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Brookhaven Science AssociatesU.S. Department of Energy
Inverse Free Electron Laser and Its Applications
X.J. Wang National Synchrotron Light Source
Brookhaven National LaboratoryUpton, NY 11973, USA
Presented at the Fist Asian Summer School on Laser Plasmas Acceleration and Radiations
August 7-11, 2006 Beijing, China
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Laser Plasma Acceleration and Radiations
Brookhaven Science AssociatesU.S. Department of Energy
OutlineOutlineIntroduction – Laser and accelerator, a marriage in heaven?Inverse Free Electron Lase (IFEL):
1. Basic principle and harmonic IFEL2. Experiments: BNL IFEL, Stella, and UCLA
Applications:1. Coherent radiation: HGHG, ESASE2. femto- to atto-second e-pulse and photon pulse: atto-s e-
beam, atto-s FEL; femto-slicing in storage ring;3. Others: heating and cooling, HHG
Summer and outlook
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Brookhaven Science AssociatesU.S. Department of Energy
1st Asian Summer S
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Laser Plasma Acceleration and Radiations
Brookhaven Science AssociatesU.S. Department of Energy
Light Source Family TreesLight Source Family TreesThe Laser Family The Accelerator Family
Masers
Lasers
CW Pulsed
High Res.Spectroscopy
CD players
SupermarketScanners
Mod. Res.Spectroscopy
Q-switched
Cutting/Welding
Surgery
Ultrafast
Pump/probe
NonlinearOptics
Linacs Storage Rings
Particle Physics
1st Gen. Synchrotron(parasitic)
2nd Gen. Synchrotron(insertion devices)
3rd Gen. Synchrotron(better insertiondevices)
FELs
Far IR
IR/Visible
UV/VUV
SASEX-ray
HHG
Ultrafast, Coherent, Intense X-Rays MaterialsScience
StructuralBiology
X-ray scattering,diffraction & spect.
E. Rolfing. DOE
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LLINACINAC CCOHERENTOHERENT LLIGHTIGHT SSOURCEOURCE
II--280280
Sand Hill RdSand Hill Rd
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LinacLinac--00L L =6 m=6 m
LinacLinac--11L L =9 m=9 m
LinacLinac--22L L =330 m=330 m
LinacLinac--33L L =550 m=550 m
BCBC--11L L =6 m=6 m BCBC--22
L L =22 m=22 m DLDL--22L L =66 m =66 m
DLDL--11L L =12 m=12 m
undulatorundulatorL L =120 m=120 m
7 MeV7 MeV
150 MeV150 MeV 250 MeV250 MeV 4.54 GeV4.54 GeV
14.35 GeV14.35 GeV
...existing linac...existing linac
newnew
rfrfgungun
2525--1a1a3030--8c8c
2121--1b1b2121--1d1d XX
LinacLinac--XXL L =0.6 m=0.6 m
2121--3b3b2424--6d6d
2 cm2 cm
10 cm10 cm
10 cm10 cm 50 cm50 cm
~120 cm~120 cm
θθ ≈≈ 5.75.7ºº 10 period undulator
Initial laser chirp
Polarizer AnalyzerEO Crystal
Bunch charge Gated spectral signal
Spectrometer
ωl
t t ωs
I
Electron bunch
Co-propagatingLaser pulse
Beam pipe
Width gives bunch lengthWidth gives bunch length
Centroid gives arrival timeCentroid gives arrival time
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Brookhaven Science AssociatesU.S. Department of Energy ANL LEUTL Undulator
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Paradise of the Laser Plasma Acceleratorhttp://fls2006.desy.de/index_eng.html
Paradise of the Laser Plasma Acceleratorhttp://fls2006.desy.de/index_eng.html
Higher gradient Lower cost, compact, and broad applications.Better beam quality smaller system
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ChallengesChallengesModern accelerators (light source in particular) usually operate more than 5000 hours/year with better than 95% reliability
Stability and Reliability
Timing jitter
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Multi-particle Coherent Radiation Multi-particle Coherent Radiation
What are requirements for CR from electrons in a bunch?• bunch (or some portion of it) has density variations on length
scale comparable to wavelength.
