 2
Figure102 Schematic of layout for Beamline 2 utilizing a two-color seeding scheme, showing main components, scale is in meters.
Figure103showsphotonsperpulseforBeamline2,
fortwocases:afixedlengthof1mradiatorundulatorfor
eachofthetwoX-raypulses,andforafixedoutputpulse
of250asdurationandinthiscasethelengthofradiator
undulatorrequiredisshown.
Phaserotationtoobtainhighharmonics(seeFigure99)
requiresafew-cycle800nmseedpulsedurationof3.5fs
FWHM,68andthetotalpulseenergyisroughly70µJ.The
resultingbeamentersasingleradiatingundulatorwith
significantlocalizedinitialbunchingofupto10%,enhanced
peakcurrentofupto3kA,andlargeenergyspread,which
canbetoleratedduetotheshortnumberofundulator
140
periodsusedtoextracttheattosecondpulse.Onebenefit
of thisschemeis that thebackgroundSASEradiation
fromthefinalradiatorwillbeverylow,leadingtohigh
contrast.Herethedesignoftheopticalmatchingismore
critical,becausecompressing theelectrons toavery
smallspotsize(withminimumbetafunctionscloseto
1m)withintheradiatorimprovesthenumberofphotons
produced,whereasextendingthelengthofundulatorso
astoamplifythepowerthroughFELgainisnotcompati-
blewiththegoalofattosecondpulsedurations.
FAt1.2nm,theresultingoutputpulsereachesapeak
power of 60 MW after only 0.8 m of undulator. For
theentirerangeofwavelengths,thepeakpowerand
pulsewidthremain fairlysimilar,producingoforder
108photonsperpulse.
Itisexpectedthattheamplifiedeffectofshotnoisewill
leadtosomejitterinoutputpulseparameterssuchasthe
totalnumberofphotons,thecentralphotonenergy,and
theprecisetimingofthepulse.Intheattosecondregime,
thebandwidthoftheresultingpulseisoforder1%,over-
lappingseveralharmonics.Thus,thereisnoneedforany
tuningofseedlaserwavelengths,andacombinationof
jumpingtodifferentharmonicsandtuningtheundulator
parametershouldallowforcontinuoustunability.
Beamline 3: High-Repetition-Rate SASE FEL
Beamline3isaSASEFELthatcanacceptthefullelec-
tronbeampower,ultimatelylimitedonlybythecapacityof
thebeamdump.Noexternallaserisrequired,andtuning
isaccomplishedsolelybychangingtheundulatorgap.The
totalbeamlineconsistsofsixteen3mlong,18.5mmperi-
odundulators.The1.5mbreakseachcontainsaquadru-
pole,aradiationdiagnosticinsert,andaphase-shifterto
maintainproperphaserelationshipbetweentheradiation
andthemicrobunches.Themagneticopticsconsistofa
simpleFODOlattice,yieldingabetafunctionthatranges
from9mto17mandaphaseadvanceperhalf-cellof42°.
AschematicofBeamline3isshowninFigure104.
Dependingondesiredwavelength,theSASEbeamline
willproduce1011–1012photonsperpulse(seeFigure105).
Notethat thenumberofundulatorsectionsneededto
reachsaturationdecreasesstronglyasthetargetwave-
lengthisincreased.Thepeakpowervariesfrom~300MW
to above 2 GW; the average power within a pulse is
reducedbyapproximatelya factorof twodueto the
stochastic nature of the characteristic SASE power
spikeswithintheoutputradiation.
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
0 0
0.2
1 1.5 4Wavelength (nm)
Photons per pulseDistance
2.5
Phot
ons
per p
ulse
(109 )
Dist
ance
to 2
50 a
s FW
HM (m
)
0.1
0.2
0.7
0.3
0.4
0.6
0.8
1
1.2
1.4
1.6
0.4
0.5
0.6
2 3.5 4.53
0 0
100Photons per pulse
Pulse FWHM
Phot
ons
per p
ulse
(109 ) a
t 1 m
Puls
e FW
HM (
as)
0.2
0.4
1.8
0.6 200
300
400
500
600
0.8
1.2
1
1.6
1.4
Figure103 Photons per pulse as a function of wavelength for two cases for Beamline 2: In the upper figure, the performance is shown for a fixed radiator of 1 m length, and the pulse duration varies as a function of wavelength. In the lower figure, the pulse length is fixed at F250 as and the length of the radiator undulator is shown. This beamline produces 2 pulses of independent wavelength.
141
5 .4 .5 .3 Undulator Design Options
Themagneticundulatorsarecriticalelementsofthe
NGLSFELs,usedinmodulatingtheelectronenergyand
providingcollimatedphotonpulsesofhighbrightness
withtailoredwavelengthandpolarizationcharacteristics.
Weplantodevelopandimplementadvanced,high-per-
formance,adjustable-gapplanarundulators,leveraging
undulatorcharacteristicstooptimizeoverallperformance
(e.g.photonflux)andtominimizeoverallfacilitycostand
risk (e.g. linac size and electron beam energy — see
Section5.4.1).
Table2describesthephotonpropertyparametersfor
thethreepreliminaryNGLSFELdesigns.Toachievethe
desired performance, various undulator technology
options were considered. As a conservative bound,
aminimum4mmvacuumapertureintheundulatorsec-
tionwaschosensoastoavoidradiationdamagetothe
undulatorandtoavoidbeaminstabilityeffectscausedby
wakefieldsinthevacuumchamberwalls.Anundulator
parameterofatleastK=0.8attheshortestwavelength
hasbeentakenasalowerboundforeffectivephotonpro-
duction.Thedesiredphotonspectral reachandrange
(approximately1nmto4nm),togetherwiththeexisting
undulator technologycapabilities, thendeterminethe
optimal undulator period λu, the required maximum
undulatorparameterK,andthebaselinemachineenergy.
Achievingthesegoals,whilesimultaneouslyreducing
technicalrisk,haveleadustoselectin-vacuumhybrid
technologyforthebaselineNGLSdesign,andleadtoa
linacenergyof1.8GeV.Parametersforthethreebaseline
undulatorbeamlinesaresummarizedinTable8.
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
MatchingX-ray production(16 undulators)
Undulator
10 4.5
3
70.5
U U U
U
U
0 0
10
20
30
40
50
60
70
1 1.5 3.52Wavelength (nm)
Photons per pulseDistance to saturation
2.5 3
Phot
ons
per p
ulse
(1012
)
Dist
ance
to s
atur
atio
n (m
)
0.5
1
2.5
1.5
2
Figure106 Prototype XFEL out-of-vacuum hybrid planar undulator (left) and SPring-8 in-vacuum hybrid undulator (right). Both technologies are mature for FEL application. NGLS will use in-vacuum devices.
Figure105 Beamline 3: Scaling of performance vs. wavelength for the SASE FEL, in terms of number of photons and saturation length.
Figure104 Schematic layout for Beamline 3, a SASE FEL, showing main components, scale is in meters.
142
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
ducting-baseddevicesandaredevelopingcomplemen-
taryand/oralternativesuperconductingtechnologies.
Bythemselves, thesehigh-fielddeviceswouldenable
expanded tuning range and modestly shorter wave-
lengths.Coupledwithacceleratorupgradestoprovide
moderatelyhigherelectronbeamenergy,suchdevices
offer the promise of much higher photon energies:
forexample,increasingbeamenergytoapproximately
2.5GeV,andusingundulatorsof~12mmperiodallows
forover3keVphotonsinthefundamental.
5 .4 .5 .4 Synchronized THz/IR Sources
Short,intensepulsesofTerahertzradiationranging
from0.5to50×1012Hzinfrequency(orequivalently,
wavelengthsfrom600μmto6μm)maybeproduced
directlyfromtheelectronbeambyathin(~1µm)metal
foilorapertureplaceddownstreamoftheFELundula-
tors.Suchsourcesareinherentlysynchronizedtothe
electronbunches,andthustotheX-raypulses.However,
therequirementthatthefoilliedownstreamoftheFEL,
toavoiddisruptionoftheFELprocessitself,limitsthe
naturaluseofsuchschemestosituationswhereatrail-
ingTHzprobepulsecouldbeused,unless theX-ray
pulseitselfcouldbedelayed.Theoppositeformat,that
ofaTHz-pumpandX-rayprobe,wouldbeexpectedto
findmanymoreapplications.Fortheseexperiments,the
NGLS timingandsynchronizationsystemwouldalso
allowtightsynchronizationoftable-topTHz,IR,oroptical
sourcestotheX-raypulses.
5 .4 .5 .5 Seed Laser Systems
TheNGLSseededFELsrequirehigh-power,tunable,
opticallasersystemswithlowpulse-to-pulsepeakpower
fluctuations(~1%),andaquasi-flat-topprofilewithlow
powerripple(~1%).Thelargeharmonicjumpsrequirethat
Table8 Undulator parameters for the three beamlines.
Beamline 1 Beamline 2 Beamline 3
Wavelengthrange(nm) 1.2–4.5(1–0.28keV)
1.2–4.5(1–0.28keV)
1–3.3(1.2-0.38keV)
MaximumundulatorparameterK(minimum0.8)
3.04 3.04 2.61
Undulatorperiod(mm) 20 20 18.5
Undulatortechnology Hybridin-vacuum
Hybridin-vacuum
Hybridin-vacuum
Permanent-magnet,planar,hybridundulatortechnol-
ogyiswelltested,utilizingNdFeBpermanentmagnet
material plus soft iron/cobalt/vanadium permendur
polepiecesinaplanararraytoproducelightwithahigh
degreeoflinearpolarization.Hybriddeviceshavebeen
used in synchrotron facilities worldwide since their
inventionatLBNLin1983.Morerecently,hybridundula-
torsarefindingapplicationastheradiatorsectionsat
FELfacilitiesatXFELinGermany(out-of-vacuum)andat
theSPring-8FEL inJapan (in-vacuum),asshownby
Figure106.Thiswell-established,yetpowerfulundulator
technology introducesnosignificant risk tobaseline
NGLSoperation.
Although the undulators will be planar, circular
polarizationcapabilitymaybeenabledbyorientingthe
finalundulatorsectionsorthogonallytothelongerbunch-
ing and radiator undulator sections upstream;69 see
Figure107.Aconventionalelectromagnetphase-shifter
positionedbetweenthecrossed-planarundulatorsec-
tionscanenablefastpolarizationswitchingandchangeof
polarizationstate.
TheperformanceoftheFELbeamlinescouldbesig-
nificantlyimprovedthroughmoreadvancedundulator
technologieswithhigherfield-strengthcapabilitiesand
shorterperiods,andwithpotentiallylowerfabrication
costs.Wehaveworkingprototypesofvarioussupercon-
Horizontal polarization Phase shifter
Vertical polarizationRadiation with
adjustable
polarization
Figure107 Planar hybrid undulator horizontal and vertical sections separated by a tunable electromagnetic phase shifter enable fast polar-ization switching and arbitrary polarization modes.
