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Bates XFEL Linac and Bunch Compressor Dynamics

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

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Bates XFEL Linac and Bunch Compressor Dynamics

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  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 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

  4. 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 (W2, W  3 ).

  5. 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.

  6. Magnet Chicane parameters

  7. 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%

  8. Before Chirp, Chirp phase -21.40 Before 3rd harmonic linearizer

  9. After 3rd harmonic Linearizer

  10. After First Chicance, Lb~2ps

  11. After second chicane.Lb~ 0.2 ps.

  12. 2.3 Wakes and CSR Wakes only(4GeV) Wakes + CSR(4GeV) Bunch length increased to ~0.4 ps

  13. 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 ?

  14. But same Chirp phase (-21.550) without Wakes & CSR –> over chirped <= ~20fs,15kA peak This can’t be real.

  15. 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:

  16. Analytic emittance growth estimation Slice analysis of emittance growth will be performed for more careful study.

  17. CSR may do more damage to emittance ? E=4 GeV, Chirp phase =-21.40 With Wakes and CSR (No Twiss matching) Linear System

  18. -21.650 -21.450 -21.550 • 2.4 Various effects • Chirp phase. Sensitive to less than 0.050

  19. -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%

  20. Injector Bunch Electron Distribution Effect. (Example of Gaussian beam …) +Wake, CSR See big energy spike Linear, Chirp phase -21.40

  21. 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.

  22. 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).

  23. 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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