1. Twin bunches at FACET-II Zhen Zhang, Zhirong Huang, Ago Marinelli … FACET-II accelerator physics workshop Oct. 12, 2015

2. 2 Outline Twin bunches at FACET-II  Twin bunches at the LCLS  2-color free-electron lasers  Wakefield effect in the compression of the twin bunches  Increase the time delay  Preliminary simulations of the twin bunches for the FACET-II  Larger charge, higher peak current and how to control the time delay  Summary

3. 3 Twin bunches & Two bunches Twin bunches at FACET-II Born/accelerated in the same RF bucket Initial time delay ~ 10ps Final time delay ~ 10s-100s fs Delay compression of ~200 Twin bunches Two bunches Born/accelerated in different RF buckets Initial time delay ~ several ns Final time delay ~ several ns Almost no delay compression

4. Twin bunches at the LCLS 4 Photocathode Laser Pulse Adjustable delay stage Double Pulse Electron Gun Linac Few ps delay Bunch Compressor 1 Bunch Compressor 2 Few fs delay ~1% energy separation UNDULATOR time Energy Splitter A. Marinelli et al., Nat. Commun. 6, 6369 (2015) T. Maxwell Power ~35 fs 10 fs E photon Power 35 eV T. Maxwell

5. 5 Wakefield effects  Wakefields in the accelerating structure complicates the compression of the twin bunches. Bunch profile Wake-induced energy loss Two effects: 1.Reduce the energy chirp over the two bunches 2.Reduce the “local” chirp of each single bunch If we fix the compression factor (or the final peak current), the time delay of the twin bunches will be Z. Zhang et al., PRST-AB 18, 030702 (2015)

6. 6 Twin bunches at FACET-II Twin bunches for FACET-II  The twin bunches at FACET-II require larger charge and higher peak current. ParametersLCLSFACET-II Charge75pC + 75pC1nC + 1nC Peak current~4kA~10kA Emittance? Time delay10fs~130fs>~100fs Energy spreadSmall for self- seeding ? BC2 4.5 GeV 10 GeV BC1 335 MeV L1S L1X L2-linac L3-linac DL1 135 MeV L0Gun R 56 =-36mm R 56 =-48mm  The layout and parameters of the beamline in the LiTrack simulation. The numbers listed here are fixed and we vary the phases of L1,L2 and L1X, and the initial distribution of the two bunches.

7. 7 Twin bunches at FACET-II Parameters range in the simulations ParameterValueScan step size Charge1nC + 1nC Initial delay6ps -10ps Profile of single bunch parabolic Phase of L1 Phase of L1X Amplitude of L1X0 ~ 25MeV0.5MeV Phase of L2 Phase of L3  For a specific initial distribution of the two bunches, we scan the phases of L1 and L2, the amplitude of L1X to vary the compression in the LiTrack simulation.

8. 8 Simulation results Twin bunches at FACET-II  If we require the final peak current >10kA, we can pick out the “good settings” in the scan results and show them in the histogram with energy separation and time delay.  Case 1: 1nC + 1nC, Initial time delay 6ps, peak current 250A

9. 9 Simulation results (time delay 100fs ~ 500fs)  Case 1: 1nC + 1nC, Initial time delay 6ps, peak current 250A

10. 10 Simulation results  Case 2: 1nC + 1nC, Initial time delay 8ps, peak current 190A When we scale the beam profile to 8ps and 190A, the final time delay ranges from 200fs to 1.2ps.

11. 11 Simulation results  Case 3: 1nC + 1nC, Initial time delay 10ps, peak current 150A When we scale the beam profile to 10ps and 150A, the final time delay ranges from 400fs to 1.4ps.

12. 12 Comments and summary  From the analysis in the reference (PRST-AB 18, 030702), wakefield effect will increase the final time delay of the twin bunches.  During the simulations for FACET-II, we find the wakefield effects are very strong and the final time delay will become large. It is easy to get >400fs, even ~ps time delay. In order to cover the small delay (~100fs), we have to optimize the initial distribution (let the two bunches initially have some overlap).  Considering the length of single bunch is ~100fs (1nC, 10kA), the two bunches will overlap in the time domain if the delay is too small (<100fs).  The simulation results show that the twin-bunch method can cover the range of the time delay from 100fs to several ps.  The two bunches cannot have the same energy and also there is always large remaining energy chirp. This is because the wakefield at L3 is very strong due to the high peak current (~10kA) and long accelerating tubes.

13. 13 Questions and comments Twin bunches at FACET-II Thanks !