Long-pulse acceleration of 1MeV negative ion beams toward ITER and JT-60SA neutral beam injectors

Pulse duration of the ITER-relevant high power density hydrogen negative ion beams at 1 MeV has been successfully extended from 0.4 s to 60 s by using a prototype Multi-Aperture and Multi-Grid accelerator for the ITER and JT-60SA neutral beam systems. This significant progress was achieved based on developments of technology for the design of a voltage holding capability and improvement of a beam optics, which are directly applicable to ITER and JT-60SA. In the design of the voltage holding capability, one of critical remaining issues was a multi-grid effect. This was experimentally investigated by using a multi-grid configuration. This result was also applied to improve the beam optics by tuning the acceleration gap. In addition, the extractor configuration was optimized. These tunings significantly contributed to reduce the grid power load and to increase the current density. As the result, 0.97 MeV, 190 A/m2 negative ion beams were generated stably without any breakdowns and any thermal damages on the acceleration grids. This pulse length of the beams with the ITER-relevant energy and current density is the longest one in the world and contributes to assure the 1 MeV accelerator for the ITER NB system.26th IAEA Fusion Energy Conferenc