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
HBEB Workshop on High Brightness Beams
San Juan, Puerto Rico
March 26th 2013
Outline
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Brief IFEL introduction
IFEL experiments
Rubicon IFEL project
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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
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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, 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, 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 design
Parameter
Value
Input e-beam energy
50 Mev
Final beam energy
117 MeV
Final beam energy spread
2% rms
Average 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 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 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
Beamline layout
Timing
S0/Sref
Coarse alignment with stripline
coincidence
Germanium used for few ps timing
σ=7.2 ps
Maximize interaction for fine timing
Δt
laser
Ge wafer
NaCl
Dipole
e-beam
S0
Polarization
0°, 4.6 J
Quarter wave plate polarizes CO2
elliptically before amplification
One handedness matches undulator
30°, 4.4 J
60°, 5.52 J
>5J
>4J
<4J
90°, 6.11 J
180°, 4.5 J
All shots have delay 1854
and 800 pC charge
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
sigma = 4.5 ps
Gradient scales proportional to the
square root of the laser power so
scale momenta
Estimated rms pulse width < 4.5 ps
Delay (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 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 energy
at focus
100 MeV
100 μm
Emittance
0.25 mm mrad
Laser spot size
240 μm
Rayleigh range
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
40 cm
18 nm rms
GeV IFEL
accelerates beam
0.18%
rms
100 MeV
20 TW Ti:Sa
1m
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
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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
slippage in
one undulator
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
Energy
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
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
Summary
Rubicon helical IFEL experiment at BNL
Observed polarization dependence
Doubled e-beam energy: >50 MeV gain
High gradient ~100 MeV/m
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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
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Backup
Space charge effect
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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
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 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
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
Mirror
To Baseler
camera (12-bit
depth)
DRZ phosphor
screen
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.
Figure of merit: charge
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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