( ) ( ) ( )[ ] ( )ωωω
ωω
ddIfNNN
ddI
clemultiparti1−+=
( ) ( )2
/ˆ∫∞
∞−
⋅= drrSef crni rωωwhere (Nodvick & Saxon)
N can be large e.g. ~ 1010
n(r)
r
bunch density λ << lbE~ N1/2; I ~ N
λ >> lb; E ~ N; I ~ N2
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Short Electron Bunch GenerationShort Electron Bunch Generation
V = V0sin(ωτ)V = V0sin(ωτ)
RF AcceleratingVoltage
RF AcceleratingRF AcceleratingVoltageVoltage
V = V0sin(ωτ)V = V0sin(ωτ)
RF AcceleratingVoltage
RF AcceleratingRF AcceleratingVoltageVoltage
Δz = R56ΔΕ/ΕΔz = R56ΔΕ/Ε
Path Length-EnergyDependent BeamlinePath LengthPath Length--EnergyEnergyDependent BeamlineDependent Beamline
Δz = R56ΔΕ/ΕΔz = R56ΔΕ/Ε
Path Length-EnergyDependent BeamlinePath LengthPath Length--EnergyEnergyDependent BeamlineDependent Beamline
Short electron bunch can be produced by: direction generation;selection; compression, and the combination
ppL Δ
=Δ 2
1γ
l
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Laser Plasma Acceleration and Radiations
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IntroductionIntroduction
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Laser Plasma Acceleration and Radiations
Brookhaven Science AssociatesU.S. Department of Energy
A brief History of IFELA brief History of IFEL
1972 – R. Palmer first proposed the idea.1980s – Courant and Pelligrini et al at BNL studied IFEL based accelerator.1990s: 1. BNL ATF experimental demonstration IFEL effect.2. HGHG based on IFEL effect.3. LBNL proposed and demonstrated femto-second electron beam slicing using IFEL effect.4. Micro-bunching by IFEL observed at the BNL ATF.5. Optical Cooling based on the IFEL6. Micro-wave IFEL7. Proposal on Ultra-high harmonic generation by IFEL effect.2000: First experimental demonstration of second generation laser accelerator using IFEL at the BNL ATF - StellaHarmonic IFEL and atto-seocnd electron beam generationAtto-second FEL pulse and Enhanced SASE
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)2
1(2
2
2
KU +=γ
λλ
Resonance Condition
Uz
zU
UUzu
uU
kkc
tztckkmc
KeEmc
Ee
cmTBKxtckK
yzBB
+=
−++=Ψ
Ψ=•
=
=−
=
=
/]))([(
)sin('
][][934.0)sin(
)2cos(
0
0
ϖβ
ϖβγ
βγ
λβγ
β
λπ
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Limitation of IFELLimitation of IFELEnergy gain (modulation)
Diffraction:
Spontaneous loss:
λπ 0
220 ])(1[
wRwhere
Rzww
=
+=
P =cZ0Ie
6Nukuγ
2K 2
( ) ( ) ( )[ ]210
0
2 2/2/32 ζζξπγ JJPPNU −=Δ 2/1
2/2
2
KK+
=ξ
laser peak power Bessel functions undulator parameter
number of undulator periods 8.7x109 W2
2
4γζ
ukkK
=
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FEL IFEL
Physics Resonance energy exchange
Same as FEL
e-beam quality
not so critical
Compression
limitation saturation Diffraction, SRapplications Coherent radiation Photon and e-
beam controlinjector
length 20Lp compact
ρσπ
λε
≤
≤
E
2
2γK
2γK
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e-
35
81.6 MHzNd:YAG
LINAC
LASER
LASERCO
2
x
RF GUN
Accelerator Test Facility
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Laser Plasma Acceleration and Radiations
Brookhaven Science AssociatesU.S. Department of Energy
BNL Inverse Free Electron LaserBNL Inverse Free Electron Laser
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Laser Plasma Acceleration and Radiations
Brookhaven Science AssociatesU.S. Department of Energy
Micro-Bunching by IFELMicro-Bunching by IFEL
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STELLA Demonstrated Staging of Laser Acceleration Process
• Staging process demonstrated for first time during Staged ElectronLaser Acceleration (STELLA) Experiment*- Used inverse free electron laser (IFEL) as laser acceleration mechanism- IFEL buncher (IFEL1) creates femtosecond microbunches- IFEL accelerator (IFEL2) accelerates microbunches
*W. D. Kimura, et al., Phys. Rev. Lett. 86, 4041-4043 (2001).
