Velocity Bunching Studies at FLASH Bolko Beutner, DESY XFEL Beam Dynamics Meeting 4.6.2007

1. Velocity Bunching Studies at FLASH Bolko Beutner, DESY XFEL Beam Dynamics Meeting 4.6.2007 Bolko Beutner, DESY

2. Contents • Introduction • ASTRA simulations • Semi-Analytic Model • CSR microbunch instability studies • Experiments at FLASH • Summary and Outlook Bolko Beutner, DESY

3. Introduction • At low beam energies ~5 MeV electron velocities are not independent of the particle energy • correlated energy spread  velocity spread • bunch compression without chicanes • First cavity of ACC1 is operated at the zero crossing (-90deg off-crest) • linear correlated energy spread L. Serafini, M. Ferrario, ``Velocity Bunching in Photo-Injectors'', AIP Conference Proceedings 581, 87 (2001) Bolko Beutner, DESY

4. Introduction Bunching stops in the second cavity since the energy increases and relativistic velocities are reached. The last cavities in the module can be used to compensate the correlated energy spread. velocity bunching compensation of correlated energy spread Velocity bunching is an additional “knob” to optimise the bunch compression system. Bolko Beutner, DESY

5. Introduction J.P. Carniero (2004) : FLASH simulations and first experiments Bolko Beutner, DESY

6. Introduction M. Krasilnikov (2007) : XFEL studies including compensation of correlated energy spread Bolko Beutner, DESY

7. ASTRA Simulations ASTRA simulations based on J.P. Carnieros files (q=0.5nC; 10k particles) Bolko Beutner, DESY

8. ASTRA Simulations At full compression emittance after solenoid optimisation is 4.5mm mrad after ACC1. Nonlinear compression longitudinal SpCh forces Bolko Beutner, DESY

9. ASTRA simulations Nonlinear bunch compression for a linear correlated energy spread due to a nonlinear dependence of the velocity on energy. Drift length Bolko Beutner, DESY

10. ASTRA Simulations • New Gun (thanks to Jang-Hui Han) • 10k particles, Nrad=24, Nlong=64 Solenoid optimsed to B=0.165T Q=0.5nC initial bunch length = 1.5mm 250A Bolko Beutner, DESY

11. Compensation of the correlated energy spread with ACC1.C5-C8 ACC1.C1 phase = -90 deg B solenoid = 0.165T Minimal energy spread (58.2 keV) at ACC1.C5-C8 phase = 17.11deg ASTRA Simulations Bolko Beutner, DESY

12. ASTRA Simulations 0.75m 3.72m Bolko Beutner, DESY

13. ASTRA Simulations 14.00m 4.56m Bolko Beutner, DESY

14. Comparison between the 32bit and 64bit paralell version of ASTRA gives an agreement better than a percent. ASTRA Simulations Bolko Beutner, DESY

15. Semi-Analytic Model d v 1 u ( ) ¯ ± 1 ¡ d u = 2 ( ) ³ ´ l l ¯ ± t + ± E E + c = f i 0 0 ( ) ¯ 0 c m 0 ( ) ¯ ± ! 2 l l d l ± ± R T + + + + ¢ ¢ ¢ = = f i i 5 6 5 6 6 ( ) ¯ 0 Scans with ASTRA to find a reasonable gradient and phase settings of the modules are time consuming => preliminary optimisation is done in an semi-analytic model longitudinal position after a low energy drift space: with drift length and velocity Bolko Beutner, DESY

16. Semi-Analytic Model ( ) ¯ ± ± 2 d m 0 R ) = 5 6 ³ ´ 2 m 2 E 1 0 ¡ 2 0 E 0 4 2 d d 3 m m 0 0 T ¡ ¡ = 5 6 6 2 ³ ´ 2 ³ ´ 2 m 2 E 2 1 0 m 4 ¡ E 2 1 0 ¡ 2 0 E 2 0 E 0 0 is expressed in a Taylor-series around zero in : Bolko Beutner, DESY

17. Semi-Analytic Model • Beam from the ASTRA simulations at the entrance of ACC1 • The cavities are divided into subintervals in which the effective longitudinal dispersion is applied • Fraction of RF-voltage is applied at the intersections Bolko Beutner, DESY

18. Semi-Analytic Model ACC1.C1 divided into 100 segments initial rms bunch length = 1.2mm ; GUN energy gain = 4.7MeV ACC1.C1 gradient = 26MV/m 0deg (blue) -> -100deg (dark read) Bolko Beutner, DESY

19. Longitudinal phase space (energy [MeV] vs. longitudinal position [mm]) Semi-Analytic Model Bolko Beutner, DESY

20. Semi-Analytic Model ACC1.C2-C4 on-crest ACC1.C5-C8 -10deg off-crest => bunch length is mapped to correlated energy spread Bolko Beutner, DESY

21. CSR microbunch instability studies LiTrack Input T. Limberg Bolko Beutner, DESY

22. Longitudinal Phase Space @ End ACC1 CSR microbunch instability studies T. Limberg Bolko Beutner, DESY

23. Longitudinal Phase Space @ End BC2 CSR microbunch instability studies T. Limberg Bolko Beutner, DESY

24. Longitudinal Phase Space @ LOLA CSR microbunch instability studies 1.8kA in BC3 with a linear energy chirp lead to a strong microbunch instability gain Corresponding slice emittance growth should be observable at LOLA T. Limberg Bolko Beutner, DESY

25. Experiments at FLASH • Modifications of the ACC1.C1 phase offset and the phase offset between ACC1.C1-C4 (up to about 100deg) and ACC1.C5-C8 (up to about 90deg) (Huening) • Indirect measurements of compression using the energy spread in BC2 with ACC1.C5-C8 off-crest • Pyro detector compression measurements ot the DBC2 diffraction radiator or TOSYLAB Bolko Beutner, DESY

26. Experiments at FLASH • Measurements of bunch length vs. ACC1.C1 phase with the streak camera at TOSYLAB using synchrotron radiation from BC2 (Grimm) • Setup of beam transport to LOLA with chicanes off • Measurements of the emittance in DBC2 FODO section • Bunch length measurements at LOLA • CSR microbunch instability studies Bolko Beutner, DESY

27. Experiments at FLASH Requirements: • For the measurements of emittance the BC2 dipoles should be switched off to prevent further compression and CSR effects • Streak camera measurements require the minimisation of the correlated energy spread to avoid additional compression in BC2 • Bunch length measurements with LOLA require transport with proper optics to LOLA while the bunch compressors are switched off (lower total energy and different phase advance) Bolko Beutner, DESY

28. Summary and Outlook • Velocity bunching operation is possible at FLASH • Compression process is analysed with ASTRA and an semi-analytic model • Beam time is requested for measurements Next Steps: • further preparations of Measurements (setup of streak camera, RF-manipulations etc.) • Detailed simulations on CSR microbunch instability studies • Measurements • Testing and optimising of different VB setups (ACC1 gradients and phase offsets) • Development of an optimised compression scheme for FLASH and XFEL including VB Bolko Beutner, DESY

29. Thank You Bolko Beutner, DESY