Analysis and experiments of a waveguide post's influence on photocathode RF gun

Several BNL/KEK/SHI type photocathode RF guns have been fabricated for high-quality electron beams in Accelerator Laboratory of Tsinghua University. This paper describes how the characteristics of a waveguide post can be chosen to correct for a mismatch in power coupling without affecting the pi-mode resonant frequency and the balance of fields between the two cells. Microwave circuit theories are used to analyze how to select the proper location and depth of the waveguide post. The tolerance on the post positioning is evaluated based on gun field quality requirements. MAFIA simulations and RF experiments have been done to confirm the theoretical analysis.