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
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.
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 dp=5KeV • BC: Bunch Compressor Chicane • SW: Switchyard • Linac Section (TESLA Cryomodules) • 3rd H: Third Harmonic Linearizer Dp: Bunch total momentum span dp: slice momentum spread SW1 3rd H 12 C. modules 6 C. modules 3 C. modules 2 Cryomodules Chirp SW2 96 MeV 200 MeV 1 GeV 2 GeV 4 GeV 561 MeV 230 MeV
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 ).
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.
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%
Before Chirp, Chirp phase -21.40 Before 3rd harmonic linearizer
2.3 Wakes and CSR Wakes only(4GeV) Wakes + CSR(4GeV) Bunch length increased to ~0.4 ps
Chirp phase adjustment: -21.400=> -21.550 Increased Peak current ~25%Bunch Length down to ~0.3ps Adjusting Chirp phase to compensate Wakefield & CSR effects ?
But same Chirp phase (-21.550) without Wakes & CSR –> over chirped <= ~20fs,15kA peak This can’t be real.
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:
Analytic emittance growth estimation Slice analysis of emittance growth will be performed for more careful study.
CSR may do more damage to emittance ? E=4 GeV, Chirp phase =-21.40 With Wakes and CSR (No Twiss matching) Linear System
-21.650 -21.450 -21.550 • 2.4 Various effects • Chirp phase. Sensitive to less than 0.050
-21.650 -21.450 -21.550 • 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%
Injector Bunch Electron Distribution Effect. (Example of Gaussian beam …) +Wake, CSR See big energy spike Linear, Chirp phase -21.40
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.
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).
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!