Generation of low emittance beams using III-V semiconductor photocathodes in an RF gun

Normalized rms emittances well below 10-6m (with thermal emittance ignored) are now predicted for a 1-nC, 10-ps, flattop beam using an S-band rf gun. The expected thermal emittance of a Cu cathode excited at 263 nm is shown to be ∼0.3 x 10-6m, which is potentially a serious limit on the overall minimum emittance. For GaAs, the photoelectron energy parallel to the emitting surface is now known as a function of the perpendicular energy. By adjusting the vacuum level for the semiconductor, it appears that the thermal emittance can be reduced (compared to Cu) by a factor of 2-even more if the cathode is cooled. The prospects for operating an rf gun with a III-V semiconductor photocathode such as GaAs are summarized