The first 6 years of The LaserElectron Accelerator Project

1
The first 6 years of The Laser-Electron
Accelerator Project at the SCA-FEL facility
knowledge gained toward the design of the E-163
experiment
The SCA-FEL facility
SLAC
T. Plettner, ORION 2003 Workshop
2
Topic for this talk
The Laser Electron Acceleration Project began
more than six years ago and has succeeded in
developing the experimental apparatus and
expertise to conduct laser-electron
acceleration experiments. During this period our
demand for beam quality and beam time have
outgrown the capabilities of the SCA-FEL
facility.
Purpose of this talk

• To present the important accomplishments
  achieved at the SCA-FEL
• Present the difficulties encountered at the
  SCA-FEL facility
• Describe the connection with the E163 experiment

• Overview
• Physics of acceleration from crossed laser beams
• The experiment at the SCA-FEL facility
• Conclusions

contents
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Overview
What is LEAP?
LEAP Laser Electron Accelerator Project
Objective
Proof of principle experiment for laser driven
particle acceleration from crossed laser beams in
vacuum
Description

• LEAP is a joint project (SLAC ARDB group, HEPL
  and the
• department of Applied Physics on campus)
• The experimental work started 6 years ago and is
  located at
• the SCA-FEL facility on campus.
• The experimental effort focused on the
  observation of laser-induced
• energy broadening of the electron beam after 1
  interaction with the
• laser beam in a single accelerator cell

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The physics of acceleration from crossed laser
beams in vacuum
P. Sprangle, E. Esarey, J. Krall, A. Ting, Laser
Acceleration of Electrons in Vacuum, Optics
Comm. 124 (1996) 69
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Key features of a laser-accelerator cell
Crossing laser beams

• Longitudinal electric field of the laser
• pattern responsible for acceleration

slits 5-10 mm
1.5 mm
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Layout of LEAP at the SCA-FEL facility
Description of the experiment

• Tisapphire
• 1 mJ/pulse
• 1 kHz rep. Rate
• 800 nm
• T 2 psec

Laser
high resolution energy spectrometer
30 MeV electron beam
accelerator cell
7
The experimental procedure Time scans of the
laser around the temporal overlap with the
electron beam
e- beam
50 100 psec time scan window
90o bending magnet
e- beam pulses
1500 mm
16 mrad
LEAP cell
140 mm
10 mm
laser pulses
0.8 mm
63
energy spectrum 2 keV resolution
7.5 x 108 V/m
energy
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The important components of the experiment
The accelerator cell The spatial and temporal
overlap monitors The energy spectrometer The
electron beam transport system The laser and the
optical transport system
The accelerator cell
The spatial and temporal overlap monitors
The energy spectrometer
The electron beam transport system
The laser and the optical transport system

• description
• problems encountered
• E163

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The accelerator cell
purpose of the cell finite interaction space
Slit of variable width for tuning purposes
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The accelerator cell
Special low-expansion Epoxy employed in the
assembly procedure
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The accelerator cell
Material Environment Surface quality Conditioning
DU 20keV
Dependence on
0.6 J/cm2 ? 0.8 J/cm2
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The beam overlap monitors
Spatial monitor
Cerenkov cell
tilt stage
pellicle
YAG screen holder
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The energy spectrometer
5 pC/ bunch
PI-MAX ICCD camera
doped YAG fluorsecent screen
spectrometer focal plane
65 cm
175 cm
55 cm
53.48 cm
86 cm
LEAP accelerator cell
focusing triplet
iron core 90o dipole magnet
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The data collection
control of the laser timing to within 1 psec
e- beam
50 100 psec time scan window
Software hardware have been developed
90o bending magnet
e- beam pulses
control of the laser timing recording of
instrument settings collection of background
images collection of data images laser
on/ laser off data rejection
stepping of the laser timing
LEAP cell
laser pulses
energy spectrum 2 keV resolution
energy
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The energy jitter
Typical spectrometer image
vertical coordinate
benefit consistent 15-20 keV FWHM peaks
drawback slowdown of collection of data
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The energy collimator
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The position jitter
Spatial monitor
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The electron transport line at the SCA-FEL
facility
Some general comments
gun

