Simulation of dark currents in a FEL RF-gun

Dark currents caused by field emission are dramatically limiting the lifetime of photocathodes used in RF-guns needed for successful FEL operation. Due to the mismatch of the dark currentelectrons to the transport channel they are also a serious hazard to beam diagnostic systems an may cause severe radiation damage. In the past high dark currents emitted from the laser driven RF-guns have been observed during substantial time of linac operation. For a better understanding of the dark current dynamics the transport through the injector part of the DESY Zeuthen (PITZ) accelerator has been investigated with the help of an appropriate tracking algorithm. Additionally, potential emission spots inside the rf-gun have been located by RF field mapping. Furthermore, the dependency of the dark current on the accelerating field inside the RF-gun as well as on the strength of the focusing solenoid has been studied.