CO2 LASER BEAM
FOCUSING LENSES
VACUUMPIPE
MIRROR WITHCENTRAL HOLE
MIRROR WITHCENTRAL HOLE
E-BEAM
E-BEAMFOCUSING
LENSES
E-BEAMFOCUSING
LENSES
UNDULATORMAGNETARRAY
DIPOLEMAGNET
SPECTROMETERVIDEO CAMERA
= QUADRUPOLE MAGNET
Buncher(IFEL1)
Accelerator(IFEL2)
ADJUSTABLEOPTICALDELAYSTAGE
UNDULATORMAGNETARRAY
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Stellar First Second Generation Laser Accelerator
Stellar First Second Generation Laser Accelerator
IFELPREBUNCHER
DRIFT & e-BEAMFOCUSING
OPTICS
IFELACCELERATOR
DRIVE LASERBEAM
BEAM SPLITTER
ELECTRONBEAM
~30 MW ~100 MW
TROMBONEDELAY LINE
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Schematic Layout of STELLAII Experiment
CO2 LASER BEAMCONVEX MIRROR
VACUUMPIPE
PARABOLICMIRROR WITH
CENTRAL HOLE
E-BEAM
E-BEAMFOCUSING
LENSES
E-BEAMFOCUSING
LENSES
DIPOLEMAGNET
SPECTROMETERVIDEO CAMERA
BUNCHER(IFEL1)
ACCELERATOR(IFEL2)
CHICANETAPERED
UNDULATORARRAY
VACUUMCHAMBER
LENSWINDOW
(1) W. D. Kimura, et al., Phys. Rev. Lett. 92, 054801 (2004).
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Examples of Experimental Results
E-beam only
80% trapping
14% trapping
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UCLA IFEL Experiment
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Laser Plasma Acceleration and Radiations
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Theory of Planar Harmonic IFELTheory of Planar Harmonic IFEL
The pendulum equation of the 1-D was extended to include the third harmonic IFEL interaction
( ) ( ) ( )[ ] ( ) ( ) ( )[ ] ξξφψξξφψγ
γ 333sin3sin 213310112 JJEJJEmceK
dzd
oo −++−+−=
⎟⎟⎠
⎞⎜⎜⎝
⎛−= 2
2
1γγψ N
wo k
dzd
2
2
4γξ
w
L
kKk
=
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IFEL Micro-Buncher ConfigurationIFEL Micro-Buncher Configuration
We have considered two options for Harmonic IFEL Micro-Buncher:
1. Single stage undulator with both fundamental and third harmonic IFEL simultaneously present.
2. Two stages of undulators, with fundamental IFEL followed by a harmonic IFEL
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Preliminary results of simulationPreliminary results of simulation
0.6 0.4 0.2 0 0.2 0.4 0.60.02
0.015
0.01
0.005
0
0.005
0.01
0.015
0.02
γ
ψran
0.0015 0.001 5 .10 4 0 5 .10 4 0.001 0.00150
20
40
60Initial Energy Distribution
f3
int3
0.6 0.4 0.2 0 0.2 0.4 0.60
20
40Initial Phase Distribution
f4
int4
We first consider a undulator now exist at the ATF:λu=3.3 cm, Lu =26.4 cm, beam energy 43 MeV, Ku=1.93P1= 150 MW, and P3 = 50 MW
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0.6 0.4 0.2 0 0.2 0.4 0.60.02
0.015
0.01
0.005
0
0.005
0.01
0.015
0.02
Δγi
ψi
0.015 0.01 0.005 0 0.005 0.01 0.0150
20
40
60Exit Energy Distribution
f
int
0.6 0.4 0.2 0 0.2 0.4 0.60
20
40
60Exit Phase Distribution
ψhist
lower2 upper2
int2
0.6 0.4 0.2 0 0.2 0.4 0.60.01
0.005
0
0.005
0.01
Δγi2
ψi2
0.008 0.006 0.004 0.002 0 0.002 0.004 0.006 0.0080
5
10
15Exit Energy Distribution
E2hist
Eint
0.6 0.4 0.2 0 0.2 0.4 0.60
50
100Exit Phase Distribution
ψhist2
lower2 upper2
ψint2
Fundamental only Third harmonic only
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0.6 0.4 0.2 0 0.2 0.4 0.60.015
0.01
0.005
0
0.005
0.01
0.015
Δγi2
ψi2
0.015 0.01 0.005 0 0.005 0.01 0.0150
10
20Exit Energy Distribution
E2hist
Eint
0.6 0.4 0.2 0 0.2 0.4 0.60
50
100
150Exit Phase Distribution
ψhist2
lower2 upper2
ψint2
0.6 0.4 0.2 0 0.2 0.4 0.60.02
0.01
0
0.01
0.02
Δγi2
ψi2
0.02 0.015 0.01 0.005 0 0.005 0.01 0.015 0.020
50
100Exit Energy Distribution
E2hist
Eint
0.6 0.4 0.2 0 0.2 0.4 0.60
100
200Exit Phase Distribution
ψhist2
lower2 upper2
ψint2
Single-Stage Harmonic IFEL Micro-Buncher
Two-Stage Harmonic IFEL Micro-Buncher
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ApplicationsApplications
High-gain Harmonic Generation (HGHG).Femto- to atto-s pulse generation:
1. atto-s e-pulse.2. E-SASE.3. Atto-s FEL pulse.4. femto-slicing in a storage ring.
