Development of Polarized Photocathodes for the Linear Collider Page: 3 of 6
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We have continued to study the GaAs/GaAsP superlattice with the goal of optimizing the
parameters with respect to the surface layer doping concentration. High surface doping gen-
erally leads to depolarization effects and thus we have studied lower p-type surface doping
levels. As stated earlier, the cold technology choice has greatly reduced the peak charge
requirements and lower doping levels that still support the required charge may be possible.
The GaAs/GaAsP superlattice structures which have been used for SLAC fixed target exper-
iments have yielded 86% polarization with a p-type surface layer doping of 5 x 1019cm-3 and
there should be a possibility for higher polarization with a lower GaAs surface layer doping
level. For this study we have obtained four samples from SVT Associates with varying p-type
doping in a 5 nm GaAs cap surface layer of a GaAs/GaAsP 14 period superlattice. The
p-type doping levels studied were 5 x 1019, 2 x 1019, 1 x 1019, and 5 x 1018cm-3. The results
are shown in Fig.1. and show no significant trend for higher polarization with reduced surface
- - 10
O - 0 733-5 (5x10^19) C
733-6 (5x10^18) s
N 50 733-7 (1x10^19)
__ - " 733-8 (2x10^19)
660 680 700 720 740 760 780
Figure 1: Polarization and Quantum Efficiency vs. Wavelength for different GaAs surface layer
p-type doping levels
Photocathode Laser with the ILC Pulse Structure
In preparation for testing photocathodes with the ILC pulse structure, Axel Brachmann of
SLAC is developing a system to produce the required ILC 300 ns bunch spacing. The system
uses a low power YAG/TiSapphire mode locked laser operating at 76 MHz which will be
Pockels Cell switched to produce about a ms pulse with a 3 MHz microstructure. We have
purchased a commercial Pockels Cell driver from Bergmann Messgerate Entwicklund KG
capable of providing the 1 ms, 3 MHz structure as the first step in this program. SLAC
has obtained the required Pockels cells. The low power stage output enters a high power
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Prepost, Richard. Development of Polarized Photocathodes for the Linear Collider, report, December 22, 2009; United States. (https://digital.library.unt.edu/ark:/67531/metadc926629/m1/3/: accessed March 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.