High Energy Gain Helical Inverse Free Electron Laser Accelerator at Brookhaven National Laboratory

1. HBEB Workshop on High Brightness Beams San Juan, Puerto Rico March 26th 2013 High Energy Gain Helical Inverse Free Electron Laser Accelerator at Brookhaven National Laboratory J. Duris1, L. Ho1, R. Li1, P. Musumeci1, Y. Sakai1, E. Threlkeld1, O. Williams1, M. Babzien2, M. Fedurin2, K. Kusche2, I. Pogorelsky2, M. Polyanskiy2, V. Yakimenko3 1UCLA Department of Physics and Astronomy, Los Angeles, CA 90095 2Accelerator Test Facility, Brookhaven National Laboratory, Upton, NY, 11973 3SLAC National Accelerator Laboratory, Menlo Park, CA, 94025

2. Outline • Brief IFEL introduction • IFEL experiments • Rubicon IFEL project • Helical undulator • Experimental setup • Electron energy spectra • 1 GeV IFEL concept • IFEL driven mode-locked soft x-ray FEL

3. Energy exchanged between laser and electrons maximized when resonant condition is satisfied IFEL interaction Undulator magnetic field couples high power radiation with relativistic electrons Undulator parameter Normalized laser vector potential Courant, Pellegrini, and Zakowicz, Phys Rev A, 32, 2813 (1985)

4. IFEL characteristics • Inverse Free Electron Laser accelerators suitable for mid to high energy range compact accelerators • Laser acceleration => high gradients • Vacuum acceleration => preserves output beam quality • Energy stability => output energy defined by undulator • Microbunching => manipulate longitudinal phase space at optical scale • Interest lost as synchrotron losses limit energy to few GeV (so no IFEL based ILC) • Recent renewed interest in compact GeV accelerator for light sources

5. IFEL experiments STELLA2 at Brookhaven - Gap tapered undulator - 30 GW CO2 laser - 80% of electrons accelerated UCLA Neptune IFEL - Strongly tapered period and amplitude planar undulator - 400 GW CO2 laser - 15 MeV -> 35 MeV in ~25 cm - Accelerating gradient ~70 MeV/m W. Kimura et al. PRL, 92, 054801 (2004) P. Musumeci et al. PRL, 94, 154801 (2005)

6. Radiabeam-UCLA-BNL IFEL CollaboratiON RUBICON Unites the two major groups active in IFEL • Past experience: UCLA Neptune, BNL STELLA 2 • Builds off UCLA Neptune experiment: strong tapering + helical geometry for higher gradient Collaboration paves the way for future applications • Higher gradient IFEL • Inverse Compton scattering • Soft x-ray FEL

7. Experimental design Parameters for the RUBICON IFEL experiment

8. Helical undulator Electrons always moving in helix so always transferring energy. Helical yields at least factor of 2 higher gradient. Especially important for higher energy (high K) IFEL's.

9. Helical undulator design • First strongly tapered high field helical undulator • 2 orthogonal Halbach undulators with varying period and field strength • NdFeB magnets Br = 1.22T • Entrance/exit periods keep particle oscillation about axis • Pipe of 14 mm diameter maintains high vacuum and low laser loses Estimated particle trajectories Laser waist

10. Beamline layout

11. Timing S0/Sref Coarse alignment with stripline coincidence Germanium used for few ps timing Maximize interaction for fine timing σ=7.2 ps Δt laser Ge wafer NaCl Dipole S0 Sref e-beam

12. Polarization > 5 J > 4 J < 4 J Quarter wave plate polarizes CO2 elliptically before amplification One handedness matches undulator 0°, 4.6 J 30°, 4.4 J 60°, 5.52 J 90°, 6.11 J 180°, 4.5 J circular polarization linear polarization circular (opposite handedness) circular polarization All shots have delay 1854 and 800 pC charge *Preliminary data

13. Laser-ebeam cross correlation Cross correlation measurement of laser and 1 ps long e-beam using IFEL acceleration as a benchmark Gradient scales proportional to the square root of the laser power so scale momenta Estimated rms pulse width < 4.5 ps sigma = 4.5 ps Delay (ps)

14. IFEL acceleration 100% energy gain *Preliminary

15. Compare spectra Looks like temporal effects at play here low power tails? 300 GW 7 GW Deficit at 52 MeV likely from phosphor damage

