an x-ray fel oscillator with erl-like e-beams kwang-je kim anl & univ. of chicago may 23, 2008...
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![Page 1: An X-Ray FEL Oscillator with ERL-Like E-Beams Kwang-Je Kim ANL & Univ. of Chicago May 23, 2008 Seminar at Wilson Lab Cornell University](https://reader030.vdocuments.us/reader030/viewer/2022032517/56649cba5503460f94982a7b/html5/thumbnails/1.jpg)
An X-Ray FEL Oscillator with ERL-Like E-Beams
Kwang-Je Kim
ANL & Univ. of ChicagoMay 23, 2008
Seminar at Wilson Lab
Cornell University
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2KJK, Cornell, May 23, 2008
Next Generation X-Ray Sources
High-gain FELs (SASE) will provide an enormous jump in peak brightness from the 3rd generation sources– Intense, low emittance bunches; Q= 1 nC, IP~ several kA, x
n~ 1 mm-mr
– LCLS, European X-FEL, SCSS, Fermi,..
Multi-GeV Energy Recovery Linacs (ERLs) will provide high average brightness with low intensity, ultra-low emittance bunches at high rep rate x
n~ 0.1 mm-mr, Q=20 pC, ~2ps, frep~1.3 GHz, IAV up to 100 mA
– Cornell, MARS, KEK-JAERI, APS,..
ERLs have so far been regarded only as a spontaneous emission source
We show that an X-ray FEL Oscillator (X-FELO) for ~1-Å based on high energy ERL beams is feasible with peak spectral brightness comparable to and average spectral brightness much higher than SASEs (to be published in PRL)
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3KJK, Cornell, May 23, 2008
ERL Plans: Cornell, KEK/JAERI , APS II
APS II concept
Cornell ERL
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4KJK, Cornell, May 23, 2008
X-Ray Cvities for Oscillators: History X-ray FEL Oscillator (XFEL-O) using Bragg reflector was first
proposed by R. Colella and A. Luccio at a BNL workshop in 1984.
This was also the workshop where a high-gain FEL(SASE) was proposed by R. Bonifacio, C. Pellegrini, and L. M. Narducci
X-Ray optical cavities to improve the performance of high-gain FELs have been studied recently:– Electron out-coupling scheme by B. Adams and G. Materlik
(1996)– Regenerative amplifier using LCLS beam ( Z. Huang and R.
Ruth, 2006)
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5KJK, Cornell, May 23, 2008
Current and Future X-Ray Sources
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6KJK, Cornell, May 23, 2008
Electron Beam Qualities Enabling X-FELO Laser-driven DC gun being developed at Cornell for frep=1.3 GHz
Thermionic cathod and bunch manipulation for frep=1-100 MHz
E=1.4 MeV, el=2ps
Q (nC) Ip (A) nx(10-7 m)
Pessimistic 19 4 1.64
Cornell High-Coherence Mode 19 4 0.82
Optimistic 40 8 0.82
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7KJK, Cornell, May 23, 2008
Principles of an FEL Oscillator
Small signal gain G= Pintra/Pintra
– Start-up: (1+G0) R1 R2 >1 (R1& R2 : mirror reflectivity)
– Saturation: (1+Gsat) R1 R2 =1
Synchronism– Spacing between electron bunches=2L/n ( L: length of the cavity)
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8KJK, Cornell, May 23, 2008
Bragg Mirrors Requiring total loss per pass to be < 20%, the
reflectivity of each Bragg mirror should be well over 90%
Possible crystal candidates are– Diamond
• Highest reflectivity & hard ( small Debye-Waller reduction)• Multiple beam diffraction in exact backscattering needs to be
avoided ( can use as a coupling mechanism?)
– Sapphire• High reflectivity without multiple beam diffraction• Small thermal expansion coefficient and large heat conductivity at
T=40K
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9KJK, Cornell, May 23, 2008
Backscattering Reflectivity's for Sapphire and Diamond ( Perfect Crystals)
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10KJK, Cornell, May 23, 2008
Diamond C(220) Reflection:E0=4.92 keV
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11KJK, Cornell, May 23, 2008
Sapphire Reflectivity @ 14.3 keV
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12KJK, Cornell, May 23, 2008
Sapphire Crystal Quality
Back-reflection topographs of HEMEX sapphire wafers cut from different boules show different dislocation densities:
(a) 103 cm-2, (b) much lower dislocation density.
