Laser driven particle acceleration experiments at SLAC
T. Plettner
Visit to DESYOctober 24,2006
The laser-driven particle accelerator project at Stanford University• The proof-of-principle demonstration• Current projects (high and low energy beams)• possible application as a light source
FWH
M e
nerg
y sp
read
(keV
)
laser timing (psec)
FWH
M e
nerg
y sp
read
(keV
)
laser timing (psec)
laser on
laser off
Participants
1 E.L. Ginzton Laboratories, Stanford University2 Stanford Linear Accelerator Center (SLAC)3 Department of Physics, Stanford University
Bob Byer1
Bob Siemann2 Chris Sears2 Jim Spencer2
Tomas Plettner1 Eric Colby2
Ben Cowan2
•Rasmus Ischebeck•Chris Mcguinness2
•Melissa Lincoln2
•Patrick Lu1
Atomic Physics collaboration
New participants
•Mark Kasevich3
•Peter Hommelhoff3
•Catherine Kealhofer3
1.
Motivation and the proof-of-principle demonstration
1947: The “Mark I”1m, 6 MeV
The linear accelerator1967: SLAC 3 km 20 GeV
2020(?) Proposed ILC: 40 km, 1 TeV
• Vacuum channel• Linear trajectory• Ultra-low energy spread (< 0.1%)• Ultra-low emittance (~10-6 m-rad)
propertiesproperties
The Livingston curve W. K. H. Panofsky, SLAC Beamline, 1997
1. near-exponential growth in the beam energy up until about 1990
2. LHC and future NLC/ILC lie below the exponential growth curve
3. Exponential curve important for new physics
For future high energy colliderfacilities beyond the LHC and ILC it becomes increasingly appealing to invest in new accelerator technologies
RF based accelerator technology is nearing its
practical high-energy limit
Future limitationMaximum gradient ~ 100 MeV/m
Proposed “afterburner”accelerator for SLAC
M. Hogan, et al, Phys. Rev. Let. 95, 054802 (2005),
Example:
1. Plasma Accelerators
possible advanced accelerator technologies
• Most “mature” of all advanced accelerator concepts• Different mechanisms of plasma wave generation
plasma wakefield accelerationUSC/UCLA/SLAC collaboration
?
Demonstrated gradient
~ 30 GeV/m
W.D. Kymura et al, Phys. Rev. ST Accel. Beams 4, 101301 (2001)
possible advanced accelerator technologies2. Inverse FEL accelerators (IFEL)• Very mature concept, first demonstrated in 1992
• First staged laser-acceleration with IFELs at λ=10 μm
• Simple setup• Vacuum• Large aperture• High charge• Extended ~ 1m interaction
I. Wernick and T. C. Marshall, Phys. Rev. A 46, 3566 (1992).
Not scalable to TeV energies
Main problem
Demonstrated gradient
7 MeV/M
gradient ⊥∝ Epropertiesproperties
W.D. Kymura et al, “Laser Acceleration of Relativistic Electrons Using the Inverse Cherenkov Effect”,Phys. Rev. Lett. 74, 546–549 (1995)
Demonstrated gradient
31 MeV/m
12 cm2.2 atm. H2
possible advanced accelerator technologies3. Inverse Cerenkov accelerators (ICA)
J. A. Edighoffer et al, Phys. Rev. A 23, 1848 (1981).
4. Other schemes
linear acceleration scheme
gradient ||E∝
• free space interaction• phase synchronicity through
index of medium• scattering from medium
• Wakefield vacuum waveguide acceleration
• particle acceleration by stimulated emission of radiation (PASER)
• Vacuum beatwave
• laser-driven cyclotron autoresonance accelerator (LACARA)
• Further R&D on RF acceleration
• Structure loaded vacuum laser-particle accelerationMiniature cousin of RF acceleration
B. Hafizi, et al, Phys. Rev. E 60, 4779-4792 (1999)
L. Schächter, Phys. Rev. Lett. 83, 92-95 (1999)
T-B. Zhang et al,Phys. Rev. E 56, 4647-4655 (1997)
T. C. Marshall et al, Phys. Rev. ST Accel. Beams 4, 121301 (2001)
Solid state diode-pumped lasers
ultrafast laser technology
< 10 fs
60 W/bar, 50% electr. efficiency
efficient pump diodes
high peak –power lasers
Klystron technology (1930s)
RF linear accelerator;based on
Diode pumped solid state lasers
E.L. Ginzton
Varian brothers
M Chodrow • very compact, tabletop systems• optical phase control • pump diodes 50% efficiency• solid state gain medium 80%
Overall wallplug efficiency > 30%
possibility for ultra-short pulse operation (100 fsec or shorter)
Features of interest to us
Es → 1010 V m
50
20
10
5
2
11 10 102 103
fused silica
CaF2
dam
age
fluen
ce(J
/cm
2 )
laser pulse duration (psec)
