Aufbau einer gepulsten Quelle polarisierter Elektronen - Institut für ...

Nuclear Instruments and Methods in Physics Research A 379 (1996) 15 – 20
Picosecond polarized electron bunches from a strained layer
GaAsP photocathode
P. Hartmann a , J. Bermuth a , J. Hoffmann a , S. Köbis a , E. Reichert b1 , H. G. Andresen a1 ,
K. Aulenbacher b1 , P. Drescher b1 , H. Euteneuer a1 , H. Fischer b1 , P. Jennewein a1 ,
K.–H. Kaiser a1 , H. J. Kreidel a1 , Ch. Nachtigall b1 , S. Plützer b1 , M. Schemies b1 ,
K.–H. Steffens a1 , M. Steigerwald b1 , H. Trautner b1 D. v. Harrach a2 , I. Altarev c2 ,
R. Geiges a2 , K. Grimm a2 , Th. Hammel a2 , E. Heinen-Konschak a2 , H. Hofmann a2 ,
E.-M. Kabuß a2 , A. Lopes-Ginja a2 , F. Maas a2 , E. Schilling a2
a Institut für Kernphysik, Joh. Gutenberg - Universität, Mainz, Germany
b Institut für Physik, Joh. Gutenberg - Universität, Mainz, Germany
c St. Petersburg Nucl. Phys. Inst., RAS, 188350 St. Petersburg, Russia
Received 19 April 1996
Abstract
At the Mainz electron accelerator MAMI a facility has been set up that allows the measurement of the bunchlength,
the energy spread and the averaged and phase resolved polarization of picosecond electron bunches
from a source of polarized electrons for MAMI. Electron bunches of typically 10 to 30 ps duration are generated
by illuminating a strained layer GaAsP photocathode with femtosecond laser pulses.
1. Introduction
In recent years the demand on polarized electron beams
for nuclear scattering experiments has constantly
risen. Experiments at MAMI [1,2], CEBAF [3],
SLAC [4] and many other sites either require tens of
microamperes of average beam current or pulse currents
of several amperes.
The sources of polarized electrons used to satisfy these
demands are usually based on the photoemission
from III-V semiconductor photocathodes. Polarizations
between 30 % and 50 % are achieved with simple
bulk GaAs-type photocathodes [5]. Recently developed
GaAs-type photocathodes, consisting of superlattice
or strained layer heterostructures, have enhanced
the polarization up to 80 % and more[6,7].
One unsolved problem since the first use of GaAs
photocathodes at an accelerator site, is the decrease
of quantum efficiency with time when current is extracted.
The quantum efficiency of these photocathodes shows
a dependence on the produced charge, that is well
described by an exponential behaviour. Experience
at MAMI shows that the cathode lifetime decreases
when beam current is increased [8].
Currently, strained layer photocathodes are installed
in the MAMI source of polarized electrons. The source
emits a d.c. beam using a cw laser [8]. Since the accelerator
is radiofrequency (rf) driven, the d.c. beam
has to be chopped and prebunched before it is injected
into the accelerator. The chopper-prebuncher system
has a capture efficiency of 16% at best (see Fig. 1).
Thus a major part of the polarized current is not accepted
at the injection point,
I
I DC
84% not accelerated
408 ps DC current from source
This work is part of the Ph.D. thesis of P. Hartmann.
Corresponding author. Address: Institut für Kernphysik,
Johannes Gutenberg, Universität Mainz, J. J. Becherweg 45,
55099 Mainz, Germany. Tel.: +49 6131 39 5808, fax: +49 6131
39 2964, e-mail: hartmannakph.uni-mainz.de.
1 B2-Collaboration.
2 A4-Collaboration.
0
65 ps
16% accelerated
Fig. 1. The transmission of the d.c. source current
through the accelerator. At best 16% of the generated
electrons reach the experiment.
t
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