Measurements on photoelectrons from a magnesium cathode in a microwave electron gun

The performance of a magnesium cathode in a one-and-half cell photocathode microwave gun was measured at the Brookhaven Accelerator Test Facility (ATF). The frequency quadrupled Nd:YAG laser (266 nm) was used to stimulate the photoelectron emission. For a normal incident laser pulse, the quantum efficiency of the magnesium at 10^-7 Torr was measured to be 5 × 10^-4, which is more than 20 times the value for copper under similar conditions. After more than 5000 h operation, and despite some optical damage on the cathode surface, no decrease in the quantum efficiency of the magnesium cathode has been observed. The Schottky effect of photoelectron emission was also studied. No electron saturation effect was observed for a train of laser pulses. The photoelectron beam bunch length was measured to be 11 ps FWHM using a microwave kicker cavity, in agreement with a streak camera measurement of the laser pulse length. The measured rms normalized emittance is 2.5 mm mrad for a bunch charge of 80 pC, and rms radius of the laser spot 0.2 mm, in agreement with a simple analytical model predictions.