High Power Diode Laser System For SHR
D.Cheever, M.Farkhondeh, W.Franklin, E. Tsentalovich, T.Zwart
MIT-Bates Linear Accelerator Center, Middleton, MA, USA
Abstract. Experiments with a polarized electron beam stored in the South Hall Ring (SHR) at
MIT-Bates Linear Accelerator Center will begin in 2003. Currently, the commissioning of
BLAST detector is under way. The polarized injector uses for the first time high power diode
array laser for photoemission. The laser operates at a wavelength of 808 nm and produces peak
power up to 150 W at a duty factor of 1.5%. Higher power is available at lower duty factor. The
laser is coupled to a fiber; laser beam emitting from the fiber has an emittance of 200 mm-mrad
and a set of lenses is used to deliver the beam through polarizing optics to a strained GaAs
crystal inside the electron gun. The photocathode has a diameter of 11 mm and the laser
illuminates the entire area. The gun optics has been specifically designed for such a large beam
spot size. The diode laser provides an excellent stability and convenience of operation. At the
same time, large divergence of the laser beam requires special attention to the transport system
in general, and to polarizing optics in particular.
INTRODUCTION
The MIT-Bates accelerator complex operates in three different modes. The electron
beam accelerated in the linac can be immediately used for the experiments ("pulse"
mode). In the second mode the beam is injected into the SHR, and then slowly
extracted into the experimental area, thus increasing the duty factor from 1% to almost
100% ("stretcher" mode). Finally, beam is stored and stacked to high average currents
in the SHR for internal target experiments ("storage" mode). The requirements for the
polarized electron injector are presented in the following table.
I (Peak), AT,
Rep.Rate, I(Aver.), P(laser)
P(laser)
mA
Hz
A
(QE-1%)
(QE-.05%)
jisec
M
Pulse
~60W
-120
- 10
-3W
-25 600 Hz
mode
~4
Stretcher
600 Hz
~60W
~ 10
-20
-3W
mode
~4
Storage
<lHz
«1
~60W
.-10
~3W
mode
The existing cw Ti:Sa laser is quite adequate for the operation with bulk GaAs
photocathodes. It is tunable, has reasonable stability, and any temporal structure can
be tailored out of the cw beam using Pockels cell based shutter. But the maximum
peak power available from this laser doesn't exceed 4 W, and it is insufficient for the
operations with high polarization (strained or superlattice) photocathodes.
Flash-lamp pumped Ti:Sa laser can be used for the "storage" mode. It provides
hundreds of Watts of laser power and is tunable. However, the stability of this laser is
CP675, Spin 2002:15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. L Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
1019
very low, and the maximum repetition rate is only about 10 Hz. Therefore, this laser
doesn't meet the requirements for the "pulse" and "stretcher" modes.
FIGURE 1. Diode array laser system. Two diodes are installed on the cooling plate to maintain a
constant temperature. The two armored fibers conducting the light from the lasers merge together into a
single fiber.
In 2001 we acquired a multimode fiber-coupled diode array laser system [1] that
meets the requirements for all modes listed.
LASER PARAMETERS AND PERFORMANCE
The diode laser can be operated in both cw and pulse modes. In cw mode the power
can be as high as 60 W. In the pulsed mode the maximum peak power is at least ISO200 W. Higher power can be achieved at low duty factor, but it may shorten the
effective life time of the laser. The rise time for the pulses is limited by the inductance
of the leads in the current design to about 0.1 jusec, which is quite adequate for our
application.
The laser provides excellent stability, extremely convenient in operation and
virtually requires no maintenance.
The drawbacks of the laser are the fixed wavelength (X = 808 nm) and a very high
emittance s = 200 mm-mrad (compare with s « Imnrmrad of the diffraction-limited
beam from the Ti:Sa laser).
The advances in the technology of photocathodes production allowed designing the
crystals matching the fixed wavelength of the laser. The first photocathodes for our
injector with a maximum of polarization obtained at about 800-810 nm have been
produced in the laboratory of Spin-Polarized Spectroscopy, State Technical
University, St. Petersburg (Russia). Fig.2 demonstrates QE and polarization
wavelength dependence measured at Bates.