143
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
for the IR laseroutput reachabout500Wofaverage
power.IRlasersystemsuptopowerlevelsof30–50Wfor
10kHzoperationsalreadyexist.Thescalabilityofexisting
systemstohigherpowerlevels,andhigherrepetition
ratesiswelldefined.Thecurrentlimitationistheavail-
abilityofhigherrepetitionratepumplaserswithsuitable
powerlevels,whichwillbedevelopedinthenextdecade.
Onceagain,efficientpulsecompression,aswellasCEP
stabilizationoftheseedlasersystemalreadyexist,but
wouldrequirefurtherexpansiontothescaledupamplifier
systemandHHGstage(typicallyachievedinagascell).
HGHGisanalternateoption,requiringseedlasers
producingapproximately10µJ,100fspulsesintheUV,
oraround100MWpeakpowerataround200nm.Soft
X-rayoutputistobeachievedbysuccessiveharmonic
generationinanumberofcascadedFELstages.Atarep-
etitionrateof100kHz,werequireaboutonewattof
averagepower in theUV,andup to100Win the IR.
CommercialIRseedlaserssystemscancurrentlyreach
upto30–50W,andspecialdesignswillprovidepower
levelsinthe100Wrange.
Thecurrentseed lasersystemsavailable for these
threeoptionsarescalabletothepowerlevelsrequired,
andmostofthecriticalopticalcomponentsarecurrently
available(e.g.largeapertureTi:Sapphirecrystals,large
thermallystablecompressiongratings,largeoptics,har-
moniccrystals,etc.).Oneof themainareas for laser
developmentwillbethatofthepowerfulpumplasers,in
ordertolimitthefootprintaswellascostoftheoverall
pumppowerrequirements.ScalabilityoftheCEPstabili-
zationtechniquesaswellasefficientsub-15fscompres-
sionwouldhave tobe furtherdeveloped tomeet the
requiredseedpulsedurations.Anotherarea thatwill
requiredevelopmentwillbetheUVpulsetemporalinten-
sityprofileuniformity.Furtherdevelopmentofexisting
techniques(e.g.phasemodulation,pulsestacking,etc.)
willberequiredtoreachstabilityrequirements.
Opticssystemswill transport theseedlaserpulses
fromaremotetemperature-stabilizedlaserroom,tothe
FELinstalledintheacceleratorradiationenclosure.There
maybelocalopticalequipmentinstalledintheFELvault,
andthismustberobustandreliable(e.g.HHGgascells
forconversionoftheIRpulsetoharmonics).Thelaser
beammustoverlaptheelectronbeamforasignificant
distanceinmanycases,andactivebeampointingstabili-
zationsystemswillbeprovidedtomeasuretheinputand
outputbeampositionsandcontrolthebeampaththrough
thephasenoisewithinthepulsebebelowapproximately
0.01radiansRMS.Additionally,thefew-cycleopticalseed
lasersusedforgeneratingattosecondX-raypulsesshould
bestabilizedwithrespecttocarrierenvelopephase,pulse
duration,andamplitude.
WehavebaselinedourpreliminarydesignsforEEHG
using theoutputofa fiberorTi:Sapphire-based laser
chain,ateither its fundamentalorup-convertedto its
3rdto5thharmonictomodulatetheelectronbeam.Seed
laserpowerofuptoapproximately30MWat200nm
isrequiredforseedinglongX-raypulses,orper-pulse
energyrequirementsof1µJ(for10fspulses)to20µJ
(for 700 fs pulses). IR systems capable of delivering
~50–100Waveragepowerat100kHzarethusrequired,to
provide temporally and spatially filtered pulses and
accommodatefortransmissionlosses.Forsub-femtosec-
ondX-raypulsegeneration,aper-pulseenergyof~70µJ
isrequiredofthefewcyclesof800nmradiation(~3.5fs),
resultinginpeakpowerupto~20GW,oraveragepower
oforder10W,at100kHz.Chirped-pulseamplification
(CPA)ordivided-pulseamplification(DPA)basedlaser
systemsofthistypealreadyexistat10–100kHz,atpeak
powerlevels~100MW,mostlylimitedbytheavailability
ofpowerfulpumplasersatthisrepetitionrate(~500W
powerlevelsofgreenpumplaserswouldberequired).In
additiontoscalingupthepumpandIRamplifierpower
outputlevels,fortherequiredIRpulsewithinafewcycles,
carrier-envelopephase(CEP)stabilizationwillberequired
(andisalreadyimplementedinsomeCPAamplifiers).
Additionally,efficientpulsecompression(downtoafew
cycles),reliablesystemswithtunability,suitablediagnos-
tics,androbustremoteoperationwillneedtobedevel-
opedaspartoftheNGLSR&Dplan.
AnotherpossibleapproachtoseedingtheNGLSFELs
wouldbetheuseofHHGlasersources(seeSections5.1.2
and5.2.4),whichwouldsignificantlyreducetheharmonic
jumprequiredbetweentheseedlaserandFELoutput
wavelength,andreduceX-rayoutputsensitivitytoseed
laserinstability.Currently,HHGsourcesoperateinthe
EUVrange,with10kHzrepetitionratepulsesofnJenergy,
atuptoabout100eV.AnFELcanfurtheramplifytheEUV
seedinanopticalklystronconfiguration,startingfrom
pulsesofaround30–50nminwavelength,5nJinenergy,
andafewtensoffemtosecondsinduration,correspond-
ingtoaround100kWpeakpower,or500µWaverage
powerat100kHzrepetitionrate.Withcurrentlyachiev-
abletypical10-6up-conversionefficiency,requirements
144
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
5.4.6 Beam Dumps
TheNGLSwillutilizebothhigh-power(MWscale)and
mediumpower(100kWscale)beamdumps.Thehigh-
powerdumpswillbelocatedattheendofthespreader
sectionandonthehigh-repetition-rateFELbranchline.
Theconceptualdesignof thehighpowerdump isa
water-cooled“window”followedbyaseriesofmetal
platessuspendedinarapidlycirculatingwaterbath.The
windowisolatestheprimaryabsorberfromthevacuum
beampipeandinitiatesanelectromagneticcascadethat
broadenstheradialextentofthebeamasittransverses
thedump.Thethicknessandspacingoftheabsorption
plateswillbeselectedtomaximizetheenergyabsorp-
tionwithin themetalwhilemaintainingsafe thermal
stresslevels,followingtheexampleoftheCEBAF5GeV,
1MWdump.70
Medium-power beam dumps will be located in the
remaining100kHzrepetition-ratebranchlines.Themoder-
atebeampowerallowstheuseofsolidabsorbermaterials,
whichsignificantlysimplifiestheconstructionandopera-
tion.Thekeydesignissuesarelocalizedheating-induced
thermalstressesandoverallheatdissipation.
Themedium-powerdumpwillutilizethin,dual,edge-
cooledwindowstoconfineanargon-gas-filledvolume.As
inthecaseofthehighpowerbeamdump,thewindowwill
initiateanelectromagneticcascadethatwillbroadenthe
beamthusdiffusingtheenergy.Followingthewindow
willbeadriftspaceupstreamoftheprimaryabsorberthat
providesroomforthebeamtospreadbeforestrikingthe
useofpiezoandmotor-controlledmirrorsandCCDdetec-
tors.Existingpointingcontrolsystemscanachievebetter
than10μradprecision,whichcorrespondtotherequire-
mentofNGLS.Suchsystemshavesuccessfullybeen
implementedalready(e.g.LBNL,SLAC,andNIFatLLNL),
andwillneedtobeadaptedtothespecificbeamtrans-
portline.Superpositionwiththeelectronbeamwillbe
accomplishedusingpop-inscreensalongtheundulator,
andultimatelyfeedbackcontrolincorporatingtheelec-
tronbeamBPMs.LaserbeamfocusingintotheFELwill
bedonewithfirstsurfacereflectiveopticsforbothIRand
shorterwavelengthbeams,allowingforadjustmentin
thewaistpositionwithintheundulator.Currentcoating
technology will support the fluence levels expected.
ForHHGseedingatEUVwavelengths,aKirkpartick-Baez
mirrorpairmaybeused.
Diagnosticssystemswillberequiredtofullycharacter-
izetheseedlaserpulses.Spatialandtemporalintensity
profileswillneedtobemeasured,aswilltheenergypro-
filealongthepulse,usingwell-knowntechniquesalready
inuse.Toolstocontroltheenergychirpwillbeneeded.
Ultimately,systemswithcapabilityforfeedbackcontrol
ofpulseparameters,basedonX-rayoutputoftheFEL,
aredesired,andwillbedevelopedinfutureR&Dprojects.
Lasersystemsaredevelopingatarapidpace,driven
by other demands, and NGLS will take advantage
ofthesetechnologiesastheyevolve.Theexistinglaser
systems represent approximately 10–20% of the IR
powerlevelsrequiredforNGLS,andthescalabilitypath
iswellunderstood.
Photocathode laser
Laserheater
Timingdiagnostics
Endstationlasers
MasterClock
Seed lasers
Stab
ilize
d lin
k
Stabilized lin
k
Stabilized link
Stabilized link
Stabilized link
RF controls
Stabilized link
Figure108 Schematic view of the distribution of the master clock to a variety of remote clients over stabilized fiber links. Seed and user lasers and timing diagnostics are all synchronized at the femtosecond level.
145
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
lengthvariationequivalenttoseveralhundredpicosec-
ondstoananosecond.Oneofthekeydevelopmentsin
femtosecondtimingdistributionoverthepastfewyears
has been the development of stabilized optical fiber
links73,74fortransmissionofthemasterclocksignalover
afacility.
Thisapproachhasbeenrealizedinsystemsinstalledat
theLCLSforsynchronizinguserlaserswiththeelectron
beam57andattheFERMI@elettralinac54forsynchroniz-
ing the relative phase of accelerating sections.
Performanceatthe10fslevelhasbeendemonstrated
overseveralmonthsofoperation.Suchasystemwould
serveasthebackbonefortimingdistributionatNGLS.
5 .4 .7 .2 Laser-laser Synchronization
GiventhatNGLSwillproducepulsesoflessthan10fs
duration, and pump/probe experiments will require
pulsedlaserstobesynchronizedwiththeX-raypulse,
NGLSwillrequirealasertimingsystemwith10fsjitteror
less.Whileitispossibletomeasuretherelativearrival
timeoftheX-rayandopticalpulsestoaprecisionwhich
islessthanthejitterandpost-processthedataaccord-
ingly,largejittercaneffectivelyreducetherepetitionrate
bymakingmuchofthedataunusablebecausethepulses
donotoverlapatall.Thus,thegoalsofNGLSlasertiming
aretoreliablymaintainprecisionwellbelow10fsandto
aimforsub-femtosecondjitteranddrift.