S. Preuss, A. Demchuk, and M. Stuke, Appl. Phys. A 61,33 (1995).
reason for ultra-short pulse operationLaser damage threshold of dielectric
materials in the near-infrared dielectric materials have a large bandgap
at near-IR (λ~1 μm) there is no two-photon absorption
At laser pulse durations of τp< 10 psec the damage
mechanism changes
Below 1 psec damage fluence ~ 2J/cm2
Below 1 psec damage fluence ~ 2J/cm2
motivation
τp~100 fsec Emax~10 GV
Gradient ≥ 1 GeV/m
Structure loaded vacuum laser-driven particle accelerator
Vacuum channelDielectric
structureLaser in
rFE
electron
• Laser beam is coupled into the micro-structure
• Electrons never traverse material
• Diffraction from the structure produces a longitudinal E-field inthe vacuum channel
•
•
• Electrons travel in a linear trajectory
v phase c= β
( )ΔU E r t drr
r= ⋅∫
r r,1
2
gradient ||E∝
Crossed laser beams in a structure loaded vacuum
Magnitude Phase
Gouy PhasePlane Wave Phase Math
( )
( ) ( ) 01
222
233
22
22ˆ
2/3220
cosˆtan2cosˆ1tancosˆ
cos
coscosˆ1sin
exp)cosˆ1(
sin2
φθθθθθωθψ
ψθθ
θθ θ
+⋅−+
⋅+⋅−⋅⋅=
×⎥⎥⎦
⎤
⎢⎢⎣
⎡
+−
+−=
− zz
ztzk
zzEE
dt
t
z
zd
†P. Sprangle, E. Esarey, J. Krall, A. Ting, Laser Accelerationof Electrons in Vacuum, Optics Comm. 124 (1996) 69
E1
E2
E1z
E2z
E1x
E2x
z
x
( ) ( ) zdzEzUz
zz ′′= ∫
0
Y.C. Huang, R.L. Byer, W.M. Tulloch, Appl. Phys. Lett. 68 (6) (1996) 753.
1995: proposed 1 GeV/m structure
• cascaded crossed-laser accelerator cells
• light guiding by micro-element MEMs structure
• possible to fabricate• could be powered with
tabletop lasers
1/3 mm
LEAParea
kickercollimator
slitsFEL
wigglersuperconducting
accelerator structures
amplified laserBeam Energy ~30 MeVTelectron ~2 psecCharge per bunch ~5 pCEnergy spread ~20 keVλlaser 800 nmElaser 1 mJ/pulse
HEPL beam parameters
The proof-of-principle experiment
electronbeam
materialboundary
θ
electronbeam
Ez
8 μm Kapton 1 μm Au
laserbeam
ΔU E dzz=−∞∫0
The reflective boundary tape drive
e-beam
steppermotor
steppermotor
estimated duration of 1 track9:12 hrs. (552 min)
Au coatedKapton
laser beam
The proof-of-principle experiment
Cerenkov cell lens
IFEL
ITR
Cerenkov cell
Motor 1
Motor 2
upstream
downstream
Laser beam
electron beam
top view of the setup
The proof-of-principle experiment
laserEU ∝
Peak Longitudinal Electric Field Ez (MV/m)
0.1 0.2 0.3 0.4Laser Pulse Energy (mJ/pulse)
0.05
Ave
rage
Ene
rgy
Mod
ulat
ion
⟨M⟩(
keV
)
( ) ( )25.035.0017.0349.0 ±−⋅±= zEM
Average FWHM energy broadening
The dependence on the laser electric field amplitude
The proof-of-principle experiment
φcos∝U
Laser Polarization Angle (degrees)
Average FWHM energy broadening
Ave
rage
Ene
rgy
Mod
ulat
ion
⟨M⟩(
keV
)
The dependence on the laser electric field polarization
T. Plettner, R.L. Byer, E. Colby, B. Cowan, C.M.S. Sears, J. E. Spencer, R.H. Siemann, “Proof-of-principle experiment for laser-driven acceleration of relativistic electrons in a semi-infinite vacuum”, Phys. Rev. ST Accel. Beams 8, 121301 (2005)
2.
