Dark current and its effects analysis in a 1.4-cell RF photocathode gun at HUST ultrafast electron diffraction

A 1.4-cell photocathode RF gun was developed for the MeV-UED to mitigate the space charge effect during electron emission through a higher acceleration gradient. However, this advancement introduces the risk of field-emitted dark current, leading to a degradation in the quality of the ultrafast electron beam. This paper investigates dark current emission within critical regions of the RF gun cavity. The results show that dark current electrons from the cathode and cathode edge escape from the electron gun, resulting in increased image background noise. The study examines the temporal characteristics of the dark current, including waveform in relation to the emission phase. Additionally, different collimator apertures are analyzed for their suppressive effect on the dark current, aiming to minimize its impact on the ultrafast electron beam.