Polarized electrons are produced by
photoemission from a GaAs photocathode as shown
in the figure above. Different laser light sources are used for the ESA and
SLC physics programs due to the different pulse structures required. For ESA,
a flashlamp-pumped Ti:sapphire laser is used to produce a 2 microsecond pulse.
For SLC operation, two Nd:YAG-pumped Ti:sapphire lasers produce two 2 ns
pulses separated by about 60 ns. One of these pulses is used to make
electrons for collisons, and the other one is used to make electrons for
positron production.
The laser beams are circularly
polarized by a linear polarizer followed by a Pockels Cell operating at its
quarter-wave voltage. A positive HV pulse on the Pockels Cell produces one
helicity, while a negative HV pulse produces the opposite helicity. The sign
of the HV pulse is set by a pseudo-random number generator, which updates
at 120 Hz (the SLAC machine pulse rate). This very effectively minimizes
false experimental asymmetries.
Reference to a detailed description of the polarized source is
available.