i.v. bazarov, beam brightness and cathode r&d, nsf annual review, oct 25, 2011 classe cornell...

I.V. Bazarov, Beam Brightness and Cathode R&D, NSF Annual Review, Oct 25, 2011

CLASSECLASSE

Cornell UniversityCHESS & ERL

Cornell Laboratory for Accelerator-based ScienceS and Education (CLASSE) Cornell Laboratory for Accelerator-based ScienceS and Education (CLASSE)

Pushing beam brightness from ERL source & photocathode R&D

world‘s highest brightness & current operating cw photoinjector

Ivan Bazarov

CLASSECLASSE

Main points

• Low emittance achieved

• Photocathode R&D

• Plans to improve the performance further

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Source brightness

• Key parameter for ERL• Improvements in the source immediately translate to a better

performance of the entire facility• Source development at Cornell

– 5-15 MeV injector

– New 500kV DC gun + beamline

5-15 MeV injector

DC gun dev lab

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Ingredients for pushing the brightness

• Theoretical insights

• Computer modeling

• Beam diagnostics

• Working hardware

• Right crew of people

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Example: analytical scaling of optics aberrations

aberrations slice-induced

Solenoid chromatic:

Solenoid geometric:

RF focusing:

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Example: high fidelity beam dynamics modeling

200-CPU computer cluster dedicated to beam dynamics simulations• Use of experimentally

benchmarked beam dynamics codes

• Pioneered the use of evolutionary multi-objective algorithms for flexible (nonlinear) beam optics design

• Cornell approach is now used in a number of labs world-wide

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Example: 6D beam diagnosticskey to success

transverse phase space (animation)

slice emittance with resolution of few 0.1ps

projected emittance

longitudinal phase space 7

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PhD students

Jared Maxson Colwyn Gulliford Siddharth Karkare Heng Li

Example: beam running crew

Luca CultreraAdam Bartnik Florian Loehl

CHESS/ERL1

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Just last month:emittance spec achieved!

• Keys to the result– Beam-based alignment (took us several months)– Working diagnostics!!– Fight jitters in the injector

• x2 thermal emittance! x1.3 simulated emittance• correct scaling with bunch charge

ny(100%) = 0.4 um @ 20pC/bunch ny(100%) = 0.8 um @ 80pC/bunch

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Understanding beam brightness

• Single RMS emittance definition is inadequate for linacs– Beams are not Gaussian

– Various groups report 95% emittance or 90% emittance (or don’t specify what exactly they report)

• The right approach– Measure the entire phase space, then obtain emittance of the

beam vs. fraction (0 to 100%)

20pC/bunch core = 0.15 um fcore = 73%

80pC/bunch core = 0.3 um fcore = 67%

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Shaking old-established notions: no Gaussians anywhere…

animation: scanning around 1st harm. ~6keV

(zero emittance)

Spectral flux (ph/s/0.1%BW/mm2) at 50m from undulator (5GeV, 100mA, p = 2cm)

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Understanding coherence: fully consistent formalism (Wigner)

Phase space (ph/s/0.1%BW/mm/mrad) at the undulator (5GeV, 100mA, p = 2cm)

animation: scanning around 1st harm. ~6keV

(zero emittance)

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Emittance vs. fraction for light

phase space of light

emittance vs fraction (of light!)

Bazarov, Synchrotron radiation in phase space (in preparation)

• Final radiation phase space: convolution with electron phase space

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Just how bright is Cornell ERL injector beam?

• Effective brightness (for comparison)

• Today’s 20mA ERL injector beam is as bright as 100mA 0.6nm-rad 0.006nm-rad storage ring Gaussian beam!

