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. High Energy Gain Helical Inverse Free Electron Laser Accelerator at Brookhaven National Laboratory. - PowerPoint PPT PresentationTRANSCRIPT
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