16. Where to go from here Doubled electron energy, now increase efficiency • Retune undulator for higher efficiency capture • Measure transverse emittance • Better characterize laser Move to Ti:Sa laser • More power => higher gradient • Shorter wavelength => shorter undulator period • >10 TW commercially available • LLNL IFEL: world's first 800 nm driven IFEL • Neptune undulator + 4 TW Ti:Sa • 50 -> 200 MeV

17. GeV class IFEL Strongly tapered helical undulator 20 TW Ti:Sa (800 nm) GeV IFEL

18. Prebunch for higher current Increase fraction captured by prebunching input beam uniform beam injected prebunched beam injected

19. Linearize ponderomotive force by coupling electrons to harmonics of the drive laser Harmonic microbunching monochromatic prebunched input harmonic prebunched input Harmonic microbunching further enhances capture and reduces energy spread of accelerated beam by increasing bunching of prebunched beam.

20. High current 1GeV IFEL B = 0.95 @ 800 nm Harmonic prebuncher 1 kA input 40 cm 18 nm rms GeV IFEL accelerates beam 0.18% rms 100 MeV 20 TW Ti:Sa 1 m 954 MeV 98% capture 13.5 kA peak current

21. Soft x-ray FEL 5 nm SASE FEL saturates in 10 m with constant current beam But IFEL beam is microbunched Requires 50 times longer to saturate with a constant undulator => ~500 m effective gain length! Some dielectric accelerators have similar bunch trains

22. Mode locked FEL slippage in one undulator • Mode locked FEL's produce short pulses with controllable bandwidth* • Microbunched beam acts as a periodic lasing medium similar to a ring resonator • Can enhance slippage by using chicanes so that pulses always see gain medium • Slippage provided by chicanes between gain sections introduces mode coupling • Periodic resonance condition controlled by energy or current modulation Micro bunches Radiation after one undulator Slippage in chicane Radiation after next undulator slippage in one chicane * Thompson and McNeil, Phys. Rev. Lett., 100, 203901(2008)

23. IFEL driven mode-locked FEL Temporal Spectra mode separation 266 as FWHM number of sidebands Pulse width controlled with number of periods per undulator Spectral width controlled by number periods per undulator

24. Summary Rubicon helical IFEL experiment at BNL • Observed polarization dependence • Doubled e-beam energy: >50 MeV gain • High gradient ~100 MeV/m Interest in IFEL's renewed for compact light source applications • GeV IFEL possible with helical undulator and 20 TW Ti:Sa laser • Natural compact driver for mode-locked soft x-ray FEL

25. Backup

26. Space charge effect • Genesis cannot do harmonic microbunching so solve DE's • Periodic boundary conditions implemented by cloning particles periodically cloned particles -1 2 -2 1 0 3 particle modeled as disc of charge laser wavelength laser wavelength 1 kA input 0 A input field of disc of charge

27. Tolerances Parameter scans in Genesis Energy fixed by tapering Deviate one parameter from ideal, lose particles Trapping sensitive to initial energy:

28. Vertical emittance measurement Measurements of vertical width of beam for different quad strengths allows calculation of vertical emittance. Quad IQ3 off Quad IQ3 maxed (10 amp) sigma = 3.4 pix or 360 um sigma = 4.5 pix or 470 um

29. Spectrometer Accepts 50 MeV to 120 MeV Energy resolution limited by beam size on screen Adding quad between undulator and spectrometer reduces rms beam size from 560um to 230um To Baseler camera (12-bit depth) Mirror DRZ phosphor screen IQ3 off dipole IQ3 on

30. Preliminary spectrometer calibration Position on screen depends on particle's radius of curvature in the bend. included in fit excluded from fit Above: spectrometer dipole field is linear in the current up to 6 amps Right: snapshots of beam positions during a dipole current sweep.

31. Figure of merit: charge • Median filter with 1 pixel radius to remove salt & pepper artifacts • Estimate noise pedestal with inactive region • Subtract noise pedestal mean from signal • Cut pixels in signal region with charge less than 5 * noise pedestal width Signal Noise pedestal

32. Rubicon Collaboration J. Duris, R. Li, P. Musumeci, Y. Sakai, O. Williams UCLA Particle Beam Physics Lab M. Babzien, M. Fedurin, K. Kusche, I. Pogorelsky, M. Polyanskiy Accelerator Test Facility, Brookhaven National Laboratory V. Yakimenko FACET, SLAC National Accelerator Laboratory Special Thanks! ATF techs and UCLA machine shop Long Ho, Joshua Moody, and Evan Threlkeld