Current experiments
Additional semi-free space laser-driven particle acceleration experiments
90o
bendingmagnet
gatedcamera
electronbeam
~8 μm thick tape
incident laser beam
reflected laser beam
reference laser beam
transmitted laser beam
phase monitor
energy spectrometer
90o
bendingmagnet
gatedcamera
electronbeam
~8 μm thick tape
incident laser beam
reflected laser beam
reference laser beam
transmitted laser beam
phase monitor
energy spectrometer
Experiment setup and expected dependence on laser crossing angle
-40 -30 -20 -10 0 10 20 30 400
5
10
15
20
25
30
35
40
laser crossing angle (mrad)
ener
gy g
ain
(keV
)
HEPL (30 MeV)θopt ~ 16.8 mradUmax ~ 18.1 keV
E163 (60 MeV)θopt ~ 8.6 mradUmax ~ 37 keV
r r r rE dx E E dslaser
Plaser rad
S
⋅ = ⋅∫ ∫?
1. M. Xie, Proceedings of the 2003 Particle Accelerator Conference (2003)
2. Z. Huang, G. Stupakov and M. Zolotorev , “Calculation and Optimization of Laser Acceleration in Vacuum”, Phys. Rev. Special Topics - Accelerators and Beams, Vol. 7, 011302 (2004)
Test with different boundaries1. Reflective2. Transparent3. Scattering4. Black absorbing
Buncher-accelerator two-stage experiment(graduate student C. M. Sears)
optical buncher
opticalaccelerator
compressor chicane
laser
IFEL
(compressor)
laser accelerator
-8 -6 -4 -2 0 2 4 60
20
40
60
80
100
120
140
160
P has e
His togram In P has e
Analytic (1-D)S imulation
0 1 2 3 4 5 6-40
-30
-20
-10
0
10
20
30
40
Phase
Mea
n En
ergy
Shi
ft (k
eV)
Net Acceleration
Fit Amp=17 keV
mea
n en
ergy
shi
ft (
keV
)
phase
Net acceleration
mea
n en
ergy
shi
ft (
keV
)
phase
Net accelerationExpected bunching Expected energy gain
• IFEL modulates energy spread• electron drift creates optical bunches• second accelerator net acceleration
Experiment features
-3 -2 -1 0 1 2 3-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Cross-correlation in time Observation of harmonic interaction
IFEL Experiments at λ=800 nm(graduate student C. M. Sears)
Dielectric waveguide vacuum channel laser accelerator structures
lowest mode
Bulk medium, high reflector, bragg-mirror or photonic bandgap
Waveguide structure
Ex
Ez
non-accelerating mode
vacuum channel
0=⋅∇ Err
zx
lowest mode
00 ≈→≈dz
dEdx
dE zx
SVEA: 0~ ≈zE
Dielectric waveguide vacuum channel laser accelerator structures
lowest mode
Bulk medium, high reflector, bragg-mirror or photonic bandgap
Waveguide structure
zx
Ex
Ez
nextmode
00 >>→>>dz
dEdx
dE zx
SVEA: 0~ >>zE
0=⋅∇ Err
accelerating mode
vacuum channel
Photonic bandgap accelerator microstructure experiments
The Blaze Photonics HC-1550-02 fiber for laser-driven particle accelerationParameter value Structure impedance 1=CZ Ω Damage factor 11.0=FD Laser wavelength 6.1=λ μm Laser pulse energy 1 μJ Laser pulse duration 1 psec Laser group velocity β ~ 0.6 Expected gradient 0.6 GeV/m Structure length 0.5 mm Energy gain 0.3 MeV
Proposed parameters for a laser-driven
particle acceleration experiment with a PBG
hollow core fiber
X.E. Lin, “Photonic band gap fiber accelerator”, Phys. Rev. ST Accel. Beams 4, 051301 (2001)
G ~ 0.4 GeV/m
Generation of an accelerating modeSimple optical tests with HeNe lasers
HeNe laser
step plate(vertical)
step plate(horizontal)
polarizer
λ/2
beamsplitter
θ
HeNe laser
step plate(vertical)
step plate(horizontal)
polarizer
λ/2
beamsplitter
θ
The generation of a TEM01* donut mode
(graduate student M. Lincoln)
(graduate student P. Lu)
Free-space to fiber coupling tests
1 mm
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛
⎟⎟⎠
⎞⎜⎜⎝
⎛+⎟⎟
⎠
⎞⎜⎜⎝
⎛+
⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛+