Toachievefemtosecondandbetterstability,weplan
todevelopsystemsbasedonthecombspectrumofa
mode-locked oscillator. Such a spectrum has two
degreesoffreedom.75Iftwooftheopticalfrequenciesin
thecombspectrumarestabilizedwithrespecttoone
another, thenthecombcharacteristicsarestabilized,
includingcontrolofthepulserepetitionrate.Acarrier-
envelope-phasestabilizedlaserhasonecombparame-
terfixed,i.e.thecomboffsetfrequencyiszero.Theother
parametercanbefixedbylockingonecomblinetoaref-
erence optical frequency. In experiments with
Ti:Sapphireanderbiumlasers,twolaserscanbelocked
towithinsub-fsjitterusingthistechnique.76,77Wehave
previouslyshownverystableopticalfrequencytrans-
missionusinganinterferometer,78whichcanbeusedto
transmitasingleoptical frequencyfromonelaserto
another to synchronize them over a long fiber.This
wouldbethebestcandidateforsub-fssynchronization,
sincethefullopticalfrequencyof200THzisused.
primaryabsorber.Theidealabsorberhasalowatomic
numberandgoodthermalconductivity.Twomaterialsthat
fitthecriteriaarealuminumandgraphite,andbothhave
beenusedsuccessfully forbeamdumpswith roughly
comparablerequirements.
5.4.7 Timing and Synchronization Systems
TheNGLSwillrequireanexactinglevelofsynchroniza-
tionbetweenacceleratorsub-systems,userandaccelera-
tor lasersystems,anddiagnostics inordertoachieve
boththedesiredelectronandX-raybeamperformance
andtoenabletime-resolvedopticalpump/X-rayprobe
studieswithfemtosecondorbetterresolution.Withsepa-
rationsofhundredsofmeters,synchronizationofthese
systemsatthefemtosecondlevelwillbechallenging,yet
criticaltoachievingoptimumfacilityperformance.
Themajorchallengestobeaddressed71inreaching
thelevelofsynchronizationrequiredforNGLSare:trans-
missionofastabletimingsignaltomultipleremotecli-
entsoverarelativelylargefacility,synchronizingremote
clientssuchaslasersandRFsystemstothestabletiming
signalatthefemtosecondlevel,andmeasurementofthe
electronbeamarrivaltime.
Aschematicdiagramofthetimingdistributionforthe
acceleratorsystemisshowninFigure108.Thesystem
comprisesaglobaltimingdistributionsystem,andlocal
systemsthatprovidesynchronizationtotheclock.Inthis
approachwesynchronizealloftheacceleratorsystems
fromthephotocathodedrivelaser,injector,linacRFsys-
tems,andseedanduserlasers.Themasterclocksignalis
distributedthroughoutthefacilityoverstabilizedoptical
fiberlinkswithrelativestabilityontheorderoftenfemto-
seconds.Systemsthatrequireextremelylow-jittersyn-
chronizationatthefemtosecondandsub-femtosecond
level,suchasuserandseedlasersandtimingdiagnos-
tics, will use an all-optical synchronization technique
describedinmoredetailbelow.
5 .4 .7 .1 Master Clock Distribution over Stabilized
Fiber Links
Oneofthemainchallengesinreachingthelevelofsyn-
chronizationrequiredfornext-generationlightsourcesis
transmissionofamasterclockreferenceoverarelatively
largefacility.72Forexample,inafacilityofakilometerin
length,diurnal temperaturevariation results in cable
146
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
betweenundulatorsections.Foroperationalstabilization
andfeedbacksystemsusethediagnosticsshouldbenon-
invasive;formachinestudiesandset-up,theymaybe
intercepting.
Transverse position tolerance through most of the
acceleratorisestimatedtobeafewtensofmicrons,andin
theFELundulatorstherequirementwillbetighter,perhaps
afewmicrons.Longitudinalarrivaltimingneedstobe
withina10fstolerance,andintegratedwiththetimingand
synchronizationsystem.Longitudinalprofilemeasure-
mentsofbetterthan10fsresolutionaredesirable,and
suchsystemswillrequirefurtherR&D.Dedicatedstudies
areneededincludingtrajectorysensitivityandcorrection
analysistodeterminediagnosticssystemsspecifications.
Theelectronbeamdiagnosticsystemsinclude:
• Beampositionmonitors(BPMs),includingwarmand
coldbuttonpick-ups,striplinesforuseintheaccelera-
torandcavityBPMsforuseintheFELs
• TransverseprofilemonitorsusingYAGscreens,opti-
caltransitionradiation(OTR),andwirescanners
• Longitudinalbunchprofilemonitorssuchasstreak
camerasandelectro-opticdevices
• Beamenergymeasurementsfollowingtheinjector,
inthedispersiveregionofthebunchcompressors,
andbeforeandaftertheFELs
• Beamarrival timemonitorsusingRFcavitiesand
electro-optictechniques
• Precisiontimingdistribution(seesection5.4.7)
• Currentmonitorsusingtoroids
• Beamlossmonitors
Anequallybroadsuiteofphotonbeamdiagnosticsare
neededbothtocharacterizeandoptimizetheFELinterac-
tionbetweentheelectronandphotonbeamsandtochar-
acterizetheresultingphotonbeamasitenterstheuser
hutches.Diagnosticsforthephotonbeamsarediscussed
furtherinSection6.5.
TooptimizetheFELinteraction,thealignmentofthe
electronandphotonbeamsmustbemaintainedtowithin
afractionofthebeamsizesovermultipleFELgainlengths
andtherelativeopticalphasebetweenundulatorsec-
tionsmustbecontrolledtowithinafractionofawave-
length.Standardinstrumentationincludesbeamposition
monitors,beamsteeringdevices,andX-raydetectors.
X-raydiagnosticswillrelyonbothon-axisandoff-axis
X-raydevices.Off-axisdiagnosticsarecomplementaryto
theon-axisX-raydiagnosticsandwillallownon-invasive
monitoringoftheradiation.
5.4.8 Instrumentation and Diagnostics
Anextremelyhighbrightnesselectronbeamisneeded
toachievethedesiredFELperformance,withexacting
pulse-to-pulsestabilityrequirementsovertimeperiodsof
hourstodays.Theserequirementsdemandanextensive
suiteofhigh-resolutionelectronandphotonbeamdiag-
nostics.Systemsarerequiredforstudyingandconfigur-
ingtheacceleratorandFELsaswellasmaintainingstable
operation,andprovidingthenecessarysensorsinfeed-
backstabilizationsystems.
Tooptimize theelectronbeam, instrumentation is
neededtomeasuretransversebeamposition,emittance,
totalcharge,chargedistribution,pulselength,energy,
arrivaltime,andbeamloss.Thesesystemsareinitially
requiredtooperatewithbunchchargesofapproximately
10–500pC,andatrepetitionrateuptothebunchrateof
1MHz.Thediagnosticsshouldbe,whereverpossible,
upgradeabletohigherrepetitionrateandlargerdynamic
rangetomeasurebuncheswithchargerangingfroma
fewpCto1nC,dependingonthemodeofoperation.
Furthermore,eachofthesediagnosticsystemsmustpro-
videsignalswiththeappropriatebandwidthtobeused
in feedback controlof theaccelerator to stabilize the
electronbeam.
Forefficientlasing,itiscriticaltocontrolthe“slice”
values,(withinafewcooperationlengthswithinabunch),
forproperties suchasemittance, charge, energyand
energyspread.Thesemustbemeasuredbeforeandafter
bunchcompression,toallowforproperoptimization.Use
ofatransversedeflectingstructurehasbeensuccessful
inmeasuringthesliceparametersbyimposingatrans-
verseangularchirpalongthebunchandprojectingthe
resultingtransversedistributiononascreen.Thetwo-
dimensionalparticledensitydistributioninadispersive
sectionenablesmeasurementoftheaverageenergyand
theenergyspreadineachlongitudinalsliceallowingsen-
sitivetuningofthebunchcompressionprocess.Because
this diagnostic is destructive, or at least disruptive,
involvingtheinterceptionandperturbationofanelectron
bunch,itshouldbeusedatamuchlowerrepetitionrate
thanotherdiagnostics.However,thehighrepetitionrate
oftheNGLSmayallowoperationofthistechniquewith
negligibleimpactontheaveragebeamintensity.
Criticaldiagnosticsgroupswillbelocatedneartheexit
oftheinjector,theinputandexitofthebunchcompres-
sor, theendofthelinac,theexitofthespreader,and
147
5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES
NGLSwillemployinterlocksthatwillimmediatelydis-
ablebeamattheinjectorinfaultconditionsanywherein
theaccelerator,FEL,andX-raybeamsystems.Integrated,
activemonitoringofmagnet,beamcurrent,andother
parametershavethepotentialtoprovidefasterfeedback
thanradiationmonitorstoreducebeamlossesduring
bothroutineoperationandinbeamlossevents.Aswith
allsimilarfacilities,radiationmonitors,vacuumlevels,
andotherstandardmeasureswillcontinuallybemoni-
toredandinterlocked.
Environmentalradiationsafetyriskshavealsobeen
evaluated.Preliminarymodelshavebeendevelopedfor
theproductionandfateofboththeshort-livedair-activa-
tionproductsaswellaslonger-livedisotopes.Standard
containmentandmonitoringofthebeamdumpswillbe
utilized.Earlycalculationsestimateoff-sitedosestobe
manyorders-of-magnitudebelowNESHAPS limits. In
addition,shieldingaroundthelinactunnelwillbethick
enoughtoeliminateanymeasurableamountsofradio-
isotopesenteringthegroundwater.
Empiricalbeam-basedalignmentwillbecriticaltothe
operationoftheNGLSandcanprovidetherequiredaccu-
racybyrecordingbeampositionexcursionsfromBPMs
underdeliberatebeamenergyvariations.Resultsfrom
BPMinformationanalysiscanfeedquadrupolerealign-
ments,transversetrajectoryfeedback,andbeamsteering
atundulatorentrances.
5.4.9 Radiation Protection
Theanalysisofradiationhazardsandcontrolshasbeen
integratedintotheprojectattheearlieststages.Experience
atothermegawattclassacceleratorshasbeenusedto
developpreliminaryradiationtransportmodelsofradia-
tionfieldsexpectedatthebeamdump,collimatorsand
otherlocationswherebeamlossisexpected.Fromthis,
appropriateshieldingstrategieswillbedevelopedwith
particularattentiontobepaidtothebeamdumpsandpho-
tonbeamlines.Thesestrategieswillbuildupontheexperi-
encegainedatsimilarfacilities.Standardsearch/secure
proceduresandinterlocktechnologiestoexcludeperson-
nelfromactivebeamareaswillbeemployed.
