Dark current studies of an L-band normal conducting RF gun

L-band normal conducting RF guns operating at long pulse mode and high gradient are installed at the European X-ray Free-Electron Laser (European XFEL) and the Free-electron LASer in Hamburg (FLASH) as high brightness electron sources. The dark current generated from the gun may cause severe issues, such as component activation, superconducting radio frequency (SRF) cavity quench and undulator degradation. The dark current is one of the parameters limiting the gun accelerating gradient and performance. In this paper, we report the dark current tracking simulations, including the detailed RF field distribution in the gap between cathode plug and copper hole in the backplane. The transmission ratios of the field emission from cathode regions were simulated for different copper hole geometries and different plug insertions. The dark current imaging measurements were conducted in an L-band normal conducting RF gun (gun 4.2) at the Photo Injector Test facility at DESY in Zeuthen (PITZ). The dark current imaging comparison before and after a 180 degree cathode plug rotation shows the main dark current emissions come from the copper hole corner and Cs₂Te film edge, which is consistent with the dark current tracking simulations.