Abstract
Abstract Thermionic-cathode radio-frequency electron gun (RF gun) has been a compact electron source of accelerators for more than two decades. In an RF gun, electrons pulled out from the cathode by the high gradient field setup in its microwave cavity are rapidly accelerated to relativistic energy such that space charge effects are less significant. Motivated by the need of a high quality electron source for the NSRRC ultrashort beam facility, we studied a 2998 MHz, 1/2+1/2+1 cell, thermionic cathode RF gun with on-axis coupled structure (OCS RF gun). OCS RF gun features simplicity of frequency tuning during fabrication due to its axial symmetry. We started with circuit analysis that provides a reliable model through the investigation. Microwave properties and electron beam characteristics of designed OCS RF gun has been evaluated by 2D simulations of SUPERFISH and PARMELA as well as 3D simulations of HFSS and CST-PS. Operating parameters are optimized to obtain a 2.5 MeV electron beam with quasi-linear energy chirp that allows aggressive bunch compression in the RF linac located downstream after proper beam selection. We have also investigated power distribution of back-streamed electrons that eventually lost onto the surface of thermionic cathode. A prototype of the designed OCS RF gun structure has been fabricated for low power microwave test. Measured frequencies and longitudinal electric field profile of the cavity structure agree well with the results predicted in the simulation studies.