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6 Experimental systems
6.1 Introduction
IntheinitialcomplementofbeamlinesfortheNGLS,we
areplanningforthreeseparateFELswithacoreoperating
rangefrom0.28to1.2keV.Thegeneralcharacteristicsof
eachofthesethreelinesareshowninTable2.Theintent
hereistooutlinethegeneraldesignissuesandpresent
directionsfortheirsolutionratherthantoprovideadefini-
tivedesignineachcase.Inthecaseofeachofthesebeam-
linesystems,ultra-shortpulseswillbeused,intothesub-fs
regime.Thisthereforepresentsoneoftheprimarychal-
lenges, the preservation of the extremely small pulse
length.Theenergyrangetobeaccessedrequiresgrating
optics,butgratingsextend the temporal lengthof the
beam.Thegrazingincidence(focusing)opticsthathaveto
beusedinthesoftX-rayenergyregionalsotypicallyinduce
wavefronttilt,againresultinginpulselengthening.Below
welookatthepracticalconsequencesoftheseeffects.
Asecondchallengeisthatinmostexperiments,there
isa requirement forsingleor twowavelengthpump-
probemeasurements,i.e.thattheprimarybeammustbe
splitwithonebeampumpingthesampleandasecond
beamwithvariabledelayprobingthesample.Inthehard
X-raydomainthisactioniseasilyperformedbycrystal
optics,wherethinpelliclepartiallytransmittingcrystals
arepractical.InthesoftX-rayenergydomain,wedonot
havetheequivalentopticalelements,andsoalternative
strategies as described below must be employed.
Thefinalchallengeisfromtransientandaveragepower
loadontheopticscreatingdamage,orwavefrontdistor-
tion.Thepeakpoweronthe1stopticalelementswillbe
similartoFLASH,butduetothemuchhigherrepetition
rate,theaveragepowerwillbemuchhigher.Theaverage
power density will exceed that experienced at
3rdgenerationsynchrotronundulatorbeamlinesandwill
presentasignificantchallenge.Mostoftheexperienceto
dateonimpulsivedamagetomirrorcoatinghasbeenon
diamond-likecarbonduetoitsexcellentreflectivityatthe
smallgrazinganglestypicallyusedforenergieslessthan
200eV.IntheNGLScasehowever,weneedtohavehigh
reflectivitythroughoutthewholesoftX-rayenergyregion
including thecarbonK-edgeregionand this imposes
somechallengesforcoatingmaterials.Inthefollowing
sectionswehaveoutlinedtheoveralldesignissuesand
suggestedsolutionsusingexistingFELtechnologyas
wellasindicatingwheretheemphasisofourR&Dactivi-
tiesneedstobeplaced.
6.2 OverallBeamlineDesign
Thedesignandoperationof theFLASHbeamlines
offerssomeguidanceon theoverall issuesandsolu-
tions,1,2buthere,oneofthemainissueswillbethepres-
ervationandmanipulationofultra-shortpulsescloseto
thetime-bandwidthlimitthatneedfurtherconsideration.
Agratingwilllengthenapulsebyatimeδt=Nmλ/c,
whereNisthenumberofilluminatedgrooves,misthe
diffractiveorder,λisthewavelengthandcisthespeedof
light.Forexample,foraresolvingpowerof10,000we
wouldneedthesamenumberofgroovestobeilluminat-
edandhenceforthe1stdiffractiveorderatthecarbon
K-edge,wewouldhaveapulsebroadeningof150fs.
Thepulselengthofatransformlimitedlightsourceis
howeverδT~λ2/δλcandsorealizingthatthefundamental
resolvingpowerofagratingisequaltothenumberofillu-
minatedgrooves,N,wecanseethatthegrating-induced
temporalsmearingoptimallycorrespondstothatimplied
by the time-bandwidth product.The design feature
requiredthereforeistoilluminatethecorrectnumberof
groovesforthewavelengthresolutionrequired.
152
6 . EXPERIMENTAL SYSTEMS
would be done with a single elliptical mirror in each
plane,butalthoughthesearefreeofpoint-to-pointpulse
stretching,afinitewidthobjectisimagedinaplanetilted
withrespecttotheopticalaxis.Tocounterthis,focusing
would require two mirrors in each plane in order to
achieveanimagefreeoftilt.Sucherectfieldimagingfor
widefieldsistypicallydonewithhyperboloid-ellipsoid
pairsasinWolterX-raytelescopedesign,buthereasthe
angularapertureissmaller,bentparaboloidcombina-
tionswillprobablybesufficientandwouldthestarting
pointforourdesignstudies.
ALShighpoweropticalcomponentshaveeitherside
orinternalwatercoolinganddealwithhighpowerden-
sities.InthecaseofNGLShoweverathighrepetition
rate,wewillbegoingtosignificantlyhigheraverage
powers.Thiscanbedealtwithbymovingopticsfurther
fromthesource,byavoidinguseofan intermediate
focusinthebeamlinesandpossiblybyusingafirstmir-
roratextremegrazingincidencethatisdivergent,thus
increasingthebeamsizeondownstreamoptics.Water
coolingcanbepushedbeyondthehighestheat load
ALSopticsbyinternaljetcooling,butbeyondthis,we
willhave tocryo-coolsomeof thehighestheat load
components.ThishasbeendoneforexampleintheALS
BL5.0wiggleropticalsystem,wherethecrystaloptics
arecooledto120Ktotakeadvantageofthezeroexpan-
sioncoefficientofSiatthistemperatureandthehigher
thermalconductivity. Inthiscase,weextractroughly
250Wofpower,similartotheNGLScase.
Due to the very high power density, intermediate
focusingtoasetofentranceslitsshouldprobablybe
avoidedandsowewillneedinterchangeofgratingswith
differinglinedensities.Thisgratinginterchangeistypi-
callydoneinsoftX-raybeamlinestodaysothatoptimum
diffractionefficiencycanbeachievedoverawidewave-
lengthrange,buthereaswellitisrequiredforselection
oftheappropriatesource-limitedresolutionandhence
time-bandwidth-limitedpulselength.Afurtherissueis
thatthewavefrontenteringtheopticalsystemistiltedat
theexit,duetotheuseofgrazingincidenceoptics.One
waytocorrectthisistousetwogratingsintheclassical
mountinginsubtractivedispersionmodeassuggested
byVilloresi3andsuccessfullydemonstratedbyseveral
groups.4Afirstfocusinggratingproducesadispersive
focusatwhichtherequiredwavebandisselectedbyaslit;
asecondgratingthenreimagesthistoafocus.Thissec-
ondaryfocuscanbefreetofirstorderofprimarypulse
stretchingandwavefronttilt.Thepenaltyisthatthenor-
mallylowdiffractionefficiencyofagratingismadesub-
stantiallyworseinthetwogratingarrangement.
Thesimplestarrangementhoweverwouldbetouse
anerectfieldvariablelinespacing(VLS)monochromator
(Figure109).5Inthissystemtheexitwavefrontisapproxi-
matelyperpendiculartotheopticalaxis.Thecombination
ofanentranceslitlessformofthisdesignwithanarrayof
gratingsdesignedtoproduceselectionofarangeofreso-
lutionswouldbeastartingpointforourdesignstudies.
Lightfromthemonochromatorwouldberefocusedtothe
sampleusinggrazing incidenceoptics.Normally this
Figure109 VLS grating monochromator as used at the ALS (left) and a K-B mirror assembly used for microfocusing.
153
6 . EXPERIMENTAL SYSTEMS
siveworkatFLASH,impulsivedamageshouldnotbean
issue.R&Dinthisareawouldconcentrateonassessing
cumulative damage due to high average power that
mightbepresentbelowthesingleshotdamagelimit.
6.4 SplitandDelay
Severaloftheexperimentsplannedfortheinitialcom-
plementofbeamlineswilluseX-raypump—X-rayprobe
techniques.Wewillthereforehavetosplitoneormore
softX-raybeamsandprovideavariabledelaypath,with
twobeamsthatareideallyco-linear.Thetwomainways
todothisaretoa)beamsplitusingagrating; inthis
methodzeroandfirstorderlightareusedandb)splitthe
wave-frontusingaknifeedgemirrorpartiallyinserted
intothebeam.Thefirstmethodhassomeadvantages,
butforultrafastpulsesisnotpractical,asasecondgrat-
ingwouldhavetobeusedtocompensatepulsestretch-
ing.ThesecondschemehasbeenpioneeredatFLASH9
andisshowninFigure110below.
ThisarrangementisbasedonagrazingincidenceMach-
Zehnderinterferometer.Therighthanddiagramshowsthe
principleinwhichamirror(SM1)ispartiallyinsertedinto
thebeam,thusgeneratingtwobeams.Thesebeamsare
directed towards mirror pairs DL1-DL3 and DL2-DL4
respectivelyandrecombinedatmirrorsSM3andSM4.
Eachpathhasfivereflectionsandadelayisintroduced
betweenthepathsbylateralmotionofthe“interferome-
ter”table.SuchasystemhasbeenbuiltattheALSand
usedforVUVFourierTransformSpectroscopybasedon
thesameoverallgeometryandcouldbeadaptedforthis
purpose.ThesystemasusedatFLASH,andplannedfor
LCLS,hasdemonstratedadelayrangeof10psandadelay
precisionof0.2fs.Themainmodificationwewouldmake
tothissystemwouldbetousemirrorsatmoreextreme
grazinganglestosupporthighthroughputtoabove1keV.
6.3 MirrorDamage
Theissueofmirrordamagehasbeenextensivelystud-
iedatFLASH,mainlyintheVUVandatLCLSforthehard
X-rayregime.NGLSpulseenergiesaresimilartothoseat
thesefacilitiesandsoinprinciplewecanadoptsimilar
solutions.Onespecialproblemhowever is that these
coatings are based on either diamond-like carbon or
boroncarbide.Inourcase,wewishtotunethroughthe
carbonK-edgeregionandnormallyuseofacarboncoat-
ingintheregionfrom280-1000eVwouldbeprecluded.
However thereare fewalternatives. Inorder toavoid
excessiveinstantaneouselectronicheating,wehaveto
usealowZcoating.AtFLASH,diamond-likecarbonis
extensivelyusedbecausethemainoperatingenergiesare
lessthan280eV.AtLCLS,B4Cisextensivelyusedbecause
thecarbonK-edgeissubstantiallylowerinenergythan
theminimumoperationalenergy.OnNGLS,thecarbon
K-edgeiswithinourenergyrange,andbyitselfaprime
targetforsomeexperiments.OtherlowZmaterialsare
limited,withperhapsBethemainalternative.Analterna-
tiveissimplytouseB4C,butatamuchlowergratingangle
thanconventionallyused.Thiswouldsignificantlyreduce
thecarbonK-edgereflectivitymodulation.Separation
betweenthevariousFELbeamlinesisprimarilyprovided
bytheFELswitchyard,notbytheoptics,andconsidering
theverylowdivergenceofthelight,mirrorlengthshould
notbeanissue.Howevershadowingofthediffraction
gratinggroovesatverysmallgrazinganglesisaconcern
andtheoptimizationofthegrooveshapeandcoatingto
optimizeefficiencywillhavetobecarefullyconsidered.
