ITER NBI DC-1MV Ultrahigh Voltage Rectifier

This paper presents the development results of a five-stage DC-1MV rectifier for an ITER neutral beam injector system. Each rectifier outputs −200kV, and the total five rectifiers are connected in series. The feasibility of both gas insulation and gas cooling for semiconductor devices was validated, to achieve DC-1MV output and 1-hour continuous operation. A simplified structure mounting a stud-type diode device on a heatsink was devised, resulting in the compatibility of sufficient insulation distance and wide cooling surface. Heat flow was analyzed to quantify the cooling effect of SF 6 gas, taking into account the gas circulation by the external cooling system. Rectification operation and temperature rise were tested with an actual-scale rectifier including a step-up transformer. Additionally, DC-1.2 MV withstand voltage was tested for a fifth-stage actual-scale rectifier. The test results demonstrated the validity of the proposed design.