Development of Negative Ion Based Neutral Beam Injector toward JT-60SA

This paper reports recent R&D results on negative ion-based NBI for JT-60SA where 10 MW neutral beam is designed to inject for 100 seconds. There are major two issues such as improvement of voltage holding capability and reduction of the grid power loading of the JT-60U negative ion source with three acceleration stages. To improve the voltage holding capability, the breakdown location with the beam acceleration was examined. In the upper ion source, breakdowns in the first gap were dominated by the cesium contamination. In the lower ion source, breakdowns occurred in the second and third gaps, since the beam optics was not optimized. Optimization of the beam perveance and reduction of the cesium contamination are required toward JT-60SA. To reduce the grid power, outward deflection of outmost beamlets, due to space charge of the inner beamlets, was suppressed by optimizing the acceleration electric field. This allowed to reduce the power loading of GRG. After the reduction of the power loading, injection pulse length was extended up to 30 s. The water temperature rise of the components saturated at 25 s. This shows that existing water-cooled components can be reused for JT-60SA.