Automation of gyrotron electron beam current for fusion devices

Abstract The electron beam current of high-power, long-pulse gyrotrons for fusion devices is typically controlled to remain constant by adjusting the power to the cathode heater by using a pre-programmed waveform. However, this pre-programmed waveform is usually developed through a time-consuming trial and error process and its precision is low. Therefore, a system to automatically control electron beam current was developed and its performance was tested with one of the JADA/QST-manufactured gyrotrons for ITER. The control system automatically adjusted beam current to a target value of 47 A within ±0.5% for 900 s. This high-precision beam current control can maintain an output power of 1 MW and, being automated, it is useful for power on/off modulation operations. If the operation is switched from continuous to modulated, the beam current can be changed to compensate for the decrease in emission cooling of the cathode. This automatic control system demonstrates the quick recovery of a stable beam current in modulation operations.