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OSCILLATOR
SINGLE PASS FEL
SASE
Dispersionnωω
HGHG
Modulator Radiator
Free Electron Laser Configurations
Challenges:1. High-quality mirrors in UV and X-ray range for oscillator.2. High-quality electron beam and long undulator for single pass FEL.
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log
(pow
er)
undulatorlength zsaturation length ~ 10 Lgain
gain
~ 1
05
low gain exponential gain(high-gain linear regime)
P(z) = Po exp(z/Lgain)
non-linear
Self Amplified Spontaneous Emission SASE
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][][][4][min mLkAI
mradmm
w
n −=Α
πελ
ρρϖϖ
ρρ
/1)/(
/
≈≈Δ
=≤Δ
U
FEL
beamFEL
N
PPEE
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Peak Brightness Enhancement From Undulator Radiation To SASEPeak Brightness Enhancement From Undulator Radiation To SASE#of photonsΩx Ωy Ωz
B = (Ωi- phase space area)
# of photons Nlc
~ 106
Enhancement FactorUndulator SASE
αΝe αΝeNlc
compressed
Δ
ΩxΩy (2πεx) (2πεy)
ΩZ ωω
⋅σ Z
c⎛ ⎝ ⎜
⎞ ⎠ ⎟ = 10 −3 ×10 ps Δω
ω⋅
σ Z
c⎛ ⎝ ⎜
⎞ ⎠ ⎟ = 10 −3 ×100 fs 210
210λ 2( )2
B 1023 1033 1010
Nlc: number of electrons within a coherence length lc
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High Gain Harmonic Generation (HGHG) Principle
HGHG has the following advantages:•Longitudinally fully coherent• Narrower bandwidth; transform limited• Larger ratio of output/spontaneous radiation• Central wavelength is stable• Pulse length is short & controllable (20 fs)• Output fluctuations can be reduced
L.H.Yu, Phy. Rev. A, (1991).
