High Energy Gain Helical Inverse Free Electron Laser Accelerator at Brookhaven National Laboratory J. Duris 1, L. Ho 1, R. Li 1, P. Musumeci 1, Y. Sakai 1, E. Threlkeld 1, O. Williams 1, M. Babzien 2, M. Fedurin 2, K. Kusche 2, I. Pogorelsky 2, M. Polyanskiy 2, V. Yakimenko 3 1 UCLA Department of Physics and Astronomy, Los Angeles, CA Accelerator Test Facility, Brookhaven National Laboratory, Upton, NY, SLAC National Accelerator Laboratory, Menlo Park, CA, HBEB Workshop on High Brightness Beams San Juan, Puerto Rico March 26th 2013

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

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

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

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 GW CO2 laser - 15 MeV -> 35 MeV in ~25 cm - Accelerating gradient ~70 MeV/m W. Kimura et al. PRL, 92, (2004) P. Musumeci et al. PRL, 94, (2005)

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

Experimental design ParameterValue Input e-beam energy50 Mev Final beam energy117 MeV Final beam energy spread2% rms Average accelerating gradient124 MV/m Laser wavelength10.3 μm Laser power500 GW Laser focal spot size (w) 980 μm Laser Rayleigh range25 cm Undulator length54 cm Undulator period4 – 6 cm Magnetic field amplitude5.2 – 7.7 kG Parameters for the RUBICON IFEL experiment

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.

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

Beamline layout

Timing Δt S 0 /S ref σ=7.2 ps Coarse alignment with stripline coincidence Germanium used for few ps timing Maximize interaction for fine timing S0S0 S ref NaCl Dipole e-beam Ge wafer laser

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

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 Laser-ebeam cross correlation sigma = 4.5 ps Delay (ps)

IFEL acceleration 100% energy gain *Preliminary

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

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

GeV class IFEL Strongly tapered helical undulator 20 TW Ti:Sa (800 nm) GeV IFEL Input energy100 MeV at focus100 μm Emittance0.25 mm mrad Laser spot size240 μm Rayleigh range20 cm

Prebunch for higher current Increase fraction captured by prebunching input beam uniform beam injectedprebunched beam injected

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

High current 1GeV IFEL GeV IFEL accelerates beam Harmonic prebuncher 1 kA input B = 800 nm 40 cm 1 m100 MeV 20 TW Ti:Sa 954 MeV 98% capture 18 nm rms 0.18% rms 13.5 kA peak current

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

Mode locked FEL * Thompson and McNeil, Phys. Rev. Lett., 100, (2008) Micro bunches Radiation after one undulator Slippage in chicane Radiation after next undulator slippage in one undulator slippage in one chicane 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

IFEL driven mode-locked FEL Energy954 MeV Relative energy spread0.18 % Bunching period800 nm Peak current13 kA Microbunch length (rms) 18 nm FEL wavelength5 nm Undulator period16 mm Periods per undulator16 Periods slipped per chicane 144 Total slippage160 Slippage enhancement10 Undulator + chicane segments as FWHM SpectraTemporal Pulse width controlled with number of periods per undulator mode separation number of sidebands Spectral width controlled by number periods per undulator

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

Backup

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

Tolerances Parameter scans in Genesis Energy fixed by tapering Deviate one parameter from ideal, lose particles Trapping sensitive to initial energy: Parameter20% capture10% capture Input energy MeV MeV Laser power> 440 GW> 370 GW Beam offset< 260 μm< 480 μm Peak current< 6 kA< 11 kA Rayleigh range < 30 cm< 37 cm Focal position cm cm

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

Spectrometer To Baseler camera (12-bit depth) 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 DRZ phosphor screen Mirror dipole IQ3 off IQ3 on

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.

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 Noise pedestal Signal

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