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10
100
80c
o
1
60-
••s
o
N
o
| 40
-- 0.1
O
20 -
0.01
740
760
780
800
820
840
Wavelength,nm
FIGURE 2. Polarization and Quantum Efficiency (QE) for the St. Petersburg photocathode.
Most recently we are using high gradient doped strained GaAsP photocathodes
grown by Bandwidth Semiconductor Inc. [2] with the SLAC specification [3]. The
phosphate fraction in these crystals («5%) is adjusted to obtain the maximum
polarization at our wavelength. The top 10 nm layer is heavily doped to minimize
surface charge effects. We measured a polarization of more than 80% and QE of 0.20.3 % at our wavelength of A, = 808 nm.
Very high divergence of the diode laser beam precluded use of the existing 20 m
long transport line for the Ti:Sa laser. A new wide aperture transport line was
designed. All optical elements are installed on an optical breadboard located next to
the gun, and the laser beam is transported straight to the cathode without any mirror
bounces. For the "storage" mode operation we are using only waveplates to produce
and reverse circular polarization. A A/4 plate following a linear polarizer transforms
linear polarization into circular. For slow reversal of the helicity a remotely controlled
A/2 plate mounted on a pneumo-driven actuator is inserted into or removed from the
laser path. The maximum rate of slow helicity reversal is about 1 Hz.
"Stretcher" and "pulse" modes might require much faster rate of helicity reversal.
For these modes, a Helicity Pockels Cell (HPG) must be used. Pockels cells
performance deteriorates with large beam divergence. To minimize the divergence of
the beam with a given emittance passing through the HPC, the beam size should be
maximized. We obtained a very large Pockels cell with a clear aperture of 75 mm [4].
A set of lenses is used to expand the beam to a diameter of about 50 mm. A 2" linear
polarizer (which is also sensitive to the beam divergence) and the HPC are located in
this section. An extinction ratio of about 140 was achieved in this set up. Two A/2
plates are installed after the HPC: one for slow helicity reverse and second for the
alignment of the direction of residual linear polarization with the photocathode axis in
order to minimize helicity-correlated effects. Since wave plates are less sensitive to
beam divergence we could use 0.5" plates. A waist was designed in the transport line
for these plates.
1021
Beam Transport
for the Diode Laser
100
-T——————
HPC
50
fD=-7!3 mm
f1=127 mm
12=600 mm
Expanded optics for
75mm HPC
X/2
1 i
I :/'':
'ib--.fl'- n
cathode
i
'-'••'.a r
-50
\
\
^st •;
^
:
-: : :
-
!
ft
... x. - .
-100
1000
2000
3000
4000
. : . .: . . . , . . . : ; • . • : • . . • • [
5000
Z.(mm)
FIGURE 3. Diode laser transport line.
The final lens focuses the beam unto the photocathode. The beam size is adjusted
by varying the lens location. Usually the laser beam illuminates the whole
photocathode (11 mm in diameter). Large beam spot size allows reducing surface
charge effect.
DRIVER FOR THE LASER
The laser requires a current pulse with sharp leading edge and amplitude of up to
200 A. We developed a driver with very low inductance. It is designed for long (up to
20-30 jisec) pulses and sharp (about 70 nsec) leading edge. This driver was
specifically designed for parity-violating experiments and allows controlling the
intensity asymmetry for different helicity states with an accuracy of better than 1 ppm.
RESULTS
Currently the polarized electron source with strained GaAs crystal and diode laser
is used for the commissioning of BLAST detector. It operates in the storage mode and
provides highly polarized (>80%) electron beam to the SHR. Excellent quality and
stability of the beam have been achieved. With very low duty factor in this mode, the
lifetime of the photocathode is very long. We perform cesiation approximately once a
week, and full activation once in 1-2 months.
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The preliminary measurements indicate that we will be able to control helicitycorrelated effects with this system to a level required for parity-violating experiments,
but further developments are needed. Also, further tests are required to prove our
ability to deliver high average current with high polarization photocathodes over long
periods of time.
REFERENCES
1.
2.
3.
4.
Spectra-Physics, Opto Power diode laser model OPC-DO60-FC.
Bandwidth Semiconductor Inc., Bedford, NH.
T. Maruyama et al, NIM A492, 199 (2002).
Electro-Optical Prod. Co., model QC-70I, max extinction ratio 280:1.
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