Surface photoemission in a high-brightness electron beam radio frequency gun

We report the experimental characterization of high-brightness electron beam generation from a magnesium (Mg) photocathode. Both the quantum efficiency (QE) and the thermal emittance of an Mg cathode are experimentally investigated. The measured QE∼0.2% is the highest reported for a metal cathode. We observed no change in the Mg cathode thermal emittance as the QE varies from 0.015% to 0.15%. The upper-limit of the thermal emittance, 0.5 mm mrad, is about 50% lower than the theoretical prediction. Our results demonstrated the feasibility of having a high QE and a low thermal emittance simultaneously for a robust metal photocathode.