High energy, high current accelerator development for ITER NBI at JADA

At JAEA as the JApan Domestic Agency (JADA) for ITER, a MAMuG (multi-aperture multi-grid) accelerator has been developed for the ITER neutral beam injector (NBI). Beam parameters achieved by the MAMuG accelerator were increased to 0.32 A H at the ion current density of 140 A/m, from the 0.2 A reported at Chengdu, and 796 keV at the beam energy, which was almost the power supply limit (0.5 A) of the test facility. This was achieved as the result of countermeasures to handle unexpected heat load by backstream positive ions. A test of SINGAP (single-aperture single-gap) accelerator was performed at JAEA under an ITER R&D task agreement. The objective of this study was to compare the two different accelerator concepts (SINGAP and MAMuG) at the same test facility. As the result, it was concluded that the MAMuG accelerator is more suitable for the ITER NBI because of its better voltage holding and less electron acceleration. A simulation is in progress for space charge repulsion of beamlets in the JT-60U accelerator. An aperture offset steering is suggested effective at exit of extractor for compensation of the beamlet deflection due to its own space charge.