Performance of an Indium-sealed S-band RF Photoelectron Gun for Time-resolved Electron Diffraction Experiments

We have developed a one-and-half -cell S-band radio-frequency (RF) photoelectron gun (photogun) fed by a coaxial coupler. The RF photogun is dedicated to ultrafast-electron-diffraction experiments by generating electron bunches of 3-MeV energy and a-few-pC charge, which is not strict condition compared to those for X-ray free-electron lasers. Brazing of RF cavities is welldeveloped process for making RF guns or RF accelerators. Sometimes, however, a failure occurs in the brazing process, causing the entire electron gun or accelerating cavity to spoil. Axial-symmetric design of the RF photogun permits indium sealing for cavity cells, a photocathode plate, and a coupling RF part. We firstly report that the indium-sealed RF photogun successfully meets the required performance and long-term stability for ultrafast electron diffraction experiments. We have stably operated the RF photogun for more than three years with the electron beam conditions of ~ 3-MeV energy, up-to-10-pC charge, and a repetition rate of 50 Hz. The quantum efficiency of the copper photocathode had improved from 10−6 to 10−5 depending on vacuum condition from 10−8 to 5 × 10−10 Torr, respectively. Measured emittance and energy spread of the generated electron beam showed 0.3 mm-mrad and less than 0.25%, respectively, for a bunch charge of ~ 2 pC, which agree well with those obtained by ASTRA simulation.