• difficult tuning procedure
• phosphor screens and current
• monitors are the diagnostics
• manual current adjustments of
• steering and focusing elements
• distance from gun to LEAP gt 100m
• 24-36 h for electrons to
• reach the LEAP spectrometer
• Superconducting accelerators complicated
• He refrigeration system
• pressure bursts
• periodic bake-outs necessary
• slow RF phase drifts in the electronics
• periodic retuning of BC,CS,PA

400 ft
Experimental time ltlt Setup time
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The optical transport line
timing
400 ft. of vacuum line to transport the laser beam
pair of laser beams produced by 50 beam splitter
PZT crystal controls relative optical phase
between the laser beams
80 MHz, 1 nJ/pulse 800 nm
Spectra Physics Tsunami
17 cm focal length lens focuses beam pair
into the accelerator cell
piezo xtal
1 KHz, 1 mJ/pulse 800 nm
Variable delay arm
Positive Light Spitfire
400 ft. transport
beam splitter
accelerator cell
f 180o
f 0o
telescope
telescope
Fixed delay arm
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The optical transport line

• location of the laser system
• inconvenient location for LEAP (gt100 m of
  vacuum transport line necessary).
• 8 mirrors, 2 vacuum windows, ½ of the laser
  power lost in the optical transport line.
• temperature gradients across the building ?
  slow drifting of the laser beam.
• mechanical vibrations ? fast position
  fluctuations on 1cm near the LEAP site.

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Summary of valuable components developed at LEAP
The concept of the variable slit with the LEAP
accelerator cell proved to work very well.
Transmission through 10 mm slits was routine.
Reduced laser damage threshold due to the special
conditions (presence of sharp edges, HR coating,
and vacuum) limited the Possible energy gain to
20 keV
The arrangement for the spatial and temporal
overlap monitors developed for the LEAP cell
worked very reliably
The combination of the high resolution
spectrometer/ doped YAG screen and PIMAX
intensified camera allowed to record clear
energy spectra of the electron beam even with the
collimators in place
Due to the observed position jitter of the
electron beam the spatial collimator proved to be
a crucial component.
The energy collimator helped reduce the energy
spread and its jitter to 20 keV
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Shortcomings encountered at the SCA-FEL facility
The electron beam at the SCA-FEL facility very
large distance between LEAP and the
accelerators qualitative diagnostics very
difficult and time consuming tuning procedure
requires constant human attention
instabilities and disruption from the He
refrigeration system
The location of the laser the assembly of
the 400 ft vacuum line was very expensive
loss of laser power position drift of the
laser
The location of the LEAP control room one
round trip to the experiment area is 1/2 km
The limited amount of beam time one run/year
in the past two years 10 days of beam time
for LEAP time demanding setup
Few hours of data collection
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Present status of LEAP
accomplishments

• manipulation and characterization of individual
  electron beam bunches
• transmission through an accelerator cell with 10
  mm entrance and exit slits
• implementation of spatial and temporal overlap
  monitors
• laser timing control to within 1 psec and the
  automated laser timing
• scan data acquisition software
• insertion of an energy filter upstream of LEAP

remaining obstacles and uncertainties

• uncertainty in the electron beam timing jitter
  (could be gtgt 1 psec)
• possible electron bunch duration gtgt 1 psec
• very low electron beam charge
• very limited availability of beam time

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Conclusions

• we have developed and tested the individual
  components for the
• proof-of-principle experiment. Most of these
  can be transferred
• to the E163 experiment.
• with a stable 2 spec electron bunch of 20 keV
  FWHM the expected
• 20 keV energy modulation from the laser should
  be observable at the
• present setup, provided there is no timing
  jitter of the e-beam.
• the quality of the beam (low charge, large
  energy and spatial jitter) present
• a serious obstacle for observing the expected
  20 keV energy modulation.
• the availability of useful beam time for
  experimentation at the present
• facility is very limited, slowing progress to
  unacceptable levels.

The LEAP project has reached a point where
substantial progress at the present facility
will be increasingly difficult, therefore making
the transition to a facility capable of
providing better, and more beam time very
feasible.