Protections against grid breakdowns in the ITER neutral beam injector power supplies

During operation of the Neutral Beam Injector (NBI) of the International Thermonuclear Experimental Reactor (ITER), breakdowns of the acceleration grids will occur routinely. The paper describes the analyses performed on the protections for the ITER -1 MV acceleration circuit, from detailed modelling including stray capacitance to circuit simulations. The study addresses normal operation and grid breakdown, in a comprehensive design approach that investigates the role of additional passive components and of inverter control. Extra resistance in series to the -1 MV conductor was found to play a crucial role in the resulting protection scheme, with a significant reduction of the power supply contribution to the energy dissipated in a grid breakdown.