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Bates XFEL Linac and Bunch Compressor Dynamics
1. Linac Layout and General Beam Parameter 2. Bunch Compressor
– System Details (RF, Magnet Chicane)– Linear bunch compressing – Wake field and CSR – Various Effects (Chirp Phase, Source..)– Further optimization and S2E Simulation
3. SummaryFuhua Wang , Dong Wang
MIT-Bates LaboratoryPresentation to MIT X-ray laser Accelerator Science Advisory
CommitteeSeptember 18-19, 2003
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• Beam From the RF Injector: 20 ps, Charge 0.2, 1 nC.
Slice emittance 0.6,1.0 um, Slice p ~ 5KeVTwo operation mode: 0.2nC for 0.1-.2ps, 1nC for 1ps.
• Linac RF: TESLA 9cell cavity, 8 cavity cryomodule. • Two (Four bends) Bunch Compressors with adjustable
R56.• Experiment Station Energy: 1,2,4 GeVWhat make this linac different from other FEL linac driver?Seeding , HGHG operations requires high energy and timing
stability of beam, high compression ratio, extraction at several energies.
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Linac Layout
• BC: Bunch Compressor Chicane• SW: Switchyard• Linac Section (TESLA Cryomodules)• 3rd H: Third Harmonic Linearizerp: Bunch total momentum spanp: slice momentum spread
BC1
R56max -122mm
Lb 20 ps
p ~4%
SW2
SW1
RF Gun p=5KeV
Chirp
3rd H
230 MeV2 GeV1 GeV561 MeV 4 GeV
96 MeV 200 MeV
BC2
R56 -45mm
Lb 1- 4ps
p 1.4%
Lb .2- 1ps
p 0.2%
2 Cryomodules 3 C. modules
12 C. modules
6 C. modules
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2. Bunch Compressor2.1 System Details
• RF Chirp : position – energy correlated
• 3rd Harmonic RF Section for RF nonlinear distortion correction
Cavity is at decelerating phase, Vh=V0/h2 .
Use 3rd harmonic reasons :
Technical and lower wake field (W2, W 3 ).
...)sin()(2
1)cos()sin(
...)sin(2
1)cos()sin(
2
002
00000
hhhhhhh VhVhVV
VVVV
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• First magnet chicane bending angles is adjustable. For 0.1-0.2 ps bunch length operation, chose R56=-122mm .
The large R56 reduces the required energy chirp.
But with issues:
more rf nonlinearity, more CSR effects, more sensitive to phase jitter?
• Chicane locations : 200 MeV and 561 MeV ( initial optimization by P.Emma, April 2003).
• Necessary of second order corrections : sextupole etc ?
Bates Energy Compressor (reverse of bunch compressor)
installed Q and S corrections later for first and 2th order system error corrections.
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Magnet Chicane parameters
Bending Angle (degree) 8.78 5.33
B field (kG) 5.11 8.70
Drift (b1-b2,b3-b4) (m) 2.423 2.447
Drift (b2-b3) (m) 0.5 0.5
R56 (mm) -122 -45
Beam parameters at last bend magnet of chicane
x 0.111 1.299
x (m) 9.22 10.759
E (MeV) 200 561
p/p =Total bunch momentum span 0.04 0.014
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2.2 Linear Compressing (0.2 ps , no wake fields , CSR etc.)
Start: Hard-edge, 0.2nC, 20 ps p=5KeV Q Variation (rms) ~ 0.2-0.3%
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Adjusting Chirp phase to compensate Wakefield & CSR effects ?
Chirp phase adjustment: -21.400=> -21.550
Increased Peak current ~25%Bunch Length down to ~0.3ps
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But same Chirp phase (-21.550) without Wakes & CSR –> over chirped
<= ~20fs,15kA peak
This can’t be real.
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Coherent Synchrotron Radiation & beam emittance growth
• Synchrotron radiation will be coherent if
>>Lbunch .
• Radiation by the tail will catch up with the head and modulate energy.
• Analytic emittance growth by P.Emma (‘Stead state’ CSR)Assuming ‘stead-state’ CSR, the incremental rms coherent energy spread at each dipole magnet slice(Lb) is (Ya. S. Derbenev):
Emittance growth:
3/43/222.0)(
z
be LNrs
)69)1((36
22.01
:Bend ChiacneLast
)(,)(
2
2223
2
4
5222
0
2616
2'
2'
220
20
2
'
BBz
B
N
e
xx
xxxx
LLLNr
dsds
dsRds
ds
dsR
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Bates Xfel-Linac Bunch Compressor : CSR effects on beam emittance
BC1 0.2nC BC2 0.2nC BC1 1nC BC2 1nC ?BC1 1nC R56=-122mm
re= 2.82E-15 2.82E-15 2.82E-15 2.82E-15 2.82E-15
Lb magnet (m) 0.2 0.2 0.2 0.2 0.2
(rad.) 0.1583 0.0931 0.0931 0.0931 0.1583
alpha 0.111 1.299 0.111 1.299 0.111
beta(m) 9.922 10.759 9.922 10.759 9.922
sigma-z(m) 6.00E-04 6.00E-05 1.20E-03 3.00E-04 3.00E-04
sigma-z(ps) 2.00 0.20 4.00 1.00 1.00
N 1.26E+09 1.26E+09 6.30E+09 6.30E+09 6.30E+09
n(mrad) 6.00E-07 6.00E-07 8.00E-07 9.00E-07 6.00E-07
gamma 391.39 1097.85 391.39 1097.85 391.39
0 1.0091 1.0059 1.0001 1.0013 1.0654
Analytic emittance growth estimation
Slice analysis of emittance growth will be performed for more careful study.
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CSR may do more damage to emittance ?
Linear SystemWith Wakes and CSR(No Twiss matching)
E=4 GeV, Chirp phase =-21.40
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-21.450 -21.650-21.550
• Chirp phase sensitive (continue)
Bunch length (Peak current A)
~ 0.4 ps (~700) ~0.3 ps(~900) ~ 0.1ps(~2200!)
p/p
~ 0.15% ~0.2% ~0.1%
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• Injector Bunch Electron Distribution Effect. (Example of Gaussian beam …)
Linear, Chirp phase -21.40 +Wake, CSRSee big energy spike
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Further optimization and S2E Simulation
More work could be done to reduce CSR and optimize compressing process, like adjusting initial bunch density, chicane parameters and the optics.
Example: lower charge, same peak current shorter pulse
0.1nC, 20 ps. Final: 50fs bunch length, ~1000A peak…
Emittance distortion comparable to above mentioned 0.2nC, 20 ps case.
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About S2E Simulation
• Start to End Simulation• Codes: PARMELA(LANL) photo injector
ELEGANT (ANL) Linac + SwitchyardGINGER(LBL) FEL
Plan: Integrate PARMELA out(beam distribution) to ELEGANT simulation. For better simulation in linac. And down to FEL get responses.
Essential for: • Design optimization. • Beam diagnostics, controls and manipulation.• System requirements (error simulations, tolerances).•
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3. Summary
• Preliminary linac optics and bunch compressor design. Beam parameters close to design requirements.
• Tough requirements to Chirp phase.
• S2E simulation required for system optimization and define tolerances
• Much work needed to reduce nonlinear effects and there is still room to work on it!