The Polarized Electron Source at ELSA
Wolther von Drachenfels, Frank Frommberger, Michael Gowin,
Wolfgang Hillert, Markus Hoffmann, Bernhold Neff
University of Bonn, Dept. of Physics, Nussallee 12, 53115 Bonn Germany
Abstract. At the electron stretcher accelerator ELSA in Bonn a pulsed 50 kV inverted gun of
polarized electrons has been in operation since February 2000. A strained-layer superlattice crystal
is used to deliver a beam with a polarization of about 80 %. A flashlamp-pumped Ti-Sapphire laser
with a pulse repetition rate of 50 Hz serves as source of light. The gun is operated in space charge
limitation. The current can be chosen by varying the distance between cathode and anode. With 1 /is
pulses of 100 mA the source was particularly used together with a polarized target for a GDH sum
rule experiment. The high photocathode lifetime allows continuous operation at 100 mA typically
for periods of about two weeks without maintenance. So far no change of the crystal was necessary.
INTRODUCTION
Polarization is of increasing importance in hadron physics and therefore efforts have
been undertaken at the electron stretcher accelerator ELSA (see Fig. 1) to provide
polarized electron beams for external fixed target experiments.
For this purpose in 1997 a pulsed source of polarized electrons came into operation at
ELSA to study depolarizing resonances which occur during acceleration of the beam in
the circular machines. These resonances might depolarize severely the beam and therefore have to be corrected for. To produce the polarized electrons the source used a GaAs
superlattice crystal illuminated by a pulsed Ti:Sapphire laser. From the measurements of
the polarization [1] schemes were developed to compensate the influences of the depolarizing resonances. But the reliability of the source was not sufficient for experiments
over several weeks. Therefore a second source was developed which started operation
in February 2000. In the following construction and operation experiences will be described.
SETUP OF THE POLARIZED SOURCE
In comparison with the old source the new source has several improvements in respect
of reliability, polarization, intensity and life time (see also [2]).
An important change is the lower high voltage of —50 kV compared with —120 kV
of the old source. The new source runs together with a second linac which meanwhile
came into operation. This linac operates with pulses of 1 /is and a repetition rate of 50
Hz but needs only 50 keV injection energy.
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
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Crystal Barrel
Dipole (horizontal)
hadron
physics
experiments
Dipole (vertical)
Quadrupole
Skew Quadrupole
Sextupole
*\ •'^ PETRA cavity
GDH
Combined-Function Magnet
Solenoid
Radio Frequency
-
stretcher ring
0.5 - 3.5 GeV
booster
synchrotron
\
bending magnet
beam lines for
I SR experiments
0.5-1.6GeV
EKSI-
LINAC1 s
<20MeV) ~-J
0
detector
FIGURE 1. Site plan of the electron stretcher accelerator facility ELS A.
In addition the source was designed as an inverted type gun, that means only the
cathode is on —50 kV, all other components are on ground potential. A careful HVdesign (surface fields below 4 MV/m) led to dark currents below 1 nA @ 60 kV and to
a very save operation of the source.
The polarization was increased by using a Be-InGaAs/Be-AlGaAs strained superlattice crystal available from Nagoya University [3]. With a flashlamp-pumped Ti-Sapphire
laser at A = 830 nm an electron polarization of about 80 % is obtained. From the 10 /is
pulses of the Ti-Sapphire laser a pulse slicer cuts 1 jUs pulses which are fed via a 85 m
long optical fiber to the source. The cathode anode arrangement allows to operate the
source in space charge limitation. The perveance of the gun is adjustable. The pulse intensity can be chosen by varying the distance between cathode and anode between 45
and 70 mm. A distance of 61 mm yields 100 mA. The strong so called "spiking behavior" of the Ti-Sapphire does not influence the shape of the electron pulse. Rectangular
pulses of 100 mA are delivered even when the quantum efficiency (QE) decreases down
to about 0.1 % during longer operation. The very slow decrease in QE means excellent
life times of up to 5000 hours for the polarized source. Therefore only seldom a new
activation of the photocathode is necessary (see Fig. 2).
The setup of the source is shown in Fig. 3. The source mainly consists of a gun
chamber and a small preparation chamber which has to be vented for crystal exchange.
