High Energy Gain Helical Inverse Free Electron Laser Accelerator at Brookhaven

National LaboratoryJ. 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 900952Accelerator Test Facility, Brookhaven National Laboratory, Upton, NY, 11973

3SLAC National Accelerator Laboratory, Menlo Park, CA, 94025

HBEB Workshop on High Brightness BeamsSan Juan, Puerto Rico

March 26th 2013

Outline

• Brief IFEL introduction• IFEL experiments• Rubicon IFEL project

o Helical undulatoro Experimental setupo Electron energy spectra

• 1 GeV IFEL concept• IFEL driven mode-locked soft x-ray FEL

IFEL interactionUndulator magnetic field couples high power

radiation with relativistic electrons

Courant, Pellegrini, and Zakowicz, Phys Rev A, 32, 2813 (1985)

Undulator parameter

Normalized laservector 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

- 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)

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 designParameter Value Input e-beam energy 50 Mev Final beam energy 117 MeV Final beam energy spread 2% rmsAverage accelerating gradient 124 MV/m Laser wavelength 10.3 μm Laser power 500 GW  Laser focal spot size (w) 980 μm Laser Rayleigh range 25 cm Undulator length 54 cm Undulator period 4 – 6 cm Magnetic field amplitude 5.2 – 7.7 kG

Parameters for the RUBICON IFEL experiment

Helical undulatorElectrons 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 Br = 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

S0/Sref

σ=7.2 ps

Coarse alignment with stripline coincidence

Germanium used for few ps timing

Maximize interaction for fine timing

S0

SrefNaCl Dipole

e-beam

Ge wafer

laser

Polarization

All shots have delay 1854 and 800 pC charge

> 5 J> 4 J< 4 J

circularpolarization

linearpolarization

circular (opposite

handedness)

circularpolarization

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 acceleration100% energy gain

*Preliminary

Looks like temporal effects at play here

Compare spectra

300 GWlow power tails?

7 GW

Deficit at 52 MeV likely from phosphor damage

Where to go from hereDoubled electron energy, now increase efficiency

o Retune undulator for higher efficiency captureo Measure transverse emittanceo Better characterize laser

Move to Ti:Sa lasero More power => higher gradiento Shorter wavelength => shorter undulator periodo >10 TW commercially availableo LLNL IFEL: world's first 800 nm driven IFEL

Neptune undulator + 4 TW Ti:Sa 50 -> 200 MeV

GeV class IFELStrongly

tapered helical undulator

20 TW Ti:Sa(800 nm)

GeV IFEL

Input energy 100 MeV

at focus 100 μm

Emittance 0.25 mm mrad

Laser spot size 240 μm

Rayleigh range 20 cm

Prebunch for higher currentIncrease fraction captured by prebunching input beam

uniform beam injected prebunched beam injected

Harmonic microbunchingHarmonic 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 = 0.95 @ 800 nm

40 cm

1 m100 MeV20 TW Ti:Sa

954 MeV98% capture

18 nm rms

0.18%rms

13.5 kA peak current

Soft x-ray FEL5 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, 203901(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 FELEnergy 954 MeV

Relative energy spread 0.18 %

Bunching period 800 nm

Peak current 13 kA

Microbunch length (rms)

18 nm

FEL wavelength 5 nm

Undulator period 16 mm

Periods per undulator 16

Periods slipped per chicane

144

Total slippage 160

Slippage enhancement 10

Undulator + chicane segments

54

266 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

0-1-2 1 2 3

Space charge effect

0 A input 1 kA input

TolerancesParameter scans in GenesisEnergy fixed by taperingDeviate one parameter from ideal, lose particles

Trapping sensitive to initial energy:

Parameter 20% capture 10% capture

Input energy 49.8 -- 53.7 MeV 49.1 -- 54.9 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 -11.8 -- 1.2 cm -16.8 -- 7.7 cm

Vertical emittance measurementMeasurements 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 off Quad 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 calibrationPosition 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 CollaborationJ. Duris, R. Li, P. Musumeci, Y. Sakai, O. WilliamsUCLA Particle Beam Physics Lab

M. Babzien, M. Fedurin, K. Kusche, I. Pogorelsky, M. PolyanskiyAccelerator Test Facility, Brookhaven National Laboratory

V. YakimenkoFACET, SLAC National Accelerator Laboratory

Special Thanks!ATF techs and UCLA machine shopLong Ho, Joshua Moody, and Evan Threlkeld