Theissueof impulsivedamageofamirrorcoating
underFELilluminationhasbeenextensivelystudiedfor
diamondlikeandamorphouscarbon6,7andforB4C.8In
bothcasestheconclusionsaresimilarinthatthesingle
shotdamagethresholdisintheregionof0.1–0.3J/cm2
wellabovethevaluesforNGLSoptics.Basedonexten-
Figure110 Mirror based split and delay line. (From Sorgenfrei et al.9)
DL1 DL3
DL2 DL4SM4
SM3
SM2
SM1
y
zSM2
154
6 . EXPERIMENTAL SYSTEMS
Nq=Eγ /ε,whereEγistheincidentphotonenergyandεvar-
iesbymaterial(ε=3.6eVforsilicon).Fluctuationsinener-
gylosslimitthespectroscopicresolutionofthedetector
toσq2=FNq,whereFistheFanofactor(F~0.12forSi).
ForNGLS, silicon is an idealdirect-detection sensor:
Figure111showstheefficiency(probabilityofphotocon-
versionwithinthesensitivevolume)forX-raydetection
ina200μmthicksiliconsensorwith3nmofnativeoxide.
Above~8keV,thesensorstartstobecometransparent—
limitingdetectionefficiency.Atlowerenergies,X-raysare
absorbedpriortoenteringthesensitivevolume.
Inordertofullydepletethesensor(andthusensure
fullchargecollectionwiththeminimalpointspreadfunc-
tion)athinconductingentrancewindowisrequired—
and the thickness of this window determines the
low-energyX-rayefficiency.Weareactivelyengagedin
R&Donthinwindows:forNGLS,thetotalthicknessofthe
deadlayerbeforethesensitivevolumemustbe≤10nmin
ordertomaintain>50%efficiencyatthecarbonK-edge.
Figure111 Efficiency for 200 μm thick silicon detectors.
Readoutspeedanddataprocessingwillbecrucialfor
NGLS.Wehavedeveloped102framepersecond(fps)
2Ddetectors,12andhavesuccessfullydeployedthese
detectorsatALS,APSandLCLS.Thisdetectorisbasedon
aunique,thick,fully-depletedMOSCCDstructuredevel-
opedatLBNL.13Wearetransformingthisdetectorfrom
102fpsto103-4fpsbyadoptingafullycolumn-parallel
architecturetogetherwithadvancedcustomintegrated
circuitreadoutin65nmCMOS.
To reach the105 fps rateneeded forNGLS,anew
detectorarchitecturewillbeneeded.Today,at200fps,the
6.5 Diagnostics
AseachofthebeamlineswillbecapableofX-raypump
—X-rayprobecapability,andmeasurementofdelayand
pulselengthwillbeanessentialcomponentofeachexper-
iment,wewillneedin-beamlinediagnosticsofthetempo-
ralcharacteristicsofthebeam.Thereareseveralwaysthat
this can be achieved, and based on on-going work at
FLASHandnowattheSXRbeamlineatLCLS,thepre-
ferredmethod,especiallyforultra-shortpulsesislikelyto
evolve.Forultrashortpulses,itislikelythatasecstreaking
methodswillbeused,butforpulsesof10fswecanuse
simplerandmorerobustmethods.Oneoftheseisbased
onthemodulationoftheopticalreflectivityofasemicon-
ductorbyaVUVorsoftX-raypulse.10Theimpulsivecre-
ation of electron hole pairs in the semiconductor
modulatesthereflectivity,andbytiltingonewave-front
withrespecttotheother,aCCDcanbeusedto“image”the
arrivaloftheX-raypulse.InanX-ray—X-rayexperiment,
thetwobeamswouldbephysicallyseparated,butwould
beimagedinthesameway;afslasersynchronizedtothe
X-raysourcewouldprovidetheimagingwindowandthe
separationof the twoX-ray inducedreflectivityedges
couldgiveaprecisemeasurementofX-raypump-probe
delay.Theprecisionofthissystemispresently40fsovera
time window of many ps, with substantial area for
improvementtothe10fsscale.Beyondthis,streakingof
photoelectronenergiesproducedbyX-rayillumination
ofagasjetimmersedinanultrafastlaserfieldcanyield
highprecisionpulseshapeanddelaymeasurementsinthe
sub-fsregime,butoververylimitedtemporalwindows.11
6.6 Detectors
Worldwide,asnewFELsourcescomeonline,corre-
spondingdetectordevelopmentsareneededinorderto
beabletorecordtheshot-by-shotdatagenerated.The
greatestchallengesarefor(2-dimensional,area)~mega-
pixeldetectorsabletoreadoutattheFELrepetitionrate,
withnomemoryofthepreviousshot.Whereasearlier
2DX-raydetectorswerebasedonscintillatingphosphors
fiber-opticallycoupledtoacharge-coupleddevice(CCD),
moderndetectorsarebasedondirectdetectionofX-rays
inasemiconductorsensor.Inafullydepletedsemicon-
ductor detector, the number of electron-hole pairs,
Nq,generatedbyanincidentphoto-convertingX-rayis
10 100 1000 10000
E (eV)
10
0
80
60
40
20
Effic
ienc
y (%
)
155
6 . EXPERIMENTAL SYSTEMS
NGLS,anadvancedversionofthehigh-speeddetector14
developedforTEAMprojecttogetherwithon-chippro-
cessingisthemostpromisingcandidate:inaggressive
CMOStechnologies,theimagingareaofareticle-scale
devicecanbesmall,leavingtherestoftheICareafreefor
imageprocessingandcompression.Aspartofouron-
goingR&Dprogram,weareprototypinganimagesensor
in65nmCMOSinordertodeterminesuitabilityforan
ultra-high-speed2DX-raydetector.Wearealsoworking
on algorithm development based on X-ray FEL data.
Thesealgorithmswillbetestedfirstinfirmware,andthen
portedtosiliconfortheNGLSdetector.
LBNLFastCCDgenerates1.5Tb/hr.Atthisdatarate,itis
stillpossible,ifnotalwaysefficient,tosimplyreadall
dataoutandstoreit.Athigherframerates,datatransmis-
siontoarchivalstoragebecomesimpossible.Forthefully
column-parallelCCDdescribedabove,hundredsofhigh-
speedseriallinkswillcarrydatatoafirmwaredatapro-
cessor, which will perform compression and feature
extractionpriortotransmittingthedatatostorage.At
105fps,amegapixeldetectorwouldgenerateaTbevery
5seconds—whichisimpossibletoreadoutconvention-
ally:thedatacansimplynotbemovedoffthedetector
silicon.On-chipprocessingisrequired.ForsoftX-raysat
References
1. Martins, M., et al., Monochromator beamline for FLASH. Review of
Scientific Instruments, 2006. 77(11): p. 115108.
2. Tiedtke, K. and et al., The soft X-ray free-electron laser FLASH at DESY:
beamlines, diagnostics and end-stations. New Journal of Physics, 2009.
11(2): p. 023029.
3. Villoresi, P., Compensation of optical path lengths in extreme-ultraviolet
and soft X-ray monochromators for ultrafast pulses. Applied Optics, 1999.
38(28): p. 6040-6049.
4. Nugent-Glandorf, L., et al., A laser-based instrument for the study of ultra-
fast chemical dynamics by soft X-ray-probe photoelectron spectroscopy.
Review of Scientific Instruments, 2002. 73(4): p. 1875-1886.
5. Hettrick, M.C., In-focus monochromator: theory and experiment of a new
grazing incidence mounting. Applied Optics, 1990. 29(31): p. 4531-5.
6. Juha, L., et al., Radiation damage to amorphous carbon thin films irradiat-
ed by multiple 46.9 nm laser shots below the single-shot damage thresh-
old. Journal of Applied Physics, 2009. 105(9): p. 093117.
7. Chalupsky, J., et al., Damage of amorphous carbon induced by soft X-ray
femtosecond pulses above and below the critical angle. Applied Physics
Letters, 2009. 95(3): p. 031111.
8. Hau-Riege, S.P., et al., Multiple pulse thermal damage thresholds of mate-
rials for X-ray free electron laser optics investigated with an ultraviolet
laser. Applied Physics Letters, 2008. 93(20): p. 201105-3.
9. Sorgenfrei, F., et al., The extreme ultraviolet split and femtosecond delay
unit at the plane grating monochromator beamline PG2 at FLASH. Review
of Scientific Instruments. 81(4): p. 043107.
10. Theophilos, M., et al., Single-shot timing measurement of extreme-ultravio-
let free-electron laser pulses. New Journal of Physics, 2008(3): p. 033026.
11. Kienberger, R., et al., Sub-femtosecond X-ray pulse generation and mea-
surement. Applied Physics B-Lasers & Optics, 2002. 74(Suppl S): p. S3-S9.
12. Denes, P., et al., A fast, direct X-ray detection charge-coupled device.
Rev Sci Instrum, 2009. 80(8): p. 083302.
13. Holland, S.E., et al., Fully depleted, back-illuminated charge-coupled
devices fabricated on high-resistivity silicon. Electron Devices, IEEE
Transactions on, 2003. 50(1): p. 225-238.
14. Battaglia, M., et al., Characterisation of a CMOS active pixel sensor for
use in the TEAM microscope. Nuclear Instruments and Methods in
Physics Research Section A: Accelerators, Spectrometers, Detectors
and Associated Equipment, 2010. 622(3): p. 669-677.
7 Future upgrades
NGLS,asdescribedinSection5,includesastate-of-the-
artelectronsource,superconductinglinac,andthreeinitial
FELbeamlines,togetherwiththeconventionalfacilities
required to house the facility.The linac and FELs are
designedtobeabletoobtainwavelengthsasshortas1nm
atrepetitionratesat1MHzorhigher.
Inordertoaccommodateinevitabletechnicaladvances,
theNGLSdesignconceptembracesastrategyofphased
implementationofFELbeamlines.TheNGLSfacilitywillbe
configuredtotakeadvantageofas-yetunrealizedadvanc-
esinthephysicsandtechnologyofhighbrightnessbeams,
newconcepts inFELoperation, and improvements in
undulatorsandX-rayoptics.Thepreliminarydesignpre-
sentedhereusesahighlyflexibleelectrongun,injector,
andlinacthatcanprovideawidevarietyofelectronbunch
structuresandtimepatterns,sothatNGLSwillbecapable
ofincorporatingawiderangeofFELtechnologies.
The electron beam spreader shown in Figure 76 is
intrinsicallymodular.Capacitycanbeincreasedbyadding
additional spreader elements, FELs, X-ray beamlines
togetherwith thecorrespondinghousingasshown in
Figure112.Capabilitycanbeincreasednotonlybyadding
advancedFELs,butalsobyincreasingtheelectronbeam
energywithacombinationofextra linacsectionsand
NGLS
Capacity Expansion
Capability Expansion
Future Buildout
Figure112 Upgrade paths.
