1 MV Rectifier With SF6-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 SF
₆
gas atmosphere was also clarified. The 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.