Beam charge feedback system for thermionic cathode RF-Gun

A feedback system to stabilize a long-time operation of thermionic cathode RF-gun has been developed in Kyoto University free electron laser (KU-FEL) facility where a 4.5-cell thermionic cathode RF-gun provides electron beam to drive a mid-infrared FEL. Since the back- bombardment effect seriously increases the temperature of the cathode surface and, consequently, increases the beam charge, a stable operation is difficult without continuous control of the thermal emission from the cathode. We have tried to stabilize the beam charge of the thermionic cathode RF-gun by using a feedback system. The beam charge was monitored with a current transformer (CT), which was located at the exit of the gun, was read by an oscilloscope. The total charge was calculated in a PC and the LabView PID-module controlled the cathode heater current. As the result, the long term stability of the beam charge was dramatically improved from 18%/h to 0.1%/h in variation. The variation at the undulator section was also improved, 1.2%/h.