=22
0
220
0
*
11
1
pf
pf
p
p
δ
δ
σβ
σβ
ββ
Focusing triplet
f = 2 cm
Electron beam focusingVacuum channel ~ λ
Photonic bandgap accelerator microstructure experiments
The Woodpile 3-dimensional photonic bandgap structure( graduate student B. Cowan)
properties
B. M. Cowan, “Three-Dimensional Photonic Crystal Laser-Driven Accelerator Structures”, SLAC-PUB 12090 (2006)
Nature 394, 251 (1998)
Aperture ~ 0.4 λGradient ~ 250 MeV/m Structure impedance ~ 400 ΩGroup velocity ~ ¼ cRequired emittance ~ 10-10 m-radCharge per bunch ~ 1 fC
Demonstration of an RF PBG acceleratorMIT group
E. I. Smirnova, A. S. Kesar, I. Mastovsky,M.A. Shapiro, and R. J. Temkin, Phys. Rev. Lett., 95, 074801 (2005).
E. I. Smirnova,* I. Mastovsky, M. A. Shapiro, R. J. Temkin, Phys. Rev. ST AB 8, 091302 (2005)
Cu – PBG structureKU-band (17.14 Ghz)
Q ~ 4000
Q>>1 Ideal for long RF pulses, but not for few-cycle pulses
Summary for waveguide laser accelerator structures
Laser mode confinement
• Very good spatial overlap of electron beam with laser beam higher efficiency than semi-open accelerator geometries
• Waveguide geometry extended phase synchronicity
• Not suitable for ultra-short pulse operation fiber segments of mm or shorter
• High dispersion: pulse chirping etcGroup velocity
Small aperture• Beam loading at low bunch charge (~1 fC)• Potential limitation from nonlinear effects:
Raman, Brilluin scattering
Mode symmetry • Difficult coupling• Potential for excitation of unwanted modes
Glass material• Poor heat conduction• Potential darkening from solarization or
from radiation damage
T. Plettner, R.L. Byer, P. Lu, “Proposed few-optical cycle laser–driven particle accelerator structure”, submitted to Phys. Rev. ST AB
xyz
laserbeam
cylindrical lensvacuum
channel
electron beam
cylindrical lens
top view
λ/2
λ
xyz
laserbeam
cylindrical lensvacuum
channel
electron beam
cylindrical lens
top view
λ/2
λ
top view
λ/2
λQuartz structure
• transparent for λ = 1 μm• good heat conductor• resistance to radiation• potential for 1 pC
FDTD calculation
( )tyxE ,,||
Transverse-pumped periodic field reversal structure
E-beam lithography + DRIE (Graduate student P. Lu)
Transverse-pumped periodic field reversal structure
1 cm long interactionGradient ~ 0.7 GeV/menergy gain: 7 MeV
Aperture ~ ½ λGradient ~ 2.5 GeV/mStructure impedance ~ 40 Ω
10 fsec,λ = 1μm
200 fsec,λ = 800 nm
E163 injector
The klystron• ~2 m tall• 1/3 MV• high power• water cooling• X-ray radiation• 10 Hz rep. rate
λ = 266 nm
The gun• ~ few cm long• Cu surface• 5 MeV at exit• QE ~ 10-4
• ~ 2 psec bunch• ε ~ 2×10-6 m-rad
Temporary solutioneventually want an all-laser driven injector
An ultrafast nanometric electron sourceAn ultrafast nanometric electron source
Peter Hommelhoff Catherine Kealhofer Mark Kasevich
Physics and Applied Physics, Stanford University
SPRC Symposium, September 20, 2006
P. Hommelhoff et al, Kasevich groupLaser
field emitter tip
Field emission tip properties
1. laser-assisted tunneling of the electrons from the atom to free space
2. Highly nonlinear3. Potential for sub-optical cycle
electron emission
metal vacuum
e
An ultrafast nanometric electron sourceAn ultrafast nanometric electron source
Single atom tip
FIM
Stable up to ~10nA(~5 1010 A/m2),
Opening angle:7° (FWHM)
Brightness:~108 A/(cm2 sr)
Invariant brightness(with U~500V ):5 1010 A/(cm2 sr)
Evaporate Pd onto tipand anneal:grow pyramid FIM
P. Hommelhoff et al, Kasevich group
Emittance ~ 10-11m
Potential for 700 as pulse
Autocorrelator with tip as (non-linear) detector
delay (a.u.)
