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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)
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 2
Goals• Produce optically spaced electron microbunches
• Obtain independent verification & measure of microbunching
• Perform net acceleration of electrons with a laser
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 3
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
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 4
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
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 5
E163 Facility and Experimental Hardware
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 6
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
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 7
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
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 8
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
)
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 9
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
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 10
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
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 11
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
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 12
Optical Microbunching and COTR Diagnostic
Experiment 1
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 13
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.
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 14
-8 -6 -4 -2 0 2 4 60
20
40
60
80
100
120
140
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
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 15
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=
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 16
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
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 17
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
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 18
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
)
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 19
-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
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 20
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
0
10
20
30
40
IFEL-ITR phase
Cen
troid
Ene
rgy
Shi
ft (k
eV)
Net Acceleration Experiment
SimulationFit
Amplitude = 25 keV
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 21
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
WEXKI02 Particle Accelerator Conference. Albuquerque, NM, June 2007 22
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