• Parity in transverse brightness with the very best storage rings is already achieved; the result can only improve

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Main points

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Photocathode R&D

• Brightness is ultimately set by the photocathode intrinsic properties

• Cornell has emerged as a leader in photocathode R&D for accelerator applications

• Expertise now covers a range of NEA materials, alkali-based: next is ‘material engineering’ of photocathodes for accelerators

x-ray topography map showing ion damage profile for a cathode with

1000 coulombs 1/e lifetime

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Photocathode research at Cornell: building up the infrastructure

multialkali growth chamber with vacuum suitcase GaAs system with load-lock in-vacuum LEED/Auger

MBE III-V system

• Also on campus AFM, EDX, SEM, STM, SIMS, ARPES

• + CHESS (XRF, x-ray topography, EXAFS, and much more)

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Photocathode research: a sample of results from the group

• Wide selection of photocathodes evaluated for MTE and response time: GaAs, GaAsP, GaN, Cs3Sb, K2CsSb

10%QE in green now routineGaAs intrinsic emittance

time response vs theory

GaN’s MTE

GaAsP response

K2CsSb MTE

Just 2011 aloneAPL 98 (2011) 224101APL 98 (2011) 094104APL 99 (2011) 152110PSTAB (2011) submittedPRL (2011) in preparation

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Photocathode research:moving forward

• Ultra-cold electrons are in the view;• About to grow our first MBE III-V

with tuned parameters;• Impact way beyond accelerators;

Transverse Energy

unique 2D energy analyzer for photoelectrons

structural studies of the bandgap structure

ps-resolution of response time

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Building collaboration on photocathodes for accelerators

• Collaboration with• ANL

• BNL

• JLAB

• SLAC

• Berkeley

• Excitement and momentum in the community;

• Cathode workshop at Cornell in 2012;

• Leading the effort on creating collaborative community- driven Internet resource;

1st workshop

2nd workshop

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Undergrads in cutting-edge photocathode research

Ashwathi Iyer: in situ work function mapping

Rick Merluzzi: Auger/LEED characterization

Yoon Woo Hwang: 2D energy analyzer, RHEED

CHESS/ERL1

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High current operation: photocathode performance

~600 coulombs delivered (same spot)

QE before QE after

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Ion back-bombardment

halo, beam losses

cathode life-time degrading mechanismscomputed ion dose

delivered to the cathode

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Real-life accelerator testing for photocathodes: high average current

• CsKSb cathode after delivering ~600C.

cathode after 20mA 8hour run SEM close-up

x-ray topography showing rings of ion back-bombardment damage

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Realistic operating conditions

• Good news: running 5mm off-center on the photocathode – same emittance (20pC/bunch) due to intrinsically low geometric aberrations in the DC gun

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Main points

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Improving brightness further

• Already close to the fundamental emittance limit (0.5 mm-mrad for given photocathode, cathode electric field)

• Detailed model/measurement verification is underway

• Merger characterization – promising simulations at ~12MeV show minimal emittance degradation

• Improvements to laser 3D shaping, element stabilityExamples of optimal laser

shapes

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3D laser shapingfor space charge control

• Optimal 3D laser shape: practical solution identified

• >50% of light gets through, emittance (sims) ~20% higher than the optimal

temporal – birefringent crystal pulse stacking

transverse – truncated Gaussian

temporal profile

transverse profile

PRSTAB 11 (2008) 040702Appl. Opt. 46 (2011) 8488

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New DC gun

• Reviewed in January 2011 by a group of external experts

• New segmented ceramic received – fully shielded from field emission

• Construction is underway

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Projected performance of the new DC gun (475kV) beamline

• Cornell’s construction of the MK II DC Gun:– Adjustable cathode/anode

gap “dial-in” a field!

– Segmented insulator: pushing the voltage limits.

– Full 6D phase space characterization out of the gun

CLASSECLASSE

Outlook: defining the state-of-the-art

• Cornell ERL photoinjector project has achieved beam brightness that exceeds that of the best storage rings;

• Diverse and aggressive program to improve beam brightness and injector performance further;– Detailed model/measurement benchmarking (6D phase space)

– Improving laser shape, element stability, diagnostics

– New gun & photocathode materials

CLASSECLASSE

i.v. bazarov, beam brightness and cathode r&d, nsf annual review, oct 25, 2011 classe cornell...

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