The materials (low carbon stainless steel, molybdenum and marcor ceramic) for the gun
chamber and the preparation chamber have been chosen carefully and vacuum fired at
1050 °C or 800 °C respectively before assembling. Silver plated copper sealings with
an additional blocking layer of nickel have been used. With combinations of ion getter
pumps and non-evaporable getter (NEG) pumps a vacuum of 1 x 10"11 mbar in the
gun chamber is obtained during operation. With a manipulator the photocathode can be
pulled back into the preparation chamber for activation. Here heat cleaning is performed
with a filament heater at 19 W (equivalent to 450 °C). The temperature measurement
was calibrated in the temperature range of 500 - 600 °C with an infrared thermometer
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0,01
Jan
Mar
May Jul
Sep Nov Jan Mar May
2000
FIGURE 2.
FIGURE 3.
Jul
Sep Nov
2001
The quantum efficiency measured at two different wavelengths.
Setup of the 50 keV source: Inverted gun and preparation chamber.
using a bulk GaAs crystal and then extrapolated towards lower temperatures . After heat
cleaning of about 1 hour and cooling down of about 6-7 hours a typical activation lasts
about I1/2 hour. Since beginning of 2000 only one crystal was in use, no change was
necessary.
To connect the source with the linac a 6.2 m long transfer line was installed (see Fig.
4). The vacuum tube has a diameter of 35 mm. Two a-magnets and an electrical bending
by 90° together with several ion getter pump stations serve for differential pumping from
10~7 mbar at the linac (vacuum dominated by water vapor) to partial pressures below
10"13 mbar at the gun for poisoning gases like H2O and CO2. Using a large number of
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10 "mbar
10~7mbar
(A) Wire Scanner
(v) Screen Monitor
Solenoid
B Quadrupole
FIGURE 4. The transfer line from the polarized source to the linac.
steerers, solenoids and additional quadrupoles a transfer efficiency to the linac of close
to 100 % is obtained. The use of wire scanners and screens is essential to align the
beam. All components of the source and the transfer line are under computer control
and integrated in the ELSA control system. Parameter sets can be stored and reloaded
easily. An optimized set was derived in February 2000 and was used for more than two
years without changes.
In table 1 the main parameters of the source are listed.
TABLE 1. Parameters of the polarized electron source.
crystal
light source
laser power at the photocathode
laser spot size on the cathode
electron polarization
electron current, pulsed
pulse length
repetition rate
time of operation
Be-InGaAs/AlGaAs strained-superlattice
flashlamp-pumped Ti:Sapphire laser
«5kW
8 mm0
about 80 % at 830 nm
100mA
1 jUS
50 Hz
typically two weeks without maintenance
OPERATING THE SOURCE WITH ELSA
The studies of depolarizing resonances have been continued with the new source. The
depolarizing influence of imperfection resonances could be compensated by special vertical orbit corrections. In addition two fast pulsed quadrupoles have been installed in the
main ELSA ring to minimize the influence of intrinsic resonances. The measurements
of the beam polarization were done by M011er scattering on the extracted beam. When
accelerating from 1.2 GeV at injection into the main ring up to 3.2 GeV nine resonances
have to be crossed. The results are shown in Fig. 5. It shows that the source together
with ELSA can provide beam polarization of more than 55 % (even above 3 GeV) for
the experiments.
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In the last two years the source delivered polarized electrons over more than 2500
hours for the Gerassimov-Drell-Hearn (GDH) sum rule experiment [4]. Single runs last
up to three weeks. During a run no maintenance at the source was necessary. A source
availability of close to 100 % has been obtained.
Polarization in ELSA
0.8
1.0
1.2
1.5
2.0
3.0
E / GeV
fya=3| |7a=8-Qz||ya=4][ya^+Q^ |ya=5] fya=iq
Depolarizing resonances
FIGURE 5. Polarization of the electron beam in ELSA.
ACKNOWLEDGMENTS
We would like to thank T. Nakanishi and his group from Nagoya University for providing
us photocathodes and for the fruitful collaboration.
This work was supported as part of the SPP 1034 by the Deutsche Forschungsgemeinschaft.
REFERENCES
1.
2.
3.
4.
Nakamura S. et al., Nucl Instr. andMeth. A 411, 93-106 (1998)
Hillert W. et al., SPIN 2000 Proc. of the 14th International Spin Physics Symposium, 961-64 (2001)
Nakanishi T. et al., Proc. Low Energy Polarized Electron Workshop, St. Petersburg, 118-124 (1998)
Helbing K. et al., see this proceedings, and Speckner T., thesis, Univ. Erlangen (2002) Zeitler G.,
thesis, Univ. Erlangen (2002)
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