Design of a cryogenic photocathode gun with a novel TM02 mode coupler for low emittance

摘要: The application of radiofrequency (RF) electron guns operating at cryogenic temperatures can enhance the
performance of a photoinjector. The low emittance and increased brightness of the electron beams resulting
from the high gradient in the cryogenic photoinjector make it possible to improve the performance of new light
sources. As an upgrading scheme for the Shanghai Soft X-ray Free-Electron Laser (SXFEL), this study explores
a novel photocathode RF gun by introducing the TM02 mode into the cathode cell of a 2.6-cell electron gun,
which improves the RF performance, particularly in terms of the monopole field. Furthermore, the optimized
cathode cell with the TM02 mode can reduce the errors resulting from installation and decrease the dark current
with alternative cathodes. In this study, the beam dynamics in a photoinjector were optimized using a 500 pC,
5 ps beam, and its feasibility was examined. This paper presents the entire RF design process, including the
adjustment of the RF structure, coupler design, local field, and multipole mode suppression to provide theoretical
guidance for subsequent manufacturing and assembly.