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 90095 2 Accelerator Test Facility, Brookhaven National Laboratory, Upton, NY, 11973 3 SLAC National Accelerator Laboratory, Menlo Park, CA, 94025 HBEB Workshop on High Brightness Beams San Juan, Puerto Rico March 26th 2013

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Outline

Brief IFEL introduction IFEL experiments Rubicon IFEL project

o o o Helical undulator Experimental setup 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 Undulator parameter Normalized laser vector potential Energy exchanged between laser and electrons maximized when resonant condition is satisfied Courant, Pellegrini, and Zakowicz, Phys Rev A, 32, 2813 (1985)

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 W. Kimura et al. PRL

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92, 054801 (2004) 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 P. Musumeci et al. PRL

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94, 154801 (2005)

R

adiabeam-

U

CLA-

B

NL

I

FEL

C

ollaborati

ON

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

Parameter Input e-beam energy Final beam energy Final beam energy spread Average accelerating gradient Laser wavelength Laser power Laser focal spot size (w) Laser Rayleigh range Undulator length Undulator period Magnetic field amplitude Value 50 Mev 117 MeV 2% rms 124 MV/m 10.3 μm 500 GW 980 μm 25 cm 54 cm 4 – 6 cm 5.2 – 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 Estimated particle trajectories Laser waist

Beamline layout

laser

Timing

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

Polarization

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

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 sigma = 4.5 ps Delay (ps) Estimated rms pulse width < 4.5 ps

IFEL acceleration

100% energy gain *Preliminary

Compare spectra

Looks like temporal effects at play here low power tails?

7 GW Deficit at 52 MeV likely from phosphor damage 300 GW

Where to go from here

Doubled electron energy, now increase efficiency o o o Retune undulator for higher efficiency capture Measure transverse emittance Better characterize laser Move to Ti:Sa laser o o More power => higher gradient Shorter wavelength => shorter undulator period o o >10 TW commercially available 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 energy at focus Emittance Laser spot size Rayleigh range 100 MeV 100 μm 0.25 mm mrad 240 μm 20 cm

Prebunch for higher current

Increase fraction captured by prebunching input beam uniform beam injected prebunched beam injected

Harmonic microbunching

Harmonic microbunching further enhances capture and reduces energy spread of accelerated beam by increasing bunching of prebunched beam.

Linearize ponderomotive force by coupling electrons to harmonics of the drive laser monochromatic prebunched input harmonic prebunched input

High current 1GeV IFEL

B = 0.95 @ 800 nm

Harmonic prebuncher

1 kA input

GeV IFEL

accelerates beam 100 MeV 20 TW Ti:Sa 1 m 40 cm 18 nm rms 0.18% rms 954 MeV 98% capture 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

• • • • • 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 undulator slippage in one chicane * Thompson and McNeil, Phys. Rev. Lett., 100, 203901(2008)

IFEL driven mode-locked FEL

Energy Relative energy spread 954 MeV 0.18 % Bunching period Peak current Microbunch length (rms) FEL wavelength 800 nm 13 kA 18 nm Undulator period Periods per undulator Periods slipped per chicane Total slippage Slippage enhancement 5 nm 16 mm 16 144 160 10 Undulator + chicane segments 54 Temporal 266 as FWHM Spectra mode separation number of sidebands Pulse width controlled with number of periods per undulator 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

Space charge effect

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

Tolerances

Parameter scans in Genesis Energy fixed by tapering Deviate one parameter from ideal, lose particles Trapping sensitive to initial energy: Parameter Input energy 20% capture 10% capture 49.8 -- 53.7 MeV 49.1 -- 54.9 MeV Laser power Beam offset > 440 GW < 260 μm Peak current < 6 kA Rayleigh range Focal position < 30 cm -11.8 -- 1.2 cm > 370 GW < 480 μm < 11 kA < 37 cm -16.8 -- 7.7 cm

Vertical emittance measurement

Measurements of vertical width of beam for different quad strengths allows calculation of vertical emittance.

Quad IQ3 off sigma = 3.4 pix or

360 um

Quad IQ3 maxed (10 amp) sigma = 4.5 pix or

470 um

Spectrometer

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

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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

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