Very high energy gain at the Neptune IFEL experiment · Period 1.5 cm 5.0 cm Field Amplitude 0.12 T...
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Very high energy gain Very high energy gain at the Neptune IFEL experimentat the Neptune IFEL experiment
P. MusumeciAdvanced Accelerator Concepts
Stony Brook, NYJune 25th 2004
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OutlineOutline
• Brief IFEL introduction• Inverse-Free-Electron-Laser accelerators around
the world• Neptune IFEL project
– Overview of Neptune Laboratory setup• Electron beam• Kurchatov Undulator• CO2 laser
• Experimental results • Conclusion
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CollaboratorsP. Musumeci, S. Boucher, A. Doyuran, R.J. England, C.Pellegrini,
J.B. Rosenzweig, G.Travish, R. YoderUCLA Department of Physics and Astronomy
S. Tochitsky, C. Clayton, C. Joshi, J. Ralph, C. Sung
UCLA Department of Electrical Engineering
S.Tolmachev, A. Varfolomeev Jr., A. Varfolomeev, T. Yarovoi
RRCKI Kurchatov Institute
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IFEL InteractionIFEL InteractionUndulator magnetic field to couple high power radiation with relativistic electrons
Br
Erx
yz
kr
wmckeBK =
kmceEKl 2
0=
Undulator magnet
Laser and electron beam
22 1
2 2w Kλγλ
≅ ⋅ + ⋅
Significant energy exchange between the particles and the wave happens when the resonance condition is satisfied.
Relative strength
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IFEL characteristics: IFEL characteristics: a solid and reliable Advanced a solid and reliable Advanced
AcceleratorAccelerator• Laser accelerator: high gradients • Vacuum accelerator: good output beam quality• Microbunching: control and manipulation of beams at the optical
scale• Efficient mechanism to transfer energy from laser to electrons• State of the art requirements on laser and magnet technology• Synchrotron losses at high energy (can be controlled by
appropriate tapering of undulator)• Gradient is energy dependent
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IFEL ExperimentsIFEL Experiments• IFELA: Wernick & Marshall 1992 (PRA, 46, 3566)
– First proof-of-principle IFEL experiment– 5 MW at λ = 1.6 mm, gradient 0.7 MV/m, gain 0.2 MeV
• BNL-IFEL: Van Steenbergen, Gallardo et al. 1996 (PRL 77, 2690)– Microbunching observed 1998 (PRL, 80 4418)– 1-2 GW at λ = 10.6 µm, gradient 2.5 MV/m, gain 1 MeV
• MIFELA: Yoder, Marshall, Hirshfield 2001 (PRL, 86, 1765)– All electrons accelerated, phase dependency of the acceleration– 6 MW at λ = 10 cm, gradient 0.43 MV/m, gain 0.35 MeV
• STELLA: Kimura et al. 2001 (PRL, 86, 4041)– First staging of two IFEL modules. – 0.1-0.5 GW at λ = 10.6 µm, gain up to 2 MeV
• STELLA 2 : Kimura et al. 2003 (PRL, 92, 054801)– Monoenergetic laser acceleration (80 % of electrons accelerated, energy
spread less than 0.5 % FWHM)– ~30 GW,at λ = 10.6 µm, gain up to 17 % of initial beam energy
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MotivationMotivation• Proof-of-principle experiments successful• Upgrade to significant gradient and energy gain
– Technical challenges: • very high power radiation• strong undulator tapering
– Physics problems:• include diffraction effects in the theory• beyond validity of period-averaged classical
FEL equation• The Neptune Laboratory at UCLA has a high-power
laser and a high-brightness electron beam
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Experimental LayoutExperimental Layout
E- beam
Laser beam
Vacuum translation stages insert in the middle of the undulator a probe for spatio-temporal alignment
High energy spectrometer
IFEL vacuum box
Kurchatov strongly tapered undulator
Final focus large aperture quadrupole magnets
To streak camera diagnostics
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Neptune IFEL Design ParametersNeptune IFEL Design ParametersLaser Power 400 GW
Laser wavelength 10.6 µm
Laser beam size (w0) 340 µm
Rayleigh range 3.5 cm
Energy 14.5 MeV
Energy spread (rms) 0.5 %
Charge 300 pC
Pulse length (rms) 4 ps Rms transverse
Emittance 10 µ
Rms beam size at the focus 150 µm
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ee--beam deliverybeam delivery
• New quadrupole magnets with larger (2.625”) aperture to avoid CO2 clipping– Up to 7 T/m– 9.1 cm effective length
The fringe field has a correction of ~ 10% on the radius of curvature
22 24 26 28 30 32 340
1x107
2x107
3x107
4x107
5x107
6x107
7x107
# of
cou
nts
(a.u
.)