P. Hommelhoff et al, Kasevich group
3.
Possible application as a light source
Conceptual freeConceptual free--electron based electron based attosecondattosecond coherent Xcoherent X--ray radiation source ray radiation source
• meter size laser-driven particle accelerator source
• cm-size micro-undulator
X-ray pulse
20 attosec
• coherent X-ray pulse• sub-fsec• spatially collimated • high rep. rate
RF accelerator
microwave(sub nsec)
electron bunch(sub psec)
λ = 1 μm (3 fsec)
electron bunch(10 attosec)
laser accelerator
Eventual objective
Coherent ultra-short wavelength sourcesRFRF--accelerator driven SASE FEL facilitiesaccelerator driven SASE FEL facilities
RF accelerator
Lu > 10 m
microwave(sub nsec)
electron bunch(sub psec)
SASE FEL X-ray
sub psec
Examples:Examples:• Brookhaven VISA FEL
• Tesla Test FacilityJ. Andruskov et. al. , “First Observation of Self - Amplified Spontaneous Emission in a Free-Electron Laser at 109 nm Wavelength”, Phys. Rev. Lett. 85, 3825–3829 (2000)
undulator
M. Hogan et al, “Measurements of Gain Larger than 105 at 12 μm in a Self - Amplified Spontaneous - Emission Free-Electron Laser”, Phys. Rev. Lett. 81, 4867–4870 (1998)
SSRL
undulator
3 km
120 m
accelerator
Experiment lines
LCLSinjector
T ~ 230 1 fsecλ ~ 1.5 – 15 ÅΦ ~ 1012 γ / pulse
SASE-FEL 14 GeV
• materials science• chemistry• atomic physics
100 m
T ~ 230 1 fsecλ ~ 1.5 – 15 ÅΦ ~ 1012 γ / pulse
SASE-FEL 14 GeV
• materials science• chemistry• atomic physics
100 m• km-size facility• microwave accelerator• λRF ~ 10 cm• 4-14 GeV e-beam
• 120 m undulator• 23 cm period• 15-1.5 A radiation• 0.8-8 keV photons• 1014 photons/sec• ~77 fsec
• separate user lines• 120 Hz pulse train
LCLS propertiesLCLS properties
Coherent ultra-short wavelength sources
TTF: Tesla Test Facility; fsec EUV SASE FEL facilityXFEL: Proposed future coherent X-ray source in Europe…TTF: Tesla Test Facility; fsec EUV SASE FEL facilityXFEL: Proposed future coherent X-ray source in Europe…
RFRF--accelerator driven SASE FEL facilitiesaccelerator driven SASE FEL facilities
H. Motz, “Applications of the Radiation from Fast Electron Beams”, J.. Appl. Phys. , 22, 527 (1951)
Undulator radiation
( )λ λγr
u K= +2
1 22
2
x K umax =
γλπ2
λu
Kekm c
Bu
e
= 0
Coherent field addition from periodic electron motionCoherent field addition from periodic electron motion
1. instantaneous
2. narrow fractional bandwidth
PropertiesProperties
amplitude of deflectionamplitude of deflection
undulatorundulator parameterparameter
radiation center wavelengthradiation center wavelength
shorter pulses?shorter pulses?
