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.
Ultrafast electron diffraction using photocathode microwave electron guns is a powerful tool for investigating ultrafast science. To improve the spatial and temporal resolution of diffraction, it is crucial to enhance the quality of the electron beam, particularly the initial quality of the electron beam emitted from the photocathode that is influenced by the driving laser. To meet the strict requirements, the performance parameters of the femtosecond laser transmission system play a significant role. In this paper, we analyze the impact of femtosecond laser system parameters on diffraction resolution and investigate the primary indicators of the femtosecond laser system. We conducted experiments to measure the primary parameters of the laser, including pointing stability, beam diameter, pulse width, and pulse energy. Based on the experimental results and considering the complexity of engineering implementation, we proposed an optical scheme for the femtosecond laser transmission path to satisfy the requirements of the ultrafast electron diffraction device for further improving the diffraction resolution. This research aims to provide valuable insights into optimizing the femtosecond laser system for ultrafast electron diffraction experiments.
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