High brightness electrostatically focused field emission electron gun for free electron laser applications

Abstract

An electrostatically focused, multistage electron gun using a cold (field emission) cathode has been developed for use in a free electron laser. This gun produces a 1.1-kA, 2.0-MV electron beam with a normalized emittance εn≂38×10−3 (π cm rad), and a brightness of 74 kA/cm2 rad2. The measured brightness is almost two orders of magnitude higher than that found in most rf linacs using conventional thermionic cathodes, and is comparable to that achieved using electron guns immersed in strong guiding magnetic fields. The emittance and brightness have been studied as a function of cathode material, surface finish, and the electric field strength at the cathode surface.