Ultrafast Electron Diffraction/Microscopy (UED/UEM) are powerful tools for directly observing ultrafast dynamic processes at the atomic level. The quality of the electron beam is crucial for image resolution, but the space charge effects can degrade parameters such as energy spread and emittance, reducing the spatiotemporal resolution. By reducing the charge per electron bunch and increasing the emission frequency, the space charge effect can be effectively suppressed, ensuring a high signal-to-noise ratio in the images. Superconducting Radio Frequency (SRF) photocathode guns can operate in continuous wave (CW) mode and generate highly stable and bright electron beams, making them promising electron sources for the next generation of UED/UEM. This paper aims to optimize the design of a 1.4-cell SRF gun using Nb3Sn for UED/UEM. The focus is on minimizing thermal losses in the cavity to enable efficient conduction cooling and ensure stable operation at 4K in the superconducting state. Furthermore, beam dynamics analysis of the electron beam inside the cavity is performed to assess beam quality for different charges and bunch sizes. This enables us to achieve a high-quality electron beam that meets the design requirements.
Accurate characterization of the longitudinal profile of ultrafast electron bunches is crucial for observing ultrafast dynamic processes in ultrafast electron diffraction (UED). Real-time monitoring of each ultrafast electron bunch's length would ideally provide a temporal basis for analyzing diffraction patterns. However, accurately and non-destructively measuring the longitudinal profile at the 10 femtoseconds level remains challenging. This paper investigates a method for monitoring the longitudinal profile of bunches using coherent Smith-Purcell radiation (cSPr). Space charge effects cause the elongation of electron bunches during flight, resulting in changes in the Smith-Purcell radiation spectrum. Therefore, the focus is on understanding the impact of space charge effects on the behavior of ultrafast electron bunches and their corresponding cSPr spectrum, with the aim of improving measurement accuracy.
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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