Sample area illuminated by x-rays is 2.1 x 1.7 mm2
Chen, McNally et al., Phys. Stat. Solidi. (a) 186 (2001) 365
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13KJK, Cornell, May 23, 2008
X-Ray Focusing
Focusing is required to adjust the mode profile Bending the Bragg mirrors for a desired curvature
(~50m) may destroy high-reflectivity Possible options:
– Grazing-incidence, curved-mirrors for non backscattering configuration
– Compound refractive lenses of high transmissivity can be constructed ( B.Lengeler, C. Schroer, et. Al., JSR 6 (1999) 1153)
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14KJK, Cornell, May 23, 2008
Options for XFEL-O Cavities (Y. Shvyd’ko)
Al2O3xAl2O3 @14.3 keV
RT=0.87, Gsat=15%, T=3%
CxCxmirror @12.4 keV
RT=0.91, Gsat=10%, T=4%
Al2O3xAl2O3xSiO2@ 14.4125 keV
RT=0.82, Gsat=22 %, T=4%
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15KJK, Cornell, May 23, 2008
Gain Calculation
Analytic formula for low signal including diffraction and electron beam profile – Sufficiently simple for Mathematica evaluation if electron beam
is not focused, distributions are Gaussian, and ZRayleigh =*
Steady state GENISIS simulation for general intra-cavity power to determine saturation power (Sven Reiche)
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16KJK, Cornell, May 23, 2008
Saturation: As circulating power increases, the gain drops and reaches steady state when gain=loss
E=7 GeV, λ=1ÅQ=19 pC (Ip=3.8A), Nu=3000 Mirror reflectivity=90%Saturation power=19 MW
E=7 GeV, λ=1ÅQ=40 pC (Ip=8 A), Nu=3000Mirror reflectivity=80% Saturation power=21 MW
0.00
10.00
20.00
30.00
40.00
50.00
60.00
70.00
0.00E+00 5.00E+06 1.00E+07 1.50E+07 2.00E+07 2.50E+07 3.00E+07 3.50E+07 4.00E+07 4.50E+07
Intra Cavity Power (W)
Gai
n (
%)
Saturation Point
0.00
5.00
10.00
15.00
20.00
25.00
30.00
0.00E+00 5.00E+06 1.00E+07 1.50E+07 2.00E+07 2.50E+07 3.00E+07 3.50E+07 4.00E+07 4.50E+07
Intra Cavity Power (W)
Gai
n (
%)
Saturation Point
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17KJK, Cornell, May 23, 2008
Examples XFEL-O
Å) E
(GeV)Q
(pC)nx
(10-7m)
K U
(cm)
NU G0 (%) RT
(%)Psat
(MW)
2.48 7 19 1.64 2.5 2.26 2600 35 83 10
1 7 19 0.82 1.414 1.88 3000 28 90 19
1 7 40 0.82 1.414 1.88 3000 66 83 21
0.84 7.55 19 0.82 1.414 1.88 3000 28 90 20
0.84 10 19 0.82 2 2.2 2800 45 83 18
=2 ps, =1.4 MeV, ZR=*=10~12 m
Electrons are not focused but matched to the optical mode determined by cavity configuration
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18KJK, Cornell, May 23, 2008
Simulation of Oscillator Start-up
Time-dependent oscillator simulation using GENO (GENESIS for Oscillator) written by Sven– Taking into account FEL interaction (GENESIS), optical cavity
layout, and mirror bandwidth (Reiche)
To reduce CPU– Follow a short time-window (25 fs) – Track a single frequency component for all radiation
wavefronts since other components are outside the crystal bandpass
– Even with these simplifications, one pass takes about 2 hr
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19KJK, Cornell, May 23, 2008
Start-up Simulation (Reiche)
Pessimistic case
Ip=4 A, mirror loss=10%
Effective net gain~6%
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20KJK, Cornell, May 23, 2008
Super-mode Analysis (adapted from G. Dattoli, P. Elleaume)
Describes gain and spectrum narrowing in the exponential gain regime taking into account the profiles of I(z-ct) and Gmono()
Eigenmode: Gauss-Hermite function opt=(2el*M)1/2/g1/4 : M=1/(2
M) : M=mirror bandwidth
Amplitude growth rate of the fundamental mode0=0.5(g-a)-(0.5u/M)2-0.5g1/2(M/el): u=pulse displacement
hM=2.8 meV, el=2 ps
Bandwidth of the fundamental mode– h
opt=2.3 meV =2 10-7!!
