Bates XFEL Linac and Bunch Compressor Dynamics

1
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
• Summary
• Fuhua Wang , Dong Wang
• MIT-Bates Laboratory
• Presentation to MIT X-ray laser Accelerator
  Science Advisory Committee
• September 18-19, 2003

2

• 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.

3
Linac Layout
BC2 R56 ?-45mm Lb? 1- 4ps Dp ? 1.4
BC1 R56max ?-122mm Lb ? 20 ps Dp? 4
Lb? .2- 1ps Dp ? 0.2
RF Gun dp5KeV
SW1
3rd H
12 C. modules
6 C. modules
3 C. modules
2 Cryomodules
Chirp
SW2
96 MeV
200 MeV
2 GeV
1 GeV
4 GeV
561 MeV
230 MeV

• BC Bunch Compressor Chicane
• SW Switchyard
• Linac Section (TESLA Cryomodules)
• 3rd H Third Harmonic Linearizer
• Dp Bunch total momentum span
• dp slice momentum spread

4
(No Transcript)
5
(No Transcript)
6
(No Transcript)
7
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, VhV0/h2 .
• Use 3rd harmonic reasons
• Technical and lower wake field (W????2, W? ? ?3 ).

8

• 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.

9
Magnet Chicane parameters
10
2.2 Linear Compressing (0.2 ps , no wake fields
, CSR etc.)
Start Hard-edge, 0.2nC, 20 ps dp5KeV
Q Variation (rms) 0.2-0.3
11
Before Chirp, Chirp phase -21.40
Before 3rd harmonic linearizer
12
After 3rd harmonic Linearizer
13
After First Chicance, Lb2ps
14
After second chicane.Lb 0.2 ps.
15
2.3 Wakes and CSR
Wakes only(4GeV)
Wakes CSR(4GeV)
Bunch length increased to 0.4 ps
16
Adjusting Chirp phase to compensate Wakefield
CSR effects ?
Chirp phase adjustment -21.400gt -21.550
Increased Peak current 25Bunch Length down to
0.3ps
17
But same Chirp phase (-21.550) without Wakes
CSR gt over chirped
lt 20fs,15kA peak This cant be real.
18
Coherent Synchrotron Radiation beam emittance
growth

• Synchrotron radiation will be coherent if
• ?gtgtLbunch .
• 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

19
Analytic emittance growth estimation
Slice analysis of emittance growth will be
performed for more careful study.
20
CSR may do more damage to emittance ?
E4 GeV, Chirp phase -21.40
With Wakes and CSR (No Twiss matching)
Linear System
21

• 2.4 Various effects
• Chirp phase. Sensitive to less than 0.050

22

• Chirp phase sensitive (continue)

Bunch length (Peak current A) 0.4 ps (700)
0.3 ps(900)
0.1ps(2200!) Dp/p 0.15
0.2
0.1
23
Injector Bunch Electron Distribution Effect.
(Example of Gaussian beam )
Wake, CSR See big energy spike
Linear, Chirp phase -21.40
24
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.
25
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).

26
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!