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The Success of the Single-Pass High-Gain FELs
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High Gain Harmonic Generation FELHigh Gain Harmonic Generation FEL
Dispersion SectionL=0.3 m
Radiator SectionBw=0.47T λw=3.3cm L=2 m
Seed Laser λ=10.6μm Ppk=0.7 MW
Electron Beam Input Parameters: E= 40 MeV εn= 4π mm-mrad dγ/γ=0.043% I = 110A τe= 4 ps
HGHG FEL λ=5.3 μm Ppk=35 MW
Modulator SectionBw=0.16T λw=8cm L=0.76 m
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NSLS SDL Facility
NSLS SDL Facility
Titanium Sapphire LaserTitanium Sapphire Laser
BNL Photoinjector IVBNL Photoinjector IV
Chicane CompressorChicane Compressor10 m NISUS Wiggler 10 m NISUS Wiggler
300 MeV S300 MeV S--Band Linac Band Linac
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Cutting Edge Science at the SDLCutting Edge Science at the SDL
SASE @ 210 nm
XUV Photochemistry
-1 0 1
-1
0
1
Y-P
ositi
on [m
m]
X-Position [mm]
Elec
tric F
ield
(kV/cm
)
-80
-60
-40
-20
0
20
40
60
80
High Intensity THz
High Brightness Beams
0.23 nm FWHMSASE x105
Wavelength (nm)
HGHG
HGHG FEL @ 266 nm
PRL
PRL
Beam Dynamics
PRSTAB
100 μJ/pulse
LCLS Laser Dazzler Exp-20 -10 0 10 200
0.2
0.4
0.6
0.8
1
Time (ps)
BlueUVIR
80fs Superradiance
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Average output: 100 μJ, 10% fluctuation
266nm HGHG Power vs. Distance in NISUS
0 2 4 6 8 10
100
102
Wiggler Length (m)
Puls
e En
ergy
(μJ
)
(a) 1.8 MW(b) 30 MWTDA
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262 264 266 268 2700
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Ene
rgy
(mJ
(% b
w)-1)
SASE × 105
HGHG
0.23 nm FWHM
x
262 264 266 268 2700
0.2
0.4
0.6
0.8
1
Wavelength (nm)
Ene
rgy
(mJ
(% b
w)-1) SASE × 4
HGHGx
Spectrum of HGHG and SASE at 266 nm
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Ultra-Violet FEL Operation: 200 nm SASEUltra-Violet FEL Operation: 200 nm SASE
185 190 195 2002000
3000
4000
5000
6000
7000
Wavelenght (nm)
Inte
nsity
(a.u
.)
194 196 198 200 202 204
1
2
3
4
5
6 x 104
Wavelength (nm)
Inte
nsity
(a.u
.)
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Histogram of electron longitudinal density
fs
Arb. units σ=55 asfs
Arb. units
2 4 6 8 10
-0.4
-0.2
0.2
0.4
2 4 6 8 10
-0.4
-0.2
0.2
0.4
fs fs
Longitudinal phase space at the exit of the undulator. Modulation amplitude = 40 σe
Longitudinal phase space at the End Station
Simulations (GINGER was used)
cut out with masks
δEin
MeV
δEin
MeV
Estimate: δE=0.6 MeV for P=10 MW, K=1.28, M=55Atto-second Electron Beam Production
In Collaboration with Zolotorev and Zholents
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Peak current, A
σ=140 as
Peak current, A
a)
b)
time, fs
Results for the ATF beam
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Energy modulation in the wiggler at 2 - 4 GeV
Enhanced Self Amplified Spontaneous Emission (ESASE)
Only one optical cycle is shown
Required:
•Laser peak power ~ few GW
•Wiggler with 10 – 20 periods
Assumed:
•Electron energy spread ~ 1.2 MeV
ChicaneLaser Wiggler Linac e-beam
x-rays
Linac Undulator
Master source Near IR pump
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Acceleration to 14.35 GeV and bunching at the laser wavelength
acceleration bunching
Only one optical cycle is shown Pea
k cu
rren
tz /λL
Pea
k cu
rren
t
Ene
rgy
spre
ad
Peak current and energy distribution within one micro-bunch
Bz L 2/λ=Δ
50 fs laser pulseλL= 2 microns
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SASE in the undulator producing x-rays synchronized to the modulating laser
The output x-ray radiation from a single micro-bunch
Power at saturation (estimate for bunching~0.5), P0~200 GW
70 as( ) xGx Mzz λˆ8Ln2/ +Δ≤Δ
• Each spike is nearly temporary coherent and Fourier transform limited.• Carrier phase for an x-ray wave is random from spike to spike.
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First Experimental Observation of ESASEFirst Experimental Observation of ESASE
195 200 205 2100
0.2
0.4
0.6
0.8
1
Wavelenght (nm)
Nor
mal
ized
Inte
nsity
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National SynchrotronLight Source
Advanced Photon Source
Stanford SynchrotronRadiation Laboratory
Advanced Light Source
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S.Khan of Bessy, FLS2006
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laser wiggler
bendmagnet
mirrorbeamline x-rays
wigglerelectron bunch inthe bend magnet
electronbunch
femtosecondlaser pulse
λW
electronbeam orbit
fs pulse
fs pulse
“dark”pulse
Selection of fs x-ray pulses (femto-slicing)
