demonstration of optical microbunching and net

WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 1

Demonstration of Optical Microbunching and Net Acceleration at 0.8 microns

(almost)

E163 Collaboration, SLACChris M.S. Sears, Eric R. Colby, Rasmus Ischebeck, Robert

Noble, Chris McGuinness, Robert Siemann, James Spencer, Dieter Walz (SLAC, Menlo Park, California),Robert L. Byer, Tomas Plettner (Stanford University,

Stanford, Califormia)


Goals• Produce optically spaced electron microbunches

• Obtain independent verification & measure of microbunching

• Perform net acceleration of electrons with a laser


Recent E163 Timeline• March 2007: First beam in hall (experimental

chamber empty) beam line commissioned

• April: Experiment installed

• May 9th: Beginning of experimental run

• May 23rd: First laser-electron interaction observed at E163 46 48 50 52 54 56

20

22

24

26

28

30

32

34

Delay (ps)

Ene

rgy

Spr

ead

(px)

Run 0094 raw data

Laser OnLaser Off


Talk Outline• Overview of E163 program, facilities, experiment hardware

• Initial results from IFEL interaction; lessons learned from first run

• Simulations of planned experiments: – Microbunching & COTR experiment– Net acceleration experiment

• The (near) future of laser electron acceleration at E163


E163 Facility and Experimental Hardware


NLCTA/E163 Facility• 60 MeV xband linac w/ sband photoinjector producing 50 pC,

0.5 ps electron pulses at 10 Hz, δE<0.1%• Two regenerative amplifiers

– 2 mJ/pulse regen to produce 100-150 μJ/pls UV for photoinjector– 1 mJ/pulse regen for light to experiment

• 2’x3’ vacuum box atop a 4’x6’ optical table for housing the experiment

• 90° energy spectrometer: ~2 keV resolution• streak camera for timing

For more info, see: Poster FRPMS072 “Diagnostic and Experimental Procedures for the Laser Acceleration Experiments at SLAC”, presenter Chris McGuinness


layout for net acceleration experiment

Experimental Area Layout

Chamber + table house...•magnetic hardware (next few slides)•accelerator structures, (THPMS080 & THPMS050)•aerogel Cherenkov radiator for timing•long working distance microscopes to view YAG screens for beam overlap and general diagnostics•laser delay & focusing optics; photodiodes & laser position monitor

ebeamdirection

1

2

3

4

5

6

7

1. Undulator with YAG screens2. Chicane3. ITR Accelerator4. Downstream YAG &

Cherenkov cell5. Laser steering, focusing, &

diagnostics6. Long working distance

microscopes7. COTR PMT

to spectrometer


The Microbunching Hardware

YAG screens for beam alignmentinserted by pneumatic actuator

Undulator:•3 periods, 1.8 cm period•Adjustable gap 4-10 mm (kw=0.3-1.3)•Upstream & Downstream YAG screens for alignment

Chicane:•3 Hybrid H-magnets (center one double thickness)•0.35 ±0.1 T•Water cooling to base plate

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

-0.06 -0.04 -0.02 0 0.02 0.04 0.06Fiel

d (T

)

Chicane Field On-Axis (no current)

-0.6

-0.4

-0.2

0

0.2

0.4

-150 -100 -50 0 50 100 150position (mm)

Fiel

d (T

)


IFEL Alignment YAG screens

flippermirror

long working distancemicroscope

•YAG scintillator + scatterer for laser•Fully automated screen insertion, image capture & analysis•7mm FOV; 25 μm resolution

~0.5 m

+y

+x

0 100 200 300 400 500 600 7000

2

4

6

8

10

0 50 100 150 200 250 300 350 400 450 5000

2

4

6

8


Initial Results

• Re-established IFEL interaction at 800nm

• electron σt=0.4ps• Some interactions up to 100

keV rms – matches simulation• temporal jitter <0.2ps• Long term stability > 5minutes

(longest data scan executed)

40 42 44 46 48 5010

20

30

40

50

60

Delay (ps)E

nerg

y S

prea

d (H

alf-w

idth

, Hal

f max

.; ke

V)

Example Data Scan

Fit ParametersAmp: 32.4 keVSigma: 0.52 ps

Laser OnLaser OffFit

Goal: Oberserve COTR radiation from microbunches


Lessons Learned• Many challenges overcome, still learning about running

NLCTA• pellicle mirror not such a good idea; found ~factor of 10

increase in emittance• Laser wavelength -> microbunch spacing: laser was running

at 785nm, 2nd harm. at 392.5nm, but using a 400±5nm bandpass filter in COTR detection (oops)

• Spot size at radiator too large: will try pinhole collimator before radiator foil


Optical Microbunching and COTR Diagnostic

Experiment 1


phototube

band pass filterat 400 nm

lenspellicles

E-beam

Experiment 1: Coherent Optical Transition Radiation for detecting Microbunching

•independent measure of microbunching for optical acceleration experiments•Allows optimization of bunching chicane R56•scan at 2nd harmonic to avoid laser background

0 20 40 60 80 10010-12

10-11

10-10

10-9

10-8

10-7

10-6COTR from 50pC, 1ps Microbunched Beam

Spot Size (μm)

Tota

l CO

TR (J

/pul

se)

800nm400nm266nm

IFEL and Chicane together.


-8 -6 -4 -2 0 2 4 60

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40

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160

Phase

Histogram In Phase

Analytic (1-D)Simulation

Experiment 1: Microbunching Characteristics

Analytic form is for plane wave and beam with no divergence/focusing (i.e. 1-D)From comparison, bunch quality clearly dominated by initial Energy spread...