Pressure in Cherenkov cell (psi)
14.5 MeV 13.9 MeV 13.2 MeV
• Cherenkov cell– Vary CO2 pressure to find
threshold for Cherenkovemission
• Calibration of 45 º two dipoles spectrometer– Alignment errors– Fringe field issue
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KurchatovKurchatov IFEL UndulatorIFEL Undulator
• Unique “double tapered” 50 cm long undulator. – Final resonant energy 250 % bigger than
initial • Hall Probe measurements. • Pulse Wire tuning.
Initial Final
Period 1.5 cm 5.0 cmField
Amplitude 0.12 T 0.6 T
Peak K parameter 0.2 2.8
gap 12 mm 12 mm
0 100 200 300 400 500-8
-6
-4
-2
0
2
4
6
8
B (k
Gau
ss)
z (mm)
Hall probe scan
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Diffraction Dominated InteractionDiffraction Dominated Interaction
2
2
( ) sin( )2( )1
l
w
r
kKK JJ Kz z z
z
γ ψγ
∂= ⋅
∂ −+
1/2 222
2
2
1( )11 ( ) cos( )
2 21
wwl
rlr
kk kz z zKK zKK JJ K z
ψ
ψ
γ
∂= + − −
∂ − ++ + + ⋅ ⋅ −
0 100 200 300 400 500
-0.003
-0.002
-0.001
0.000
0.001
0.002
Beam centroid trajectory CO2 beam size ( w0 )
X (m
m)
Z (mm)0.0 0.1 0.2 0.3 0.4 0.5
20
40
60
80
100
120
Ele
ctro
n en
ergy
(γ)
Distance along the undulator ( m )
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DiagnosticsDiagnostics• Charge
– ICT in the folding box and Faraday Cup at the end of beamline.
• Energy spectrometer – Browne and Buechner geometry
to have the energy spectrum over as wide a range of energies as possible.
– 1.5” pole gap for CO2 dump– 11º edge angle for controlling
vertical size of the beam• Spatial and temporal
synchronization– Mid-of undulator graphite-coated
phosphorous screen– Ge crystal on the beam path for
temporal synchronization using e-beam controlled transmission of CO2
Experimental 2d field map of Browne-Buechner poles
200 400 600 800
0
2
4
6
8
10
12
x0 459.4±4.9σt 91.0±11.2a.
u.
Time delay (ps)
Measured cross-correlation curve fit reconstructed CO2 laser pulse
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Pulse propagation in a COPulse propagation in a CO22 amplifieramplifier
0
50
100
150
200
250
0 5 10 15 20
Pulse delay in the MARS amplifier
Tim
e de
lay,
ps
Amplification - g x L, m -1
nactive medium = nres + nnonres
Classical Vgr = c/(n+νdn/dν)
nnonres
= 1+ 2π Nmα m +m
∑ ∆nvibr − ∆ne
m = CO2 , N2
nres
(ν ) = n0 +c(ν − ν 0 )g(ν )
2πν∆ν
Vgr = c
n0 +cg0
2π∆ν
t delay = Lg0
2π∆ν
tmin
tmax
Time delay of 100 ps < tdelay < 220 ps in the active medium of MARS high-power amplifier has to be compensated.
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Streak camera: Streak camera: ““Live from the bunkerLive from the bunker””• Shot-to-shot measurements of the laser pulse length and the timing
between two pulses necessary because of the complex dynamics of the final amplification of the CO2 pulse.
