1 MV Rectifier With SF 6 -Gas Insulation and Cooling for ITER Neutral Beam Injector System

This article presents the development results of a five-stage dc-1 MV rectifier for an international thermonuclear experimental reactor neutral beam injector system. Each rectifier outputs −200 kV, and the total five rectifiers are connected in a series. The feasibility of both gas insulation and gas cooling for semiconductor devices was validated to achieve dc-1 MV output and 1-h continuous operation. We devised a novel structure of cylindrical dense alignment of the heatsinks with stud-type diodes, which enables both the mitigation of electric-field concentration on the heatsinks and the improvement of heat dissipation from diodes. The difference in the temperature rise for three different directions of the heatsink in an SF6 gas atmosphere was also clarified. The heat flow was analyzed to quantify the cooling effect of SF6 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.