λr < 1/10 Å
(10 attosec)
1% bandwidth
ωω c
Δω ω 0 1= N
( )I ω
SASE – FEL processMATLAB based finite element analysis
( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛−=′
∂∂
ΔΔ∈
−∑ vetzEt j
i jψχ2,~
ηψuk
dtd 2=
ψχη sin01Edtd
−=
Electron pendulum equationsElectron pendulum equations
Optical field growthOptical field growth
Undulator period-2000 -1500 -1000 -500 0
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1x 10-3
electrons
( )tzE ,~ ′
elec
tron
ener
gy
• 1-D model • no space charge effects• slowly varying field envelope
assumptions
Undulator radiation acting on e-beam
Proposed parameters for laser driven SASE–FEL
Laser accelerator undulator
~ 2 m
~ 1 GeV
Input electron beam Input electron beam ~ 1-2 GeV beam energy~ 10 10 attosecattosec pulse durationpulse duration~ 1 pC bunch charge~ 0.05% energy spread
undulatorundulatorλu ~ 200 μmLu ~ 20-40 cmB0 ~ ½ - 1 T
( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛−=′
∂∂
ΔΔ∈
−∑ vetzEt j
i jψχ2,~
Field envelope growthField envelope growth
electrons per unit volume
Smallest possible
beam size
φb < 500 nm
Solid state laser
λ ~ 1 μm
3 fsec10 attosecpulse structure
λ
F e Nz rr
e
b
=⋅ ⋅
2
022πε γΔ
rb
Fr
Δz
H. Wiedemann, “Particle Accelerator Physics I”, 2nd
ed. Springer, p. 21 (1999)
Proposed parameters for laser driven SASE–FEL
Space charge repulsionSpace charge repulsion
0 0.5 1 1.5 2 2.5 3 3.5 40
100
200
300
400
500
600
drift (mm)
spot
siz
e (n
m)
Effect of transverse emittanceEffect of transverse emittance
ε = 10-11 m-rad
aN
z rre
b
∝⋅ ⋅Δ γ 3
ε θ∝ ⋅Δ Δx
( ) ( )s t a t dtr
tt= ∫∫ 00
( )
Electron beam guiding Electron beam guiding
( )
( )2
2
1
0
20
0
0
2
0
D
DZ
ZsDzD
dεθ
ε
=
=
+=
Proposed parameters for laser driven SASE–FEL
λu= 200 μm10 mm (50 λu)
50 μm
40 cm
400 nm
Proposed Proposed undulatorundulator geometry for 2 geometry for 2 GeVGeV electron beamelectron beam
LCLS laser-driven SASE-FEL
bunch charge 1 nC 1 pC
transverse emittance 1.2 x 10-6 m-rad 10-10 m-rad
undulator strength 1.38 T ~ 1.0 T
undulator period 3 cm 200 μm
undulator parameter K 3.71 0.019
transverse beam size 96 μm 400 nm
total length 120 m 40 cm
FODO length ~7 m (230 periods) ~1 cm (50 periods)
LCLS laser-driven SASE-FEL
bunch charge 1 nC 1 pC
transverse emittance 1.2 x 10-6 m-rad 10-10 m-rad
undulator strength 1.38 T ~ 1.0 T
undulator period 3 cm 200 μm
undulator parameter K 3.71 0.019
transverse beam size 96 μm 400 nm
total length 120 m 40 cm
FODO length ~7 m (230 periods) ~1 cm (50 periods)
High strength Nd:Femicromagnets
0 50 100 150 200 250 300 350 400 450 5000
20
40
60
80
100
120
140
beam spot size (nm)
leng
th (m
m)
LG
LR
L LR G>
Proposed parameters for laser driven SASE–FEL
LR
LCLS laser-driven SASE-FEL
beam energy 14 GeV 2 GeV
bunch charge 1 nC 1 pC
transverse emittance 1.2 x 10-6 m-rad 10-10 m-rad
transverse beam size 96 μm 400 nm
Rayleigh range 190 m 8 cm
Beam divergence ~1 μrad ~5 μrad
gain length 4.8 m (160 periods) 18.8 mm (94 periods)
LCLS laser-driven SASE-FEL
beam energy 14 GeV 2 GeV
bunch charge 1 nC 1 pC
transverse emittance 1.2 x 10-6 m-rad 10-10 m-rad
transverse beam size 96 μm 400 nm
Rayleigh range 190 m 8 cm
Beam divergence ~1 μrad ~5 μrad
gain length 4.8 m (160 periods) 18.8 mm (94 periods)
leng
th (
mm
)
L LR G>
10 20 30 40 50 60 700
0.5
1
1.5
2
2.5
3
x 109
10 20 30 40 50 60 700
0.5
1
1.5
2
2.5
x 109
0 10 20 30 40 50 60 70 800
2
4
6
8
10
12
14
16
18x 10
8
10 20 30 40 50 600
0.5
1
1.5
2
2.5
3
3.5
x 109
time (attosec)
pow
er (W
)
Proposed parameters for laser driven SASE–FELEffect of initial energy spreadEffect of initial energy spread
5% 1%
0.2% 0.1%
0 500 1000 1500 2000 2500 3000-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1x 10-5