-4 -2 0 2 4 6 8

126.75

126.8

126.85

126.9

126.95

127

127.05

127.1

127.15

127.2

127.25

Phase

Fina

lEne

rgy

Effect of Transverse Position on Bunching

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

x 10-4

No initial E spread

Rad

ial P

ositi

on (m

)

-4 -2 0 2 4 6 8

126.75

126.8

126.85

126.9

126.95

127

127.05

127.1

127.15

127.2

127.25

Phase

Fina

lEne

rgy

Effect of Transverse Position on Bunching

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

x 10-4

No initial E spread

Rad

ial P

ositi

on (m

)

No initial energy spread


Experiment 1: Possible ExtensionMulti-color COTR & determining longitudinal profile

( ) ( ) ( )2

01

1 2 exp cos2IFEL n B

n

nz J n nk zγσρ ρ β

η

∞

=

⎡ ⎤⎡ ⎤⎛ ⎞⎢ ⎥= + −⎢ ⎥⎜ ⎟⎢ ⎥⎢ ⎥⎝ ⎠⎣ ⎦⎣ ⎦

∑

Microbunching density should be given by:

Longitudinal distribution for β=1, η/σγ=2.5, 5 optical cycles shown.

0 1 2 30

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6x 10-8 COTR As Function of Chicane Strength

β (prop. to Chicane Strength)

CO

TR E

nerg

y (J

/pul

se)

800nm400nm x10266nm x 100

scanning chicane strength and finding max’s for each color will inform on longitudinal profile

Peak FWHMfor η/σγ=2.5

FWH

M (λ

)

β

minimum=0.12λat β=1.23

Note: peak harmonic content ≠ densest bunch

56lk Rβ η=


Net Acceleration of Electrons with Light

•Combine microbunching hardware with ITR accelerator to obtain net acceleration•laser power split between IFEL and ITR•Careful beam control and electron filtering to avoid interference and obtain signal

Experiment 2


Exp 2: IFEL-ITR Experiment Simulation

Code Overview:•Euler method Integration of Lorentz eqs. No emission/absorption•1-D field profiles of undulator & chicane from magnetostatic code Radia

•ignores focusing & edge effects of magnets; previous studies found these to be negligible

•Analytic form for full TEM00 laser field for both lasers

2038

Parameter ValueE-beam

Beam Energy 60 MeVSpread 0.1%Emittance 1.5 π-mm-mradFocused Size 50 μm (rms)

IFEL LaserEnergy 0.4 mJ/pulseFocused Size 100 μm (rms)Focus Location 10 cm after und.

ITR LaserEnergy 0.6 mJ/pulseFocused Size 50 μm (rms)Angle 7 mrad


0 0.5 1 1.5 20

0.2

0.4

Total Power Available (mJ)

Power to IFEL for Optimal Acceleration

Pow

er to

IFE

L (m

J)

Exp 2: Laser Power Optimization

0 0.2 0.4 0.6 0.8 10

5

10

15

20

25

30

Power to IFEL (mJ/pulse)

Pea

k N

et A

ccel

erat

ion

Laser Power Distribution Optimization

0.50.7511.25

Power Available

(keV

)


-1 -0.5 0 0.5 1

x 10-4

-25

0

25

Mean Energy Shift w/ Horizontal Position

Horizontal e- Position (m)

Mea

n E

nerg

y S

hift

(keV

)

Compare to Wavelength/sin(angle)=96 μm

So, need to collimate e-beam or focus very tightlyto see net acceleration

Exp 2: Interaction at Tape & effect of e- beam spot size


Exp Parameters:•Ebeam

•γ=117, Δγ=0.1%•50 micron focus (on tape)•εN=4e-6 m

•IFEL:•0.4 mJ/pulse•100 micron focus•z0=10 cm (after center of und.)•0.5 ps FWHM•Gap 8mm

•Chicane 20 cm after undulator•ITR:

•38 cm after undulator•0.6 mJ/pulse•50 micron focus•0.5 ps FWHM

Total Energy Spread After Tape ~180 keV FWHM.

Experiment 2:Full Net Acceleration Experiment Simulation

Collimation of ±25μm about center (38% acceptance)

0 1 2 3 4 5 6-40

-30

-20

-10

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10

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30

40

IFEL-ITR phase

Cen

troid

Ene

rgy

Shi

ft (k

eV)

Net Acceleration Experiment

SimulationFit

Amplitude = 25 keV


The Near Future of E163• Observe COTR/perform net acceleration

experiment

• Inverse Transition Radiation (ITR) studies – T. Plettner, THPMS080

• Ultra strong focusing & wakefield from PBG fibers – C. Sears, THPMS052

• First optical scale acceleration - THPMS050

Thorlabs HC-1550-02 Photonic Crystal Fiber


Many Thanks

• PAC07 Organizing Committees

• Janice Nelson, Doug McCormick, Justin May, ToneeSmith, Keith Jobe, and Richard Swent

• Zach Wolf & rest of magnetic measurement group

• Denise Larsen

• Roger Carr

RasmusIschebeck

Ben Cowan

Jim Spencer

Chris McGuinness

Bob Siemann

Eric Colby

Bob Byer

Tomas plettner

Chris Barnes

Chris Sears

Bob NobleDieter WalzNot in photo

The E-163 Collaboration

demonstration of optical microbunching and net

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