selection of fs-xrays
A. Zholents, M. Zolotorev, Phys. Rev. Lett. 76, 912, (1996).
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1) by coordinate: x( ),2) by angle: x’( ), 3) by time-off-flight: t( ), 4) by spectra: ω( ) 1)
Selection options:
3) time-off-flight selection
-δ
+δ
4) spectral selection works only for undulator source Mn
1≈
Δωω
harmonic number
number of undulator
i d
Width of the spectral peak of the undulatorradiation
1) H Padmore private discussion
Selection of fs x-ray pulses (2)
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-1200-1000 -800 -600 -400 -200 0 20060
80
100
120
140
160
180
delay (fs)
coun
ts
-1200-1000 -800 -600 -400 -200 0 200
100
150
200
250
coun
ts
-1500 -1000 -500 0 500 1000 1500
2000
2500
coun
ts
0
.2
.4
.6
.8
1.0
x/σ x
time (fs)
Electron Density Distribution
+3σx to +8σx
+4σx to +8σx
Fitted with amplitude of modulation = 6.4 MeVΔ
Laser correlation with visible synchrotron pulse (2)1
R.W.Schoenlein et. al , Science, March 24, 2000
-3σx to +3σx
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1.2
1.0
0.8
0.6
-0.4 -0.2 0.0 0.2 0.4 1086420
1.02
1.00
0.98
0.96
0.94
-6 -4 -2 0 2 4 6
Time (ps)
1.00.80.60.40.20.0
Frequency (THz)
AE=2 AE=4 AE=8
Am
plitude (arb. units)Rel
ativ
e ch
arge
den
sity
a) b)
c) d)
Holy Bunches1/24 ring after slicing
3/4 ring after slicing
Calculated distributions for ALS with nominal and twice nominal momentum compaction.
Holes spread due to time of flight disperson(i.e. momentum compaction)
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Slicing CSR signals
Raw bolometer signal shows a signal synchronous with the laser repetition rate.
1 msec laser rep rate
long slice short sliceThe signal spectra extend up to 2 THz and depend on the initial laser pulse and proximity to the slice. Fine structure in spectra is due to measurement details.
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Generation of attosecond pulses based on SASE FEL: slicing method (Proposal B)
Monochromator: δλ/λ~10-4
Saldin, Schneidmiller, Yurkov, Optics, Com., 237,(2004)
x-rays
UndulatorWigglerLinac
e-beam
%1.02
0
>=Δ
ωδω
EE
~300 asec, ~5 μJ
Selection of attosecondpulse with contrast > 1:1 requires ΔE=40 MeV and laser: 5-fs, 4-mJ
Attosecond x-ray pulse
ω0
ω0+δω
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Laser HeaterLaser Heater
2 cm2 cm
10 cm10 cm
10 cm10 cm 50 cm50 cm
~120 cm~120 cm
θθ ≈≈ 5.75.7ºº
• Laser-electron interaction in an undulator induces rapid energy modulation (at 800 nm), to be used as effective energy spread before BC1 (3 keV 40 keV rms)
• Inside a weak chicane for easy laser access, time-coordinate smearing (emittance growth is completely negligible)
10 period undulator
800 nm laser pulse800 nm laser pulse
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Transient Time Method of Optical Stochastic Cooling
. ..radiation pulse amplifier
energy gain/loss
undulator undulator
bypass.
~1 μm
transverse kick
( )
EExxx c Δ
−=−=Δ ηββ
energy kick
A pick-up and a kicker should be installed in a position with a nonzerodispersion function for a simultaneous cooling of energy and transverse coordinates (similar to the Palmer’s method of the momentum cooling).
after kickEEx Δ
=Δ η
before kick
⎟⎠⎞
⎜⎝⎛ zE δ
λπδ 2sin~
δz is particle delay
Mikhalichenko, Zolotorevand Zholents (1993-94)
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Summary and OutlookSummary and OutlookIFEL is one of the oldest and most matured laser acceleration techniques, It works.
The potential of IFEL is only limited by our imaginations. It will be continued to explored for:
1. Injector for other laser plasma accelerator.2. Femto- to Atto-s e-pulse and photon pulses.3. Improve the FEL performance.4. Control and manipulating the charged
particle beam.
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AcknowledgementAcknowledgement
BNL NSLS and ATF colleagueLBL: Zolotorev, Zholents, John ByrdSTI:W. D. KimuraUCLA: D. Cine, C. Pellegrini and P.MusumeciMany other whose work are cited here
Thank you!
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