Space-Charge Dominated Photoemission in High Gradient Photocathode RF Guns

The cathode emission physics plays a crucial role in the overall beam dynamics in an RF gun. Interplays between intricate emission mechanisms in the cathode vicinity strongly influence the cathode quantum efficiency (QE) and the intrinsic emittance. The presence of strong space-charge effects in high gradient RF guns further complicates the emission process. A proper modeling of photoemission and a careful treatment of the space-charge contribution is thus of great necessity to understanding the formation of the beam slice emittance. In this article, emission measurements are carried out using the L-band cesium-telluride photocathode RF gun at the Photo Injector Test Facility at DESY in Zeuthen (PITZ)and the S-band copper photocathode RF gun at Tsinghua University. Following the Dowell model a simple so-called space-charge iteration approach is developed and used to determine the QE through temporal- and spatial-dependent electromagnetic fields. An impact of the space-charge cooling on the intrinsic emittance is presented. Measurementdata are shown and discussed in comparisons to preliminary simulation result