1. Velocity bunching from S-band photoinjectors Julian McKenzie 1 st July 2011 Ultra Bright Electron Sources Workshop Cockcroft Institute STFC Daresbury Laboratory, UK

2. Introduction Normal conducting S-band RF guns are often the gun of choice for modern FELs Have provided very low emittance beams However, FELs typically require multiple stages of magnetic compression Velocity bunching schemes have been proposed for low bunch charge applications such as electron diffraction Can we apply the same techniques to 100 pC bunches to serve as an FEL driver?

3. S-band RF gun ALPHA-X / Strathclyde (TU/e + LAL) 2.5 cell, 2998.5 MHz Cu photocathode 266nm laser Courtesy Bas van der Geer, Marieke de Loos, Pulsar Physics

4. ASTRA simulations Take on-axis field-maps, feed into ASTRA Assume thermal emittance as per LCLS measurements: 0.9 mm mrad per mm (rms) of laser spot* Assume gun can achieve peak on-axis field of 100 MV/m Beam energy on exit ~6 MeV Start with low charge (10 pC) and scale up to high charge... * D.Dowell, “Unresolved emittance issues of the LCLS gun”, 5/08/2010, LBNL workshop on “Compact X-Ray FELs using High-Brightness Beams” Red = Ez field from gun Blue = Bz field of combined bucking and focussing solenoids

5. Shortest bunch from gun At the multiple-picosecond level, it is safe to assume that the bunch length from the gun is equivalent to that of the drive laser This assumption breaks down sub-ps due to space charge limitations Assumed a 0.5mm diameter beam, Gaussian temporal profile, and scanned laser pulse length. Minimum of ~250 fs electron bunch. Similar figure to results from Osaka University, K. Kan et al at IPAC’10/Linac’10

6. Shortest bunch from gun Can reduce bunch length by increasing laser diameter However, there is a trade-off with emittance Previously assumed linear correlation between laser spot size and emittance This emittance cannot be improved but bunch length can Therefore initially use 0.5 mm spot, assume best case laser, 10 fs rms

7. Add bunching cavity 2m long S-band cavity Operating at the bunching zero-cross phase 7.5 MV/m Bunch length continues to increase after gun Operate gun at -15°to help mitigate this effect Place bunching cavity as close to gun as possible Buncher Gun NB// using different gun/solenoid fieldmaps here Bunch length comes to a focus ~6m from cathode Minimum of 27 fs rms

8. Buncher cavity length RED = 1m GREEN = 2m BLUE = 3m Don’t gain anything by increasing to 3m, therefore utilise 2m long buncher

9. Capture cavity Add linac cavity at waist to capture the short bunch length For simulations used 2m long S-band cavity operating at 20 MV/m Beam energy on exit ~ 50 MeV Buncher Linac Gun

10. Buncher Linac Gun Buncher Linac Gun RED = no solenoids BLUE = solenoid around buncher and before linac GREEN = small solenoid at end of buncher Transverse focussing schemes

11. Optimisation Utilise genetic/evolutionary optimisation algorithm Multi-objective shows trade-off between transverse emittance and bunch length Uses non-dominated sorting technique, based off NSGA-II* 100 generations of 60 runs each, takes overnight *Kalyanmoy Deb et al., IEEE Transactions on Evolutionary Computation 6 (2) 2002, pp183-197

12. Optimisation parameters Laser spot size (flat-top) Laser pulse duration (Gaussian) Gun field strength Gun phase Gun solenoid strength Buncher field strength Buncher solenoid strength

13. Optimisation front @ 10 pC RED = small solenoid at buncher exit BLUE = solenoids all around buncher Buncher Gun Buncher Gun

14. Manual versus genetic optimisation Linac Buncher Gun Linac Buncher Gun

15. Manual optimisationGenetic optimisation NB// head of bunch to the right 10 pC 50 MeV

16. 100 pC optimisation RED = small solenoid at buncher exit BLUE = solenoids all around buncher Buncher Gun Buncher Gun

17. Optimisation fronts at various bunch charges RED = 10 pC GREEN = 100 pC BLUE = 250 pC

18. Selected bunches (100pC) Bunch A Bunch B ParameterABunits Emittance0.711.10mm mrad Bunch length15323fs Peak current3313340A Energy spread58187keV Energy4850MeV

19. Bunch A: 300A Bunch B: 3kA 100 pC 50 MeV NB// head of bunch to the right

20. Optimised parameters 100 pC, 50 MeV ParameterBunch ABunch Bunits Laser radius rms0.400.45mm Laser length rms14050fs Gun peak field9771.5MV/m Gun phase-2-10° Gun solenoid peak field0.2480.246T Buncher peak field15.614.6MV/m Buncher solenoid peak field0.0380.039T Linac operated on-crest, 20 MV/m

21. Bunch B to 240 MeV 6 MeV 240 MeV50 MeV Linac 0 BuncherGun Linac 1Linac 2

22. Jitter 500 runs 10,000 macroparticles per run Random jitter based on the following sigmas (cut off at 3 sigma) Parameter(s)SigmaUnit Bunch charge1pC Laser position (x,y)0.1mm Laser timing100fs RF gradients0.1% RF phases0.1° Solenoid strengths0.1% NB// all RF jitter applied individually to each cavity, similarly with solenoids (except bucking and gun solenoid locked)

23. Jitter ~ 0.2 mm mrad ~ 15 fs ~ 600 fs ~ 0.6 MeV

24. Tolerances: Arrival time

25. Summary A velocity bunching scheme was presented based around an S-band gun and followed by a 2m long S-band buncher and a further S-band capture cavity This scheme can provide 100 pC bunches to the sub-ps level, kA peak current and 1 mm mrad emittance Simulated beam parameters are capable of delivering beam to an FEL However, jitter remains a big issue