PARMELA simulations of a PWT photoinjector

Conventional rf guns such as the BNL/SLAC/UCLA 1.6-cell gun require a high operating peak field typically above 120 MV/m. By contrast, the PWT gun can operate at a much lower peak field with excellent beam properties. A 10+2(1/2)-cell PWT photoinjector can achieve an emittance of 0.56 mm-mrad for a 1 nC bunch charge at a peak field of 55 MV/m; and 1.04 mm-mrad for 2 nC at 60 MV/m. By operating the PWT gun at a low peak field, dark current production from back bombardment of electrons emitted elsewhere in the rf cavity is mitigated as the number of electrons which could back stream to hit the photocathode and the cathode holder is significantly curtailed. The quantum efficiency and lifetime of a semiconductor photocathode, such as GaAs are significantly improved if the surface of the cathode is subjected to a lower peak field.