0 10 20 30 40 50 60 70 80 90 1000
2
4
6
8
10
12
14x 109
time (attosec)
pow
er (W
)
0 500 1000 1500 2000 2500 30000
1
2
3
4
5
6x 1028
1 pC, 2 GeV, λu = 200 μm,3000 periodsrb ~ 200 nmB ~ 1 T
Photon field buildupPhoton field buildupm
illion
s of
pho
tons
Time profile of Time profile of SASE FEL pulsesSASE FEL pulses
Pow
er (G
W)
0 20 40 60undulator position (cm)
6x106 photons1 photon/electron10 GW peak power~15 attosec FWHM~20 attosec timing jitterEγ~ 190 kev
6x106x1066 photonsphotons1 photon/electron1 photon/electron10 GW peak power10 GW peak power~15 ~15 attosecattosec FWHMFWHM~20 ~20 attosecattosec timing jittertiming jitterEEγγ~ 190 ~ 190 kevkev
Loss of kinetic energyLoss of kinetic energy
322
0
343
1
euG nkeK
mLμ
γ=
Comparison with the analytical steady state field growth model
Analytical Gain length expressionAnalytical Gain length expression 1 pC, 2 GeV, λu = 200 μm,rb ~ 200 nmB ~ 1 T 18.8 mm
(94 periods)
0 500 1000 1500 2000 2500 30000
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5x 1028
LCLS laser-driven SASE-FEL
gain length 4.8 m (160 periods) 18.8 mm (94 periods)
cooperation length 25 nm 0.56 nm
Lb/Lc 920 5.4
FEL parameter 5 x 10-4 4.9 x 10-4
LCLS laser-driven SASE-FEL
gain length 4.8 m (160 periods) 18.8 mm (94 periods)
cooperation length 25 nm 0.56 nm
Lb/Lc 920 5.4
FEL parameter 5 x 10-4 4.9 x 10-4
200
L Leff G≈ 2
small signal gain lengthsmall signal gain length
Leff
SASE–FEL electric field amplitude evolution
-3500 -3000 -2500 -2000 -1500 -1000 -500 0
-1
-0.5
0
0.5
1
x 10-3
lower mean energy
kinU
0 10 20 30 40 50 60 70 80 90 1000
0.5
1
1.5
2
2.5
3x 1010
time (attosec)
pow
er (W
)
-3500 -3000 -2500 -2000 -1500 -1000 -500 0
-1
-0.5
0
0.5
1
x 10-3
0 10 20 30 40 50 60 70 80 90 1000
0.5
1
1.5
2
2.5
3x 1010
time (attosec)
pow
er (W
)
( )tzE ,~ ′( )tzE ,~ ′
kinU
Pulse 1 Pulse 2
( )tzP ,′ ( )tzP ,′
Proposed parameters for laser driven SASE–FEL
oscillator laser
field emission
tip
laser accelerator
amplifiers
low energy high energy
undulator
beam dump
2 m ½ m
xyz
laserbeam
cylindrical lensvacuum
channel
electron beam
cylindrical lens
top view
λ/2
λ
xyz
laserbeam
cylindrical lensvacuum
channel
electron beam
cylindrical lens
top view
λ/2
λ
top view
λ/2
λ
~200 μm
~50 μm
~ 40 cm
~200 μm
~50 μm
~ 40 cm
1 pC, 2 GeV, λu = 200 μm,Lu = 40 cmrb ~ 200 nmB ~ 1 T
Summary1. There are many different advanced particle
acceleration concepts
2. Structure loaded vacuum laser acceleration• Similar to RF, but λ ~ 1μm• Possibility for relativistic, high energy
attosecond electron bunches
3. Proof-of-principle demonstration• Linear scaling law• Single laser-electron interaction
4. Short-term objectives• Staged acceleration• First test of dielectric accelerator structures• Low energy laser particle acceleration
5. Eventual goals• Eventual tabletop all-laser accelerator• Utilization of attosec electron bunches for
light sources
Backup slides
Simulation: time-dependent flux
Driving laser electric field: 8 fs pulse
Electron current: A single 700 as pulse
Electron current:Double pulse
P. Hommelhoff et al, Kasewich group
Light induced electron emission processes
Conduction band
Metal Vacuum
Energy
distance
Conduction band
Metal Vacuum
Energy
distance
e-
μ
Φωh
γ >> 1: multiphoton emission Optical field emission
Conduction band
Metal Vacuum
Energy
distance
e-ωh
Photo-assisted field emission
Both processes are prompt
Increased tunnel current due to heating of electron gas: slow, typ. 100fs – 1ps
SASE – FEL processCartoon of the principleCartoon of the principle
Interaction between the undulator radiation and the particlesInteraction between the undulator radiation and the particles
• can there be coherent amplification of the undulator radiation?