• Optical Kerr Effect to get CO2 streaks• E-beam reference pick off the photocathode drive laser
532 nm pulse
CO2 pulse
0 500 1000 1500 200005
101520
250 ps1170 psa.
u.
time (ps)
Streak lineout
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Measurement of timingMeasurement of timingbetween CO2 and e beambetween CO2 and e beam
900 950 1000 1050 1100 1150 1200 1250 1300 135016
17
18
19
20
21
22
23
24IF
EL
Out
put e
nerg
y (M
eV)
Relative timing between Photo-Injector Driver and CO2 laser (ps)
0
20
40
IFEL output energy vs timing Typical laser pulse profile
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Optimization of IFEL outputOptimization of IFEL output
13.0 13.5 14.0 14.5 15.0
18
20
22
24
26
28
30
IFEL output vs. Injection energy
Injection energy (MeV)
Fina
l ene
rgy
(MeV
)
0
1
2
3
4
5
Fraction of particles trapped (%)
100 150 200 250 300 350 400 450
20
22
24
26
28
30
Laser Power (GW)
Fina
l ene
rgy
(MeV
)0
1
2
3
4
5
6IFEL output vs. Laser Power Fraction of captured particles (%
)
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Output energy vs. focus positionOutput energy vs. focus position
-6 -5 -4 -3 -2 -1 022
24
26
28
30
32
34
36
Intensity at undulator entrance P= 400 GW, zr= 1.8 cm Intensity Threshold for trapping
Focus position (cm)
Fina
l ene
rgy
(MeV
)
20
30
40
50
60
70
80
I (GW
/cm2)
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SideSide--effects of effects of very high power laser beamsvery high power laser beams
• Increase in laser intensity could be accomplished by increase inRayleigh range, or increase of power in the pulse…
Single crystal NaCL windowsSingle crystal NaCL lens
Copper mirrors
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Single shot spectrum (laser on)Single shot spectrum (laser on)
10 15 20 25 30 350
400
800
1200
1600
2000
a.u.
Electron energy (MeV)
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Single shot spectrumSingle shot spectrum(laser polarization 90(laser polarization 90ºº off)off)
10 15 20 25 30 350
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000a.
u.
E lectron energy (MeV)
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Single shot spectrum (laser on)Single shot spectrum (laser on)
10 15 20 25 30 350
400
800
1200
1600
2000
a.u.
Electron energy (MeV)
24 26 28 30 32 340
50
100
150
200
a.u.
Energy (MeV)
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Where are the energy peaks coming from?Where are the energy peaks coming from?2
2,
12
2
w
res n
K
n
λγ
λ
⋅ + =
⋅
Unfortunately we were not able to follow the red curve because of missing laser intensity, but if you slip out of the first resonance, the undulator is tapered enough that electrons can start to exchange energy with 10 µm photons through second harmonic coupling !!!
0 0.1 0.2 0.3 0.4 0.5
50
100
Fundamental2nd harmonic3rd harmonic
Distance along the undulator (m)
Res
onan
t ene
rgy
Neptune IFEL undulator resonant energy
Resonant conditions for energy transfer between particles and e.m. wave: Higher Harmonic IFEL
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Higher Harmonic IFEL theoryHigher Harmonic IFEL theory
( )( ) sin (1 )2
nl w
n
JJ K KkK k z n kz tzγ ω ϕ
γ∂
= ⋅ ⋅ + + − +∂ ∑
( ) ( ) ( )( )2 2 2( ) ( ) ( ) ( )n m m n m nm
JJ K J K J K J Kξ σ σ∞
+ + +=−∞
= ⋅ +∑2
2( )
4 12
KKK
ξ =
+
( )w
KKk w
σγ
=where
0 1 2 3 4 50
0.10.20.30.40.50.60.70.80.9
1
Fundamental2nd harmonic3rd harmonic
Coupling coefficients for harmonics
K undulator
JJ fa
ctor
The even harmonics are coupled through the diffraction angle
In the limit σ(Κ)→0 we found the known result for odd harmonics
Higher Harmonic IFEL gives a lot of Higher Harmonic IFEL gives a lot of flexibility in flexibility in undulatorundulator design !!!design !!!
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3d simulation3d simulation• Energy gain is in the first section of undulator. (20 MeV in 25 cm !! )• Higher Harmonic IFEL in the second section
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Summary & ConclusionSummary & Conclusion
• IFEL Advanced Accelerator at the Neptune Laboratory– > 20 MeV energy gain ( + 150 % ) !!– trapped up to 10 pC in accelerating buckets !– accelerating gradient ~70 MeV/m !
• First experimental study of Strong Tapering & Diffraction Effects in IFELs
• Observation of Harmonic IFEL interaction in second section of undulator