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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.

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hbeb workshop on high brightness beams san juan puerto rico march 26th 2013
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

  • 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
ifel interaction
Energy exchanged between laser and electrons maximized when resonant condition is satisfiedIFEL 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)

ifel characteristics
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
IFEL experiments

STELLA2 at Brookhaven

- Gap tapered undulator

- 30 GW CO2 laser

- 80% of electrons accelerated


- 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)

r adiabeam u cla b nl i fel c ollaborati on rubicon
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
Experimental design

Parameters for the RUBICON IFEL experiment

helical undulator
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
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



Coarse alignment with stripline coincidence

Germanium used for few ps timing

Maximize interaction for fine timing

σ=7.2 ps



Ge wafer







> 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 (opposite handedness)



All shots have delay 1854 and 800 pC charge

*Preliminary data

laser ebeam cross correlation
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)

ifel acceleration
IFEL acceleration

100% energy gain


compare spectra
Compare spectra

Looks like temporal effects at play here

low power tails?

300 GW

7 GW

Deficit at 52 MeV likely from phosphor damage

where to go from here
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
gev class ifel
GeV class IFEL

Strongly tapered helical undulator

20 TW Ti:Sa

(800 nm)


prebunch for higher current
Prebunch for higher current

Increase fraction captured by prebunching input beam

uniform beam injected

prebunched beam injected

harmonic microbunching
Linearize ponderomotive force by coupling electrons to harmonics of the drive laserHarmonic 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.

high current 1gev ifel
High current 1GeV IFEL

B = 0.95 @ 800 nm



1 kA input

40 cm

18 nm rms

GeV IFEL accelerates beam



100 MeV

20 TW Ti:Sa

1 m

954 MeV

98% capture

13.5 kA peak current

soft x ray fel
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

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)

ifel driven mode locked fel
IFEL driven mode-locked FEL



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


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
space charge effect
Space charge effect
  • Genesis cannot do harmonic microbunching so solve DE's
  • Periodic boundary conditions implemented by cloning particles periodically

cloned particles







particle modeled as disc of charge





1 kA input

0 A input

field of disc of charge


Parameter scans in Genesis

Energy fixed by tapering

Deviate one parameter from ideal, lose particles

Trapping sensitive to initial energy:

vertical emittance measurement
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


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)


DRZ phosphor screen

IQ3 off


IQ3 on

preliminary spectrometer calibration
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
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

rubicon collaboration
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