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

P. Hartmann et al. / Nucl. Instr. and Meth. in Phys. Res. A 379 (1996) 15 – 20 19
(a)
(b)
(c)
Bunch duration / ps
Energy spread / eV
Polarization / %
30
25
20
15
10
5
0
500
400
300
200
100
0
90
80
70
60
50
40
30
20
10
0
1 2 3 4 5 6 7 8
1 2 3 4 5 6 7 8
Systematic error
0 1 2 3 4 5 6 7 8 9
Bunch charge / fC
Fig. 6. Polarization, bunch duration and energy spread
of the bunches vs. the bunch charge. These three measurements
were not taken at the same time. The strong
dependence of the bunch duration on the bunch charge
is considered to be an external space charge effect.
Current / a.u.
100
90
80
70
60
50
40
30
20
10
0
Polarization
Current in Faradaycup
-10 -8 -6 -4 -2 0 2 4 6 8 10
Rel. phase / degree
100
90
80
70
60
50
40
30
20
10
0
Polarization / %
Fig. 7. Phase resolved measurement of the polarization
of the electron bunches. The spin polarization (upper
points) and the transmitted current (curve on the
bottom of the plot) are shown as a function of the deflector
cavity phase.
enlarged systematic error due to detector rate effects,
starting at 3000 counts/s.
To investigate the phase resolved polarization, we examine
the transmitted current from the phase analyzer
(phase mode) with the Mottdetector. Fig. 7 shows the
spin polarization (upper points) and the transmitted
current (curve on the bottom of the plot) as a function
of the deflector cavity phase. Within the error bars no
dependence of the polarization on the position in the
bunch is observed.
4. Conclusions
This work demonstrates the feasibility of picosecond
photoemission of polarized electrons from a strained
layer GaAsP photocathode. The emitted bunches are
short enough for an injection into the electron accelerator
MAMI, even though there is a strong dependence
of the bunchlength and the energy spread on the
bunch charge due to an external space charge effect.
No dependence of the average beam polarization on
the bunch charge was seen up to 8 fC. This bunch
charge corresponds to a mean beam current of 20 µA
at a pulse repetition rate of 2.45 GHz (MAMI-rf).
With the possibility of measuring the phase resolved
polarization a powerful tool for investigations in photocathode
technology has been set up. A first measurement
of the phaseresolved polarization of electron
bunches from a strained layer GaAsP photocathode is
presented. Within the error bars no dependence of the
polarization on the bunch phase was observed. This
result is not expected for bulk GaAs photocathodes.
The work will be continued by studying the space
charge spreading of bunches as a function of primary
light pulse duration. Bulk cathodes and samples with
different layer thicknesses will be investigated.
Acknowledgments
This project is supported by the Deutsche Forschungsgemeinschaft
(SFB 201).
References
[ 1 ] M. Meyerhoff et al., Phys. Lett. B 327 (1994) 201.
[ 2 ] D. v. Harrach. MAMI–Proposal A4/1-93, 1993.
[ 3 ] J. M. Finn and P. Souder. CEBAF Experiment E–91–010, 1991.
[ 4 ] S. Ecklund. In Proc. of the Workshop on Photocathodes for
Polarized Electron Sources for Accelerators, Stanford, 1993;
SLAC–report 432.
[ 5 ] D. T. Pierce et al. Appl. Phys. Letters, 26:670, 1975.
[ 6 ] T. Nakanishi et al. Phys. Lett. A, 158:345, 1991.