THE SOURCE OF EMITTANCE DILUTION AND PHOTOEMISSION TUNNELING EFFECT IN PHOTOCATHODE RF GUNS

Experimental data on HoBiCaT SRF photoinjector give an emittance which is much larger than the predicted for the idealized setup. Modeling of photocathode RF gun beams with the different imperfections of experimental setup (alignment errors, inhomogeneity of quantum efficiency and laser power distributions on the cathode) is given. The main reason for the beam emittance dilution is photocathode field imperfections induced by field emitters that change the local electric field. Some field models of such photocathodes are tested in the simulations. The dependence of photocathode beam currents on the surface electric field was measured with the HoBiCaT SRF Photoinjector. The dependence can be explained by the tunneling effect described by Fowler-Nordheim like equation and is difficult to explain with the Schottky effect.