Development of negative ion based neutral beam injector toward JT-60SA

Tanaka, Yutaka; Hanada, Masaya; Kobayashi, Kaoru; Kamada, Masaki; Kisaki, Masashi

This paper reports recent R&D results on negative ion-based NBI system for JT-60 Super Advanced where 10 MW neutral beams is designed to be injected for 100 seconds. There are major two issues to realize such as a high-power and long-pulse injection, i.e., the improvement of voltage holding capability and the reduction of the grid power loading of the JT-60U negative ion source with three acceleration stages. As the first step for improving voltage holding capability, the breakdown location has been examined on site of JT-60U by measuring the lights emitted from spark gaps that are installed outside of the ion source in parallel with acceleration stages. To reduce the grid power to an allowable level, outward deflection of outmost beamlets, due to space charge of the inner beamlets, was suppressed by distorting the acceleration electric field at the edge of the grids. This allowed to reduce the highest grid power loading to acceptable level of 5 %.