158
5 . FUTURE UPGRADES
onlysmallincrementstobeamenergy.Atsufficientlyhigh
repetitionrates,FELoscillatorscombinedwithharmonic
radiatorsmightprovidethebenefitsofseededoperation
withouttheneedforexternallasers.Significantimprove-
mentsinHHGforEUVorsoftX-rayproductioncanbelev-
eragedtoprovideseedingsignalsrequiringlessstringent
harmonicjumps.Ifthelinaccouldoperatereliablyatmul-
tipleparameterpoints,soastodeliverbunchesofdiffer-
ent charge, duration, and/or energy to different FEL
beamlines,theflexibilityandversatilityofthemachine
wouldberedoubled.
Manyoftheseareasofimpactare,orwill,bethesub-
jectofsignificant researcheffortsworldwide,and the
NGLSprojectwillbepoisedto takeadvantageofany
incrementalorrevolutionaryimprovements.
gradientincreases.Forexample,increasingbeamenergy
to approximately 2.5 GeV, and using undulators of
~12mmperiodwillproduceover3keVinthefundamen-
tal,and10keVinthe3rdharmonicatpowerlevelsesti-
matedtobe1%ofthoseachievableinthefundamental.
Similarly,anincreaseinbeamenergytoapproximately
4GeV,togetherwithdevelopmentsinundulatortechnol-
ogythatprovidefor10mmperioddevices,wouldpermit
lasinginthefundamentalat10keV.
Inaddition,modest improvements incavityquality
factorscouldtranslateintosignificantlyhigherbeamener-
giesatfixedoperatingcosts.Novelundulatordesigns—
including very-short-gap superconducting undulators
orpossiblyevenelectromagneticundulators—could
dramaticallyexpandtheNGLSwavelengthwithperhaps
8 Management
8.1 CostEstimate
ThepreliminaryTotalEstimatedCostwithoutcontin-
gency(TEC)oftheprojectis$635M,andthepreliminary
TotalProjectCost (TPC) is$997Minthen-yeardollars
(withthescheduleshowninTable11,andthefundingpro-
fileshowninTable10).Theestimateincludesallcosts
associatedwithConceptualDesignandR&D,together
withalltechnicalandconventionalconstruction,acceler-
atorcommissioning,projectmanagementandcontin-
gency.These estimates were developed during pre-
conceptual design, and are based on recent US
experiencewithsuperconductingaccelerators,freeelec-
tronlasers,andrecentUniversityofCalifornia/LBNL
conventionalconstructionexperience.Abreakdownof
thecostisshowninTable9,withapreliminaryfunding
profileshowninTable10.
Apreliminaryannualoperatingcostestimatehasbeen
performedbasedonexperiencewiththecurrentoperat-
ingcostsfortheAdvancedLightSource.Thepreliminary
estimateof$97MinFY11dollarsislargerthantheALS
operatingcostduetothelargeracceleratorandtechnical
staffrequiredforthemorecomplexNGLS,togetherwith
higherpowercosts.
Table9 Cost breakdown in escalated M$.
Construction Cost (M$)
Injector $31
Linac $233
X-rayProduction $44
ExperimentalSystems $52
ConventionalFacilities $201
ProjectManagement $74
TotalEstimatedCost(TEC) $635
ConceptualDesign $10
R&D $54
Commissioning $27
OtherProjectCosts(OPC) $91
Contingency 37% $271
TotalProjectCost(TPC) $997
Profile FY11 FY12 FY13 FY14 FY15 FY16 FY17 FY18 FY19 FY20 FY21 FY22
OPC $2 $10 $10 $15 $16 $7 $4 $0 $0 $0 $6 $21
TEC $0 $0 $15 $20 $34 $93 $156 $250 $160 $110 $64 $4
TPC $2 $10 $25 $35 $50 $100 $160 $250 $160 $110 $70 $25
Table10 Funding profile in escalated M$.
160
8 . MANAGEMENT
• Theelectronbeamspreader,whichdistributesthe
electronbeamtotheindividualFELs
- High switching rates with minimum beam
perturbationandbeamlossarerequired—see
Section5.4.4
• Electronbeamdumps,which requireengineering
design,notR&D—seeSection5.4.6
• AdvancedFELoperation(seeSection5.4.5),including:
- Seedingschemes:inparticular,EEHG-seededFELs
radiating at a high harmonic of the seed laser
requiredemonstrationofphysicsandtechnology
atbothshorterwavelengthsandfarhigherhar-
monicjumpsthancurrentlyachieved.
- Polarizationcontroland tunabilityusingcross-
polarizedundulators.
- Synchronization,timing,andfeedback-and-control
systemsforsub-femtosecondoperation.
Inaddition,inordertotakefulladvantageofthetrans-
formationalcapabilitiesofNGLS,advancesinkeytechni-
calareaswillenhancethescientificproductivityofNGLS.
Forthisreason,R&Dwillalsobeperformedon
• Shortperiodundulators
AsdescribedinSection5.4.5.3,thebaselineundula-
torsforNGLSwillbebasedonwell-provenhybridperma-
nentmagnettechnologies.Superconductingtechnologies
havethepromisetoeliminatemovingpartsandreduce
costsandprovideellipticalpolarization,alongwiththe
capabilitytoprovideshortperiods,andthusextendthe
wavelengthreachofNGLS
• X-raybeamtransport
AsdescribedinSection6.3,thehighaveragepowerof
NGLSwillrequireadvancesinsomeopticalcomponents.
• High-speeddetectors
AsdescribedinSection6.6,advancesinhighframe
ratedetectorsarerequiredinordertobenefitfromthe
NGLSrepetitionrate.
• High-powerseedlaserandpumplasersystems
AsdescribedinSection5.4.5.5high-power,andcarri-
er-envelopephase-stabilized lasersare required,with
robustandreliableperformance.
• High-dynamic-rangediagnostics
AsdescribedinSection5.4.1,thecapabilitiesofNGLS
tooperateatverylowcharge(afewpC)andveryhigh
charge(upto1nC),willallowsignificantadvancesinper-
formance,andrequiredevelopmentsindiagnosticssys-
tems(seeSection5.4.8andSection6.5).
8.2 Schedule
The preliminary schedule of major milestones is
showninTable11.TheNGLSconceptualdesignreport
(CDR)wouldbedevelopedforCD-1,andtheperformance
baselinewouldbeapprovedatCD-2.Transportofelectron
beamthroughthelinacwouldmarkthestartofcommis-
sioning (CD-4a).The project would be complete with
deliveryofphotonstotheexperimentalhall(CD4-b),fol-
lowedbythestartofoperationasauserfacility.
Table11Preliminary Major Schedule Milestones.
Milestone Date
CD-0 ApproveMissionNeed FY11
CD-1 PreliminaryBaselineRange FY13
CD-2/3a PerformanceBaseline/Long-leadProcurement FY15
CD-3b StartofConstruction FY16
CD-4a StartofCommissioning FY21
CD-4b StartofOperations FY22
8.3 RiskManagementandR&DNGLScost,scheduleandperformanceriskswillbe
minimizedbyusingproventechnologyinthebaseline
designwhereverpossible.Nonetheless,inordertodeliv-
ertheperformancedescribedabove,NGLSwillrequire
certain,specificR&Dandengineeringadvancesassum-
marizedinthissection.Onacostbasis,themajorityof
NGLSislowrisk:theNGLSlinacisaconservativeimple-
mentationofcurrentsuperconductingacceleratortech-
nology,anddoesnotrepresentasignificanttechnical
risk;similarly,theconventionalconstructioniscompara-
bletootherequivalentfacilities.
Fourprincipletechnicalsystemsrequiretechnology
maturation,whichwillbeaddressedbyearlyR&Dinthe
project,andnoneofwhichareexpectedtoimpactthe
proposedconstructionschedule:
• The high-repetition-rate, low-emittance electron
injector.Asdescribed inSection5.4.2 the injector
requires
- High quantum efficiency, long lifetime photo-
cathodes—seeSection5.4.2.2
- Ahigh-powerdrivelaserthatmatchesthephoto-
cathodematerialspropertiesandthathastrans-
verseandlongitudinalpulseshapingcapabilities
—seeSection5.4.2.3
- Ahigh-powerelectrongun—seeSection5.4.2.4
161
8 . MANAGEMENT
8.5 Environment,SafetyandHealth
8.5.1 Integrated Safety Management SystemEnvironment,SafetyandHealth(ES&H)requirements
willbesystematicallyintegratedintomanagementand
workpracticesatalllevelssothattheNGLSprojectisexe-
cutedwhileprotectingthepublic,theworker,andtheenvi-
ronment.NGLSIntegratedSafetyManagementSystem
documentsandpolicieswillmakeitclearthattherespon-
sibilityforsafetyandenvironmentalprotectionstartswith
theNGLSDirectorandflowsthroughthemanagement
chainfromseniormanagementtolinesupervisors,and
finallytotheworkers.ItistheresponsibilityofNGLSman-
agementtoensurethatstaffaretrainedandareresponsi-
bleforES&Hintheirassignedareas.TheNGLSproject
workwillbeexecutedinaccordancewithdefinedinstitu-
tionalES&Hpoliciestoensurehazardsareidentifiedand
mitigated;workisauthorizedafterES&Hanalysisiscom-
pleted;andoversightofworkisconductedbyNGLSman-
agementandstaff.Continuousassessmentandoversight
oftheprojectwillbeconductedbytheproject,institutional
EH&Sandassuranceorganizations,andtheDOE.
8.5.2 National Environmental Policy ActTheDOEwill complywith the requirementsof the
NationalEnvironmentalPolicyAct(NEPA)anditsimple-
mentingregulationspriortotakinganyactiononthepro-
posedproject that couldhaveadverseenvironmental
effects.ANEPAevaluationwillbepreparedtoevaluatethe
potentialenvironmentalconsequencesofconstructing
andoperatingtheNGLS.Itisplannedthatthiswilltake
theformafullEnvironmentalImpactStatement(EIS).
8.5.3 Fire Hazards AnalysesAfirehazardanalysis(FHA)willbedevelopedtodeter-
minethefiresafetyrisksassociatedwiththeNGLSproject.
8.5.4 Safety Assessment DocumentIn compliance with DOE Order 420.2B “Safety of
AcceleratorFacilities,”aSafetyAssessmentDocumentwill
bepreparedthatidentifiesthespecificES&Hhazardsand
themeansfortheirmitigation.Inparticular,theradiation
hazardsassociatedwiththisfacilitywillbefullyanalyzed
andallappropriateshielding,interlock,andadministrative
controlswillbedevelopedandimplemented.
8.4 Organization
NGLSwillbeexecutedasaprojectwithinthePhoton
SciencesDirectorateatLBNL.Forthedesign,construc-
tion and operation of NGLS, technical staff will be
matrixed from theAccelerator and Fusion Research,
EngineeringandFacilitiesDivisions—analogoustothe
constructionandoperationoftheAdvancedLightSource.
Scientific coupling to LBNL’s Physical Biosciences,
Genomics, Life Sciences, Chemical Sciences,
EnvironmentalEnergyTechnologies,MaterialsSciences,
EarthSciences,ComputingandGeneralSciencesand
AdvancedLightSourcedivisionswillguidethedevelop-
mentofthefacilityandtheinitialexperimentalprogram.
