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Velocity bunching from S-band photoinjectors

This summary discusses velocity bunching from S-band photoinjectors for FEL applications, focusing on low bunch charge schemes and optimization techniques using genetic algorithms. The presentation covers gun and cavity designs, bunch length control, and beam optimization for high current, low emittance beams. A velocity bunching scheme is proposed for generating 100 pC bunches suitable for FEL drivers, with simulations showing potential for FEL beam delivery. However, challenges related to jitter control are highlighted. The workshop was held at the Cockcroft Institute, STFC Daresbury Laboratory in the UK on July 1st, 2011.

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Velocity bunching from S-band photoinjectors

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  1. Velocity bunching from S-band photoinjectors Julian McKenzie 1st 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... Red = Ez field from gun Blue = Bz field of combined bucking and focussing solenoids * D.Dowell, “Unresolved emittance issues of the LCLS gun”, 5/08/2010, LBNL workshop on “Compact X-Ray FELs using High-Brightness Beams”

  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 fsrms

  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 Bunch length comes to a focus ~6m from cathode Minimum of 27 fsrms NB// using different gun/solenoid fieldmaps here

  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 Linac Buncher Gun

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

  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 Buncher Gun BLUE = solenoids all around buncher Buncher Gun

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

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

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

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

  18. Selected bunches (100pC) Bunch A Bunch B

  19. 100 pC50 MeV Bunch B: 3kA Bunch A: 300A NB// head of bunch to the right

  20. Optimised parameters • 100 pC, 50 MeV Linac operated on-crest, 20 MV/m

  21. 240 MeV Bunch B to 240 MeV Linac 1 Linac2 50 MeV Linac 0 6 MeV Buncher Gun

  22. Jitter • 500 runs • 10,000 macroparticles per run • Random jitter based on the following sigmas (cut off at 3 sigma) 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

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