Observation of a charge limit for semiconductor photocathodes

The Stanford Linear Accelerator Center is currently operating with a photocathode electron gun (PEG) to produce polarized electrons for its experimental program. Bunch intensities of up to 1011 electrons within 2 ns (8 A) are required from the electron gun. Operation of PEG has demonstrated a charge limit phenomenon, whereby the charge that can be extracted from the gun with an intense laser beam saturates at significantly less than 1011 electrons (the expected space-charge-limited charge) when the photocathode quantum efficiency is low. Studies of this charge limit phenomenon observed with a GaAs photocathode are reported.