Polarized Source Information

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.