Transcript Linac and Bunch Compressor Dynamics

Bates XFEL Linac and Bunch
Compressor Dynamics
1.
2.
Linac Layout and General Beam Parameter
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. Summary
Fuhua Wang , Dong Wang
MIT-Bates Laboratory
Presentation to MIT X-ray laser Accelerator Science Advisory
Committee
September 18-19, 2003
1
• Beam
From the RF Injector: 20 ps, Charge 0.2, 1 nC.
Slice emittance 0.6,1.0 um, Slice dp ~ 5KeV
Two 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 GeV
What 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.
2
Linac Layout
RF Gun
dp=5KeV
BC1
BC2
R56max -122mm
R56 -45mm
Lb  20 ps
Lb 1- 4ps
Dp ~4%
Lb .2- 1ps
Dp  0.2%
Dp  1.4%
SW1
3rd H
12 C. modules
2 Cryomodules
Chirp
96 MeV
3 C. modules
6 C. modules
SW2
200 MeV
230 MeV
561 MeV
1 GeV
2 GeV
4 GeV
•
BC: Bunch Compressor Chicane
•
SW: Switchyard
•
Linac Section (TESLA Cryomodules)
•
3rd H: Third Harmonic Linearizer
Dp: Bunch total momentum span
dp: slice momentum spread
3
4
5
6
2. Bunch Compressor
2.1 System Details
• RF Chirp : position – energy correlated
• 3rd Harmonic RF Section for RF nonlinear distortion
correction
1 2
D V0 sin(0 )  ...
2
1
Vh  Vh sin(h )  hDVh cos(h )  (hD ) 2 Vh sin(h )  ...
2
Cavity is at decelerating phase, Vh=V0/h2 .
V0  V0 sin(0 )  DV0 cos(0 ) 
Use 3rd harmonic reasons :
Technical and lower wake field (W2, W  3 ).
7
• 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.
8
Magnet Chicane parameters
Bending Angle (degree)
8.78
5.33
B field (kG)
5.11
8.70
2.423
2.447
0.5
0.5
-122
-45
0.111
1.299
9.22
10.759
200
561
0.04
0.014
Drift (b1-b2,b3-b4) (m)
Drift (b2-b3) (m)
R56 (mm)
Beam parameters at last bend magnet of chicane
x
x (m)
E (MeV)
Dp/p =Total bunch momentum span
9
2.2 Linear Compressing (0.2 ps , no wake fields , CSR etc.)
Start: Hard-edge, 0.2nC, 20 ps
dp=5KeV
Q Variation (rms) ~ 0.2-0.3%
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Before Chirp, Chirp phase -21.40
Before 3rd harmonic linearizer
11
After 3rd harmonic Linearizer
12
After First Chicance, Lb~2ps
13
After second chicane.
Lb~ 0.2 ps.
14
2.3 Wakes and CSR
Wakes only(4GeV)
Wakes + CSR(4GeV)
Bunch length increased to ~0.4 ps
15
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
16
But same Chirp phase (-21.550) without Wakes & CSR –> over chirped
<= ~20fs,15kA peak
This can’t be real.
17
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(DLb) is (Ya. S. Derbenev):
D d (s)  0.22
Emittance growth:
re NDLb
 2 / 3 z4 / 3

 2   02   0  2 D x2  2  D x D x '    2 D x2'
D x 
R
16
( s)
d d
ds,
ds
D x ' 
R
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( s)

d d
ds
ds
Last ChiacneBend :
D
0

0.222 re2 N 2
1
36  N
  5 LB

 4
z

2
3 2
2
2

 ( L B (1   )  9   6L B )

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Analytic emittance growth estimation
Bates Xfel-Linac Bunch Compressor : CSR effects on beam emittance
BC1
re=
0.2nC
BC2
0.2nC
BC1 1nC
BC2 1nC
?BC1 1nC R56=-122mm
2.82E-15
2.82E-15
2.82E-15
2.82E-15
2.82E-15
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
1.26E+09
1.26E+09
6.30E+09
6.30E+09
6.30E+09
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
Lb magnet (m)
N
n(mrad)
Slice analysis of emittance growth will be performed for more careful study.
19
CSR may do more damage to emittance ?
E=4 GeV, Chirp phase =-21.40
With Wakes and CSR
Linear System
(No Twiss matching)
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2.4 Various effects
• Chirp phase. Sensitive to less than 0.050
-21.450
-21.550
-21.650
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• Chirp phase sensitive (continue)
-21.450
-21.550
-21.650
Bunch length (Peak current A)
~ 0.4 ps (~700)
~0.3 ps(~900)
~ 0.1ps(~2200!)
Dp/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, CSR
See big energy spike
23
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 + Switchyard
GINGER(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!
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