Development of a high-gradient X-band RF gun with replaceable field emission cathodes for RF breakdown studies

The contradiction between people’s infinite pursuit of charged particle beam energy and finite laboratory space has led to the development of high gradient acceleration structure. As X-band high gradient structures are worldwide used in Collider, FELs and compact light sources, a more detailed observation and deeper understanding of the bottleneck of gradient improvement—RF breakdown are strongly demanded. To this end, an X-band electron gun with a replaceable field emission cathode has been developed at Tsinghua university to serve as the observation object. Said gun is composed of 2.37 cavities and a pluggable cathode, with a tapered tip that was machined on the cathode centre to increase the local emission current. To ensure the cathode replaceability while eliminating the leaking field, a novel design of the first cavity working at TM02 mode was adopted. After conditioning of around 107 pulses, the peak electric field of the cathode tip achieved more than 1 GV/m, and a stable current of 3 mA averaged over rf pulse was detected. These results lay a foundation for the observation of RF breakdown in our next stage.