Low-emittance radio-frequency electron gun using a gridded thermionic cathode

Abstract

We developed an electron gun system capable of delivering high brightness beams to drive soft x-ray free-electron lasers (SXFEL). We modified the conventional radio-frequency (rf) gun concept by using a 50-kV gun with a gridded thermionic cathode connected with a 238-MHz acceleration cavity. We achieved a low target emittance by applying the following two methods: (1) to obtain a parallel beam after the control grid, we optimized the grid voltage to compensate for the electric potential distortion near the grid (i.e., the lens effect); and (2) to avoid emittance growth due to the space charge effect at low energy regions, the 50-kV gun was connected to a 238-MHz rf cavity with a minimum distance, which immediately accelerated the beam energy to 500 keV. We built a gun test stand to verify the high beam performance, i.e., a normalized emittance of less than 4 mm mrad with a bunch charge of 1 nC and a pulse length shorter than 0.7 ns, as predicted by cst and parmela codes. Our proof-of-performance experiments demonstrated that a projection emittance of 1.7 mm mrad was obtained as a core part, representing 60% of all extracted electrons and the measured emittance agreed well with the simulation value of 2.0 mm mrad. This paper presents an overview of the electron gun system and our proof-of-performance experimental results.