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21KJK, Cornell, May 23, 2008
Tolerances
Reduction in gain<1%– Pulse to pulse overlap u<20 fs– Cavity detuning (tolerance on cavity length)<3m
Change in optical axis<0.1*mode angle– angular tolerance of crystals <8 nrad
LIGO technology?
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22KJK, Cornell, May 23, 2008
X-FELO Repetition Rate frep= 1.5 MHz when one x-ray pulse stored in 100 m
optical cavity– I=60 A (Q=40 pC), P=0.4 MW May not need ERL
frep=100 MHz with ERL?– Thermal loading on crystals is tolerable (probably)– Electron rms energy spread increases from 0.02 % to 0.05%– With increased energy spread, the loss in the ERL return pass
becomes 2 10-5
– These problems may be solved by increasing the minimum recovery energy to 30 MeV (higher than usual 10 MeV)
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23KJK, Cornell, May 23, 2008
Tunability with two crystals
Tuning range is very limited (<2 10-6) due to the need to keep 2< 4 mr for high reflectivity of grazing incidence miror
L
2
Grazing Incidence Mirror
Crystal
H
Crystal
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24KJK, Cornell, May 23, 2008
Tunable Cavity Scheme (KJK)
For tuning; increase H and decrease S keeping the round trip path length the same
– L=100m, H0=1m, S0=0.1m=5 10-4 (Hmax=3.3 m)
– L=100m, H0=1m, S0=2m =1% (Hmax=14.3 m)
With this scheme, diamond may be used for most cases:
– With 2max~60 degree: (444) for 12<<15 keV
– (220) for 5<<6 keV
S
A B
C D
H
2
L
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25KJK, Cornell, May 23, 2008
Gun technologies
SCSS: CeB6, thermionic, pulsed gun
LBL 50 MHz, laser driven
Cornell laser-driven 750 kV, DC
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26KJK, Cornell, May 23, 2008
Ultra-Low Emittance <50 MHz Injector(P. Ostroumov, Ph. Piot, KJK)
Use a small diameter thermionic cathode to extract low emittance beam Provide 500 kV extracting voltage using low frequency ~50 MHz room
temperature RF cavity Using chicane and slits form a short ~ 1 nsec bunch Remove energy modulation by a 6th harmonic cavity Use a pre-buncher an booster buncher to form low longitudinal emittance of
the bunched beam Accelerate to ~50 MeV using higher harmonic SC cavities Use an RF cosine-wave chopper to form any required bunch repetition rate
between 1 MHz and 50 MHz. (ANL Invention Application, IN-06-093)
1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10
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27KJK, Cornell, May 23, 2008
Performance of X-FELO Spectral range: 5 keV<<20 keV
Full transverse and temporal coherence in ~1 ps (rms) /FWHM=2.5 10-7 ; h=2 meV (rms)
Tunable 109 photons (~ 1 J) /pulse
– Peak spectral brightness~LCLS
Rep rate 1-100 MHz average spectral brightness (1026 -1029) #/(mm-mr)2(0.1%BW)
The average spectral brightness is higher by a factor of– 105-107 than other future light sources considered so far, ERL-
based or high-gain FEL-based – Current APS about 100-1000 less than ERL
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28KJK, Cornell, May 23, 2008
Science Drivers for XFEL-O Inelastic x-ray scattering (IXS) and nuclear resonant
scattering (NRS) are flux limited experiments! Need more spectral flux in a meV bandwidth!
Undulators at storage rings generate radiation with ≈ 100−200 eV bandwidth. Only ≈ 10−5 is used, the rest is filtered out by meV monochromators.
Presently @ APS: ≈ 5 × 109 photons/s/meV (14.4 keV)
XFEL-O is a perfect x-ray source for:– high-energy-resolution spectroscopy (meV IXS, neV NRS, etc.), and
– imaging requiring large coherent volumes.
– Expected with XFEL-O ≈ 1015 photons/s/meV (14.4 keV) with 107 Hz repetition rate.
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29KJK, Cornell, May 23, 2008
Concluding Remarks
A X-FELO around 1-Å is feasible with high-quality e-beams contemplated from future ERLs
However, the rep rate of an X-FELO can be 100 MHz or lower to 1 MHzInjector may be less challenging
An X-FELO is new type of future light sources ( in addition to high-gain FELs and ERLs)