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