Analysis of Transient Response of RF Gun Cavity Due to Back-Bombardment Effect in KU-FEL

Abstract

Thermionic RF guns have advantageous features, such as compactness and high brightness of output beam, against electrostatic guns because of their high acceleration fields. For these reasons, thermionic RF gun has been ­chosen as electrons injector for Kyoto University free electron laser facility. The most critical issue of the thermionic RF gun is the transient cathode heating due to the electron back-bombardment when the gun is used as a driver linac of an MIR–FEL oscillator, which requires macropulse duration of more than 5 μs and bunch charge of more than 20 pC. Under the condition, the transient cathode heating results in rapid increase of beam current and rapid decrease of beam energy during a macropulse. Several important parameters, such as cathode temperature, current density of the cathode surface, are difficult to be measured during the experiment. A numerical simulation is the best way to expect the behaviour of such parameters. Numerical calculations were carried out to evaluate the cathode temperature and current density during the macropulse, by a simulation code which was developed at Kyoto University. The code simultaneously solves two differential equations; the differential equation of the equivalent circuit of the RF gun and one dimensional thermal equation of the thermionic cathode which includes the heat input from back-bombardment electrons. The results of transient response of the RF cavity due to the back-bombardment effect are presented with conclusions.