Figure113showstheorganizationofNGLS.NGLSwillbe
executed as a scientific and technical collaboration
betweennumerouslaboratories.Inparticular,itisantici-
patedthatthesuperconductinglinacwillbedeveloped
byaDOEpartnerlaboratory.
Department of Energy
Deputy Secretary, Acquisition ExecutiveUnder Secretary of EnergyDirector, Office of Science
Director, Office of Basic Energy SciencesDirector, Scientific User Facilities Division
NGLS Program Manager
Berkeley Site Office
Site ManagerFederal Project Director
Lawrence Berkeley National Laboratory
Laboratory Director
Project Advisory Committee
NGLS Project Office
Project DirectorProject Manager
Deputy Project Manager
Accelerator Systems
Experimental Systems
LINACInjector FELs
ConventionalFacilities
Advisory Committees
ScienceMachine
ManagementConventional Facilities
Figure113 NGLS Organization.
Appendices
Appendix1
X-ray Interactions and Non-Disruptive Probing
SoftX-raysareanincisiveprobeofelectronicstruc-
ture.However,animportantrequirementisthattheprobe
pulsenotdisruptthesystemthatweseektounderstand.
Thisisequallytrueforsystemsinacorrelatedground
state,andforsystemspreparedinaperturbativenear-
equilibriumexcitedstatebyatailoredexcitationpulse.
TheX-rayprobeinteractionwiththematerialunderstudy
mustremaininthelinearregime,andthisplacesrestric-
tionsonthetolerablepulsefluenceforultrafastprobes.
NonlinearX-rayprobeinteractionmaybemanifestin
severalformsdependingontheexperimentaltechnique,
andonthematerialpropertiesunderinvestigation.For
example,photoelectronspectroscopyisoneofthemost
informativeprobesofelectronicstructure—reportingon
boththeenergyandmomentumoftheelectronicstates
of an ordered solid. However, at fluences above
~5x106ph/pulse(50µmspot)space-chargeeffectsdis-
tortthephotoelectronspectrumanddegradetheenergy
resolution.Thushighrepetitionrateatmoderatefluxper
pulseisessentialtoachievetherequiredcountratesfor
photoemissionspectroscopy.
Photon-inphoton-outtechniquescantoleratesomewhat
higherpulsefluenceandstillremaininthelinearinteraction
regime.Forexample,XESmeasurementsonSisamples
atFLASHindicateanacceptableupperpulsefluencelimit
ontheorderof10mJ/cm2.Recentresonantdiffraction
studiesofcharge-orderinginnickelatesamplesatLCLS
indicateasafeupperboundof~1mJ/cm2.Thesefluence
levelsareconsistentwithultrafastvisiblespectroscopy
researchoverthepastseveraldecades,wheretheimpor-
tanceofmaintainingalinearprobeinteractioniswell
established.For1keVphotonsandcharacteristicspotsizes
of50µm(e.g.forprobingheterogeneousmaterialseven
smallerfocalspotsmayberequired),thiscorrespondstoan
upperlimitontheusablefluenceof~108photonsperpulse.
However,photon-inphoton-outspectroscopy tech-
niquesarephotonhungry,owingtothesmallinelastic
cross-sections.The most demanding experiments at
3rdgenerationsoftX-raysynchrotronsourcesrequirean
averagefluxinexcessof~1012ph/s/(10meVbandwidth).
Inordertoachievetheseaveragefluxlevelswithsoft
X-raylasers(whilerestrictedtolessthan108ph/pulseas
describedabove),theymustoperateinthe10-100 kHz
regime.The most demanding and most informative
experimentsofthefuture,experimentsthatarepresently
wellbeyondourreach,willpushthisrequirementintothe
MHzregime,andwillrequireevenbetterenergyresolution.
Similarestimatesontheappropriatefluencelimitcan
bederivedbasedonsimpleconsiderationsoftheX-ray
absorptioncross-section,andthedepositedenergyper
atom.InthesoftX-rayrange(0.1-1keV),typicalatomic
absorptioncross-sectionsareontheorderof1018 cm2.
Ifoneconsiders109photons/pulseat1keV,ina30µm
focalspot(10-5cm2area),thiscorrespondsto1017eV/cm2
(16mJ/cm2),or0.1eVperatom.Thetypicalenergyofa
covalentbondis~1eVperatom,sothisfluencelevelis
sufficienttobreak~10%ofthecovalentbondsinamole-
culeorsolid.Thisinteractionlevelisfarfromlinearor
non-disruptive.
Fromanotherperspective, this fluence levelcorre-
spondsto~10%valencetoconductionbandexcitationin
a1eVgapsemiconductor.Thisisthenominalthreshold
atwhich“non-thermal”melting isknown tooccur in
semiconductors.Theseelectronicexcitationlevelsare
sufficientlyhightodirectlydestabilizethelattice.Based
ontheseconsiderations,theincidentfluenceperpulse
shouldbe~1 mJ/cm2or less inordertobesafely ina
linear interaction regime. High repetition rate will be
essentialtoprovidethehighaverageX-rayfluxrequired
bytheexperiments,whilekeepingthefluxperpulsesafely
inthelinearinteractionregime.
164
APPENDICES
Nanoscale Coherent Imaging
and Microscopy with a Soft
X-Ray Laser
Date:October16–17,2009
OrganizingCommittee:
JohnCorlett(LBNL),Robert
Schoenlein(LBNL)
Attendees: 102 from Lawrence Berkeley National
Laboratory, University of California-Berkeley, Davis,
LawrenceLivermoreNationalLaboratory,PacificNorthwest
NationalLaboratory,SLACNationalAcceleratorLaboratory,
Sandia National Laboratory, Heimholz Center Berlin
(Germany),DOEOfficeofScience,CanadianLightSource
Inc.(Canada),PrincetonUniversity,StonyBrookUniversity,
PrincetonUniversity,ArizonaStateUniversity,Stanford
University, University of Wisconsin-Milwaukee,
Brookhaven National Laboratory, Royal Holloway
UniversityofLondon(UK),McMasterUniversity(Canada),
SincrotroneTrieste(Italy)
Imaging and Defining
Function: Chemical Sciences
Drivers for Next Generation
Soft X-ray Light Sources
Date:November30–
December3,2009
OrganizingCommittee:
OliverGessner(LBNL)
Attendees: 92 from Lawrence Berkeley National
Laboratory, SLAC National Accelerator Laboratory,
Lawrence Livermore National Laboratory, Argonne
NationalLaboratory,UniversityofNebraska,Louisiana
StateUniversity,UniversitätKassel(Germany),Western
Michigan University, University of Arizona, Vienna
UniversityofTechnology(Austria),UniversityofHeidelberg
(Germany),KansasStateUniversity,BrownUniversity,
NorthwesternUniversity,WashingtonStateUniversity,
UniversityofColorado,PrincetonUniversity,ETHZurich
(Switzerland),USDepartmentofEnergy,Universityof
California-Berkeley, and Davis, Stanford University,
ImperialCollegeLondon(UK),NationalResearchCouncil
Canada,FrankfurtUniversity(Germany),TohokuUniversity
Appendix2–Workshops
The international user community has been fully
engagedindefiningthescience-basedrequirementsfor
anextgenerationlightsource.Aseriesofworkshopshave
been,andcontinuetobe,organizedatLBNLwiththe
goalsofrefiningtheserequirements,andunderstanding
newareasofsciencethatNGLSwillenable.Asummary
ofrecentworkshopsislistedbelow.
Toward Control of Matter:
Energy Science Needs for
a New Class of X- Ray
Light Sources
Date:October8–10,2007
OrganizingCommittee:
AliBelkacem(LBNL),John
Corlett(LBNL),RogerFalcone
(LBNL/UC Berkeley),Graham
Fleming(LBNL/UC Berkeley),
BillMcCurdy(LBNL/UC Davis),DanNeumark(LBNL/UC
Berkeley),RobertSchoenlein (LBNL)
Attendees:89fromArgonneNationalLaboratory,Arizona
StateUniversity,BESSY(Germany),BrookhavenNational
Laboratory, EPF Lausanne (Switzerland), ETH Zurich
(Switzerland),FrontierCollaborativeResearchCenter,High
Energy Accelerator Research Organization, Lawrence
BerkeleyNationalLaboratory,KansasStateUniversity,
Louisiana State University, University of Groningen
(Netherlands), University of Oregon, University of
Washington,UniversityofWisconsin-Madison,Universite
PierreetMarieCurie(France),WesternMichiganUniversity,
MaxPlanckInstitute(Germany),MassachusettsInstituteof
Technology,NationalResearchCouncil,OakRidgeNational
Laboratory, Ohio State University, Oxford University,
PaulScherrerInstitut(Switzerland),PrincetonUniversity,
Radboud University (Netherlands), Sandia National
Laboratory,SLACNationalAcceleratorLaboratory,Stanford
University,UniversityofCalifornia-Berkeley,Davis,Irvine,
Santa Barbara, University of Oregon, University of
Washington,UniversityofWisconsin-Madison.
