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.
New accelerator technologies such as laser wakefield accelerators (LWFA) or dielectric laser accelerators (DLA) have pushed the electron bunch length down to femtosecond or sub femtosecond regime. These ultrashort electron bunches find many applications, e.g. seeding for free-electron lasers (FEL), ultrafast electron diffraction (UED) and coherent Smith-Purcell radiation (cSPr) sources etc. The characterization of such ultrashort bunches is becoming a challenging task, especially at low energy regime due to the space charge effects. Usually, the streak cameras based on RF cavities are used to obtain accurate bunch length. However, the phase jitter between the incident beam and the electromagnetic field phase in the cavity set a resolution limit. A bunch length diagnostic based on a self-emission THz driven split-ring resonator (SRR) is proposed to reach the sub-picosecond (ps) or femtosecond (fs) resolution. Since the coherent SmithPurcell radiation from the incident electron beam produces the driving THz pulse, it can essentially eliminate the time jitter between the incident beam and the deflection THz field in the SRR gap. Besides, this THz pulse frequency can be tunable to easily match the SRR resonance frequency. In this paper, we describe the mechanism of the THz generation method and present the simulation results of the novel bunch length measurement based on a THz-driven SRR. The results show that this novel method can successfully measure the bunch length with the temporal resolution of 2-10 fs.
The advances in electron accelerator science and technology continue to reach shorter bunch lengths, even down to femtosecond, paving a way to generate coherent Smith-Purcell radiation naturally, taken as one of the most promising THz sources. In order to design a high power and broadly tunable THz radiation source, we make theoretical and numerical analysis of the characteristic of coherent Smith-Purcell radiation, which demonstrates good agreement between them. In the paper, we also present the comparison of spectra of coherent Smith-Purcell produced from the interaction of a single bunch and a train of microbunches.
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