Toward a Multipactor-Free Cavity Design for a Very-High-Frequency Continuous-Wave Electron Gun

A very-high-frequency (VHF) photocathode electron gun serves as an electron source capable of generating high-repetition-rate, high-brightness electron bunches. It operates in continuous-wave mode with a 100% microwave duty cycle, a configuration where multipactor emerges as a critical operational challenge requiring meticulous mitigation. The exponential amplification of secondary electrons within the electron gun may cause power dissipation, cavity material degradation, and beam quality deterioration, ultimately compromising the performance and operational lifetime of the VHF gun. Analysis of electron trajectories inside the cavity reveals that the energy dispersion and angular distribution of emitted electrons significantly affect their subsequent motion. Although the multipactor behavior is non-resonant, multiple resonance modes can still be identified from the statistical distribution of particle flight times. This study examines the influence of radio-frequency (rf) cavity geometry on multipactor behavior, with the underlying mechanisms elucidated through statistical analysis of particle trajectories. The effect of the secondary electron yield of copper on multipactor is also discussed. Finally, an optimized rf cavity profile exhibiting very low multipactor intensity is presented, providing a promising approach to achieving stable VHF electron gun operation.