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 3.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  p ~ 5KeV Two operation mode: 0.2nC for ps, 1nC for 1ps. Linac RF: TESLA 9cell cavity, 8 cavity cryomodule. Two (Four bends) Bunch Compressors with adjustable R 56. 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 BC: Bunch Compressor Chicane SW: Switchyard Linac Section (TESLA Cryomodules) 3rd H: Third Harmonic Linearizer  p: Bunch total momentum span  p: slice momentum spread BC1 R56 max  -122mm L b  20 ps  p  ~4% SW2 SW1 RF Gun  p=5KeV Chirp 3 rd H 230 MeV 2 GeV1 GeV 561 MeV 4 GeV 96 MeV200 MeV BC2 R56  -45mm L b  1- 4ps  p  1.4% L b .2- 1ps  p  0.2% 2 Cryomodules 3 C. modules 12 C. modules 6 C. modules

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7 2. Bunch Compressor 2.1 System Details RF Chirp : position – energy correlated 3 rd Harmonic RF Section for RF nonlinear distortion correction Cavity is at decelerating phase, V h =V 0 /h 2. Use 3rd harmonic reasons : Technical and lower wake field ( W   2, W    3 ).

8 First magnet chicane bending angles is adjustable. For ps bunch length operation, chose R 56 =-122mm. The large R 56 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

Linear Compressing (0.2 ps, no wake fields, CSR etc.) Start: Hard-edge, 0.2nC, 20 ps  p=5KeV Q Variation (rms) ~ %

11 Before Chirp, Chirp phase Before 3rd harmonic linearizer

12 After 3 rd harmonic Linearizer

13 After First Chicance, L b ~2ps

14 After second chicane. L b ~ 0.2 ps.

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: => Increased Peak current ~25%Bunch Length down to ~0.3ps

17 But same Chirp phase ( ) without Wakes & CSR –> over chirped <= ~20fs,15kA peak This can’t be real.

18 Coherent Synchrotron Radiation & beam emittance growth Synchrotron radiation will be coherent if >>L bunch. 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(  L b ) 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 ? Linear System With Wakes and CSR (No Twiss matching) E=4 GeV, Chirp phase =

Various effects Chirp phase. Sensitive to less than

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%

23 Injector Bunch Electron Distribution Effect. (Example of Gaussian beam …) Linear, Chirp phase Wake, CSR See big energy spike

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!