Development of Radio-Frequency Electron Gun Using a Gridded Thermionic Cathode with High Reliability and High Maintainability

Abstract

Electron guns using a gridded thermionic cathode have been used in many accelerator facilities because of their ease of handling. However, due to the lens effect of the electron beam passing through the grid, the gridded cathode has been considered unusable as an electron source for free electron laser, which require a normalized emittance of less than 2 mm mrad. We researched the low-emittance beam generation using a gridded thermionic cathode, focusing on its advantages such as high maintainability and long lifetime. Both numerical analysis and simulation using physical models were used to study the distortion of the electric field and electron beam trajectory near the grid. It was found that there exist conditions of voltage between the cathode and grid under which the electron trajectory passing through the grid are parallel when the distortion of the electric field is at a minimum. Low emittance of less than 2 mm mrad/1 nC can be produced with an electron gun high voltage of 50 kV, obtaining a uniform particle distribution and minimum emittance at the gun exit. The low-emittance beam can be accelerated 500 keV by using the rf cavity immediately after the electron gun to avoid emittance increase due to space-charge effect. We constructed an electron gun system based on the scheme and confirmed that the target performance of less than 2 mm mrad emittance was achieved in the beam test. The emittance dependence due to the electric field distortion of the grid was also obtained in agreement with the simulation.