Three-dimensional analysis of beamlet deflection in a MeV accelerator for ITER NBI

Kashiwagi, Mieko; Taniguchi, Masaki; Umeda, Naotaka; Mizuno, Takatoshi; Tobari, Hiroyuki; Dairaku, Masayuki; Watanabe, Kazuhiro; Inoue, Takashi

An accelerator which generates deuterium negative ion beams of 1 MeV, 40 A