Yes!! SASE-FEL mechanism
“Self-Amplified Spontaneous Emission Free-Electron Laser”
• what are the conditions for this self-starting radiation mechanism?
λu
λ r
Electron trajectory
( )( )L x z dzu
u
≈ + ′∫λλ
2
0
ΔL vvu
x
zλ≈
⎛
⎝⎜
⎞
⎠⎟
12
2
ΔL eBk mcu uλ
≈⎛
⎝⎜
⎞
⎠⎟
12
02
2
Electron time delay
Δτ τ τ= −e c
Δτβ
λ= −
Lc ce u
ΔΔφ π τ
τ= 2
opt
ΔΔ
φ πλ
δ δz
LL
vvr
= −⎛⎝⎜
⎞⎠⎟
2
1. Electron position in the optical field 1. Electron position in the optical field
SASE – FEL process
The electron slips behind by one λr for every undulator period of travelThe electron slips behind by one λr for every undulator period of travel
The resonance condition:
3. The pendulum equation3. The pendulum equation
The amplitude of the optical field is
fixed
ψ
ηcEku
osc01
2~χ
πτ
ηψuk
dtd 2=Electron phase advance
Electron energy change ψχη sin01Edtd
−=
221 2 mceKc
γχ =
0 1 2 3 4 5 6
-0.01
-0.005
0
0.005
0.01
SASE – FEL process
3. The Maxwell Equation3. The Maxwell Equation
Rate of change of the optical field due to the electronRate of change of the optical field due to the electron’’s motions motion
⎥⎦⎤
⎢⎣⎡
∂∂
+∂
∂−=
⎥⎥⎦
⎤
⎢⎢⎣
⎡∇−⎟
⎠⎞
⎜⎝⎛
∂∂
+⎟⎠⎞
⎜⎝⎛
∂∂
⊥ xc
tJ
cE
ztcex
xρ
ε2
20
222 11
( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛−=⎟
⎠⎞
⎜⎝⎛
∂∂
+∂∂
ΔΔ∈
−∑ veKqtzEztc j
i jψ
γεβ
02,~1
Galilean Transformation
( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛−=′
∂∂
ΔΔ∈
−∑ vetzEt j
i jψχ2,~
( ) ( )tctzEtzE
ctzz
,~,~~ +′=→
+′=
γεβχ0
2 2Kqc
=
SASE – FEL process
Àmpere’s law
Field evolution Current density & phase( )~ ,E z t
tail head
R. Bonifacio, C. Pellegrini, L.M. Narducci, “Collective Instabilities and high-gain regime in a free electron laser”, Opt. Comm. 50, 373-378 (1984)
40 45 50 55 60 65 70 75
1
2
3
4
5
6
7
8
9
10
x 109
0 10 20 30 40 50 60 70 80 90 100-5
-4
-3
-2
-1
0
1
2
3
4
5x 1012
0 10 20 30 40 50 60 70 80 90 100-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
15 attosec
powerpower
Electric fieldElectric field phasephase
Snapshot of a particular Snapshot of a particular SASE FEL pulseSASE FEL pulse