165
APPENDICES
FEL Design Workshops
Workshop on X-Ray FEL R&D
Date:October23–25,2008
OrganizingCommittee:
JonathanWurtele (LBNL/UC
Berkeley),AlexanderZholents
(LBNL)
Attendees: 40 from Lawrence Berkeley National
Laboratory, SLAC National Accelerator Laboratory,
UniversityofOregon,UniversityNijmegen(Netherlands),
UniversityofWisconsin,ArgonneNationalLaboratory,
UniversityofIllinois,MaxPlanckResearchDepartment
(Germany),UniversityofHamburg(Germany),University
ofColorado,WashingtonStateUniversity,HongDing
InstituteofPhysics(China),ChineseAcademyofSciences
(China), Helmholtz-Zentrum Berlin (Germany),Tokyo
Institution ofTechnology (Japan),Tokyo Institute of
Technology(Japan),UniversityofCalifornia-Berkeley,
Davis,andSanDiego,LosAlamosNationalLaboratory,
StanfordUniversity
Compact X-Ray FELs Using
High-Brightness Beams
Date:August5–6,2010
OrganizingCommittee:
JonathanWurtele(LBNL/UC
Berkeley),JohnCorlett(LBNL),
MarcoVenturini(LBNL)
Attendees: 45 from Lawrence Berkeley National
Laboratory, SLAC National Accelerator Laboratory,
ArgonneNationalLaboratory,UniversityofCalifornia-
Los Angeles, Naval Postgraduate School, RAND
Corporation,DaresburyLaboratory(UK),Universityof
Wisconsin, Cockroft Institute (UK), University of
Strathclyde,Glasgow(UK)
(Japan),MaxPlanckResearchDepartment (Germany),
UniversityofHamburg(Germany),AMOLF(Netherlands),
Helmholtz-Zentrum Berlin (Germany), University of
Hamburg(Germany)
Condensed Matter Science
for the Next Generation
Light Source
Date:May5–7,2010
OrganizingCommittee:
RobertSchoenlein(LBNL),
ZahidHussain(LBNL),Robert
Kaindl(LBNL)
Attendees: 58 from Lawrence Berkeley National
Laboratory, SLAC National Accelerator Laboratory,
UniversityofOregon,UniversityNijmegen(Netherlands),
UniversityofWisconsin,ArgonneNationalLaboratory,
UniversityofIllinois,MaxPlanckResearchDepartment
(Germany),UniversityofHamburg(Germany),University
ofColorado,WashingtonStateUniversity,HongDing
InstituteofPhysics(China),ChineseAcademyofSciences
(China), Helmholtz-Zentrum Berlin (Germany),Tokyo
Institution ofTechnology (Japan),Tokyo Institute of
Technology(Japan),UniversityofCalifornia-Berkeley,
Davis, San Diego, Los Alamos National Laboratory,
StanfordUniversity
166
APPENDICES
Appendix3–ListofAcronyms
1D ............... one-dimensional2D ............... two-dimensional3D ............... three-dimensional4D ............... four-dimensional6D ............... six-dimensionalACP ............ amorphous calcium
phosphateADK............ adenylate kinaseAF ............... antiferromagneticAFM ........... atomic force microscopyAH .............. aromatic hydrocarbonALS ............ Advanced Light SourceAMOLF ...... Foundation for
Fundamental Research on Matter’s Institute for Atomic and Molecular Physics
APEX.......... Advanced Photoinjector EXperiment
APPES ....... Ambient Pressure Photoelectron Spectroscopy
APPLE........ Advanced Planar Polarized Light Emission
APS ............ Advanced Photon Source
ARPES ....... Angle-Resolved Photoemission Spectroscopy
ASTRA ....... A Space-charge TRacking Algorithm
ATP ............ adenosine triphosphateBCS ............ Bardeen-Cooper-
SchriefferBES ............ Basic Energy SciencesBESSY ....... Berliner
ElektronenSpeicherring-gesellschaft für SYnchrotronstrahlung
BNL ............ Brookhaven National Laboratory
BPM........... beam position monitorBW ............. bandwidthBZ............... Brillouin ZoneC-band....... “compromise” band CA .............. carbonic anhydraseCAD............ computer aided designCARS ......... Coherent Anti-Stokes
Raman SpectroscopyCCD ............ charge coupled deviceCD .............. circular dichroismCD .............. Critical DecisionCDR ............ Conceptual Design
Report
CEBAF ....... Continuous Electron Beam Accelerator Facility
CEP ............ carrier-envelope phaseCFEL ........... Center for Free Electron
Laser Science, DESYCMOS ........ complementary metal–
oxide semiconductorCMR ........... colossal
magnetoresistanceCNT ............ Classical Nucleation
TheoryCO-LIF........ carbon monoxide laser
induced fluorescenceCOLTRIMS COLd Target Recoil Ion
Momentum Spectroscopy
CPA ............ chirped pulse amplification
CRS .............. coherent (i.e., stimulated) Raman scattering
cryo-EM .... cryogenic Electron Microscopy
CSR ............ coherent synchrotron radiation
CT ............... charge transferCW ............. continuous waveCXDI........... Coherent X-ray
Diffractive ImagingDC .............. direct current (i.e., non-
oscillatory)DESY .......... Deutsches Elektronen
SYnchrotronDNA ........... deoxyribonucleic acidDNS ........... direct numerical
simulationDOE ............ Department of EnergyDPA ............ divided pulse
amplificationEEHG.......... Echo-Enabled Harmonic
GenerationEF-G ........... Elongation Factor-GEGR ............ exhaust gas
recirculationEIS.............. Environmental Impact
StatementELEGANT .. ELEctron Generation
ANd Tracking codeEM.............. electron microscopyEPAC .......... European Particle
Accelerator ConferenceEPFL ........... École polytechnique
fédérale de LausanneERL ............. energy recovery linacES&H ......... Environmental, Safety,
and HealthESRF .......... European Synchrotron
Radiation FacilityET ............... electron transferEUV ............ extreme ultraviolet
EXAFS........ Extended X-ray Absorption Fine Structure
FEL ............. free electron laserFERMI ........ Free Electron Laser for
Multidisciplinary Investigations
FET ............. field effect transistorFHA ............ Fire Hazards AnalysisFLASH........ Free Electron Laser in
HamburgFM .............. ferromagneticFODO ......... FOcusing-DefOcusingFROG.......... Frequency Resolved
Optical GatingFTIR............ Fourier Transform
InfraRed spectroscopyFWHM ....... full width at half
maximumfXS ............. fluctuation X-ray
scatteringFY ............... fluorescence yieldFY ............... Fiscal YearGTP ............ guanosine triphosphateHAP............ hydroxyapatiteHGHG......... High Gain Harmonic
GenerationHHG ........... High Harmonic
GenerationHOM .......... higher-order modeHOMO........ Highest Occupied
Molecular OrbitalHTC ............ high temperature
superconductorIC ................ integrated circuitID................ insertion deviceILC .............. International Linear
ColliderIMPACT ..... Integrated-Map and
Particle ACcelerator Tracking code
INFN .......... Instituto Nazionale di Fisica Nucleare
IOT ............. Inductive Output TubeIR ................ infraredISR ............. incoherent synchrotron
radiationIXS ............. Inelastic X-ray
ScatteringJLAB .......... Thomas Jefferson
National Accelerator Facility (Jefferson Lab)
KEK ............ Kō Enerugī Kasokuki kenkyū kikō (High Energy Research Organization)
L-band ....... “long” waveLASA.......... Laboratorio Acceleratori
e Superconduttività Applicata (MIlano)
laser........... Light Amplification by Stimulated Emission of Radiation
LBNL .......... Lawrence Berkeley National Laboratory
LCLS........... Linac Coherent Light Source
LES ............. large eddy simulationLIF .............. Laser Induced
FluorescenceLINAC ........ LINear ACceleratorLLNL........... Lawrence Livermore
National LaboratoryLLRF ........... low-level radio-
frequencyLSC............. Longitudinal Space
ChargeLUMO ........ lowest unoccupied
molecular orbitalMAX........... National Electron
Accelerator Laboratory for Synchrotron Radiation Research (Sweden)
MBE ........... molecular beam epitaxyMD ............. molecular dynamicsMOSFET .... metal-oxide-
semiconductor field-effect transistor
mRNA ........ messenger RNAMtrC .......... outer membrane
decaheme cytochrome c lipoprotein
NA .............. numerical apertureNEG............ non-evaporable getterNEPA ......... National Environmental
Policy ActNESHAPs .. National Emission
Standard for Hazardous Air Pollutants
NEXAFS..... Near Edge X-ray Absorption Fine Structure
NGLS ......... Next Generation Light Source
NIF ............. National Ignition FacilityNIR ............. near-infraredNLC ............ Next Linear ColliderNLS ............ Next Light SourceNM ............. non-magnetic metalNMR .......... nuclear magnetic
resonanceNSF ............ National Science
FoundationNSLS.......... National Synchrotron
Light SourceOCP ............ octacalcium phosphateOEC ............ Oxygen Evolving
Complex
167
APPENDICES
OmcA......... outer membrane decaheme cytochrome
OO .............. orbital orderOPC ............ Other Project CostsOTR ............ optical transition
radiationPAC ............ Particle Accelerator
SchoolPAH ............ polycyclic aromatic
hydrocarbonPEEM ......... PhotoEmission Electron
MicroscopyPES ............ PhotoElectron
SpectroscopyPGA............ phosphoglyceratePITZ ........... Photo Injector Test
Facility – ZeuthenPS............... photosystemQE............... quantum efficiencyqp ............... quasiparticleR&D ........... Research &
Developmentredox ......... oxidation-reductionRF ............... radio-frequencyRIXS ........... Resonant Inelastic X-ray
ScatteringRMS ........... root-mean-squareRNA ........... ribonucleic acidRST ............ Reference Structure
TomographyRT ............... room temperatureRuBisCo .... ribulose
1,5-bisphosphate carboxylase oxygenase
RuBP.......... ribulose 1,5-bisphosphate
S-band....... “short” waveSASE.......... Self-Amplified
Spontaneous EmissionSAXS ......... Small-Angle X-ray
ScatteringSBA............ “Santa Barbara”
mesoporous silicateSC............... superconductingSCRF .......... superconducting
radio-frequencySCSS.......... Spring-8 Compact SASE
SourceSFL ............. Stabilized optical Fiber
LinkSHG............ second harmonic
generationSLAC .......... SLAC National
Accelerator LaboratorySMD........... single molecule
diffractionSNS............ Spallation Neutron
SourceSONICC ..... Second-Order Nonlinear
optical Imaging of Chiral Crystals
SPARC ....... Sorgente Pulsato Auto-amplificata di Radiazione Coerente
SPIM.......... Selective Plane Imaging Microscopy
SPring-8 .... Super Photon Ring – 8 GeV
SR............... storage ring
SRS ............ Spontaneous Raman scattering
STM ........... scanning tunneling microscopy
SXPCS ....... Soft X-ray Photon Correlation Spectroscopy
SXR ............ soft X-rayTEAM......... Transmission Electron
Aberration-corrected Microscope
TEC............. Total Estimated CostTEM ........... transmission electron
microscopyTESLA ........ Tera-Electron-volt
Superconducting Linear Accelerator
TEY ............. total electron yieldTJNAF ....... Thomas Jefferson
National Accelerator Facility
TMV ........... Tobacco Mosaic VirusTOF............. time-of-flightTPC ............ Total Project CostTR ............... time-resolvedTR- XAS ..... Time-Resolved X-ray
Absorption Spectroscopy
tRNA .......... transfer RNATRPES........ Time-Resolved
PhotoElectron Spectroscopy
TRS ............ time-reversal symmetryUHV............ ultra-high vacuumUK .............. United Kingdom
URA............ uniformly redundant array
USA............ United States of AmericaUV .............. ultravioletVHF ............ very high frequencyVLS............. variable line spacingVUV ............ vacuum ultravioletWAXS ........ Wide-Angle X-ray
ScatteringX-band....... “cross” bandXANES....... X-ray Absorption Near
Edge StructureXAS ............ X-ray Absorption
SpectroscopyXCARS ....... X-ray Coherent Anti-
Stokes Raman Spectroscopy
XES ............ X-ray Emission Spectroscopy
XFEL ........... (European) X-ray Free Electron Laser
XFROG ....... Cross-Correlated Frequency Resolved Optical Gating
XMCD ........ X-Ray Magnetic Circular Dichroism
XPCS.......... X-ray Photon Correlation Spectroscopy
XPS ............ X-ray Photoelectron Spectroscopy
XUV ............ eXtreme UltraViolet, or X-ray UltraViolet
YAG ............ Yttrium Aluminum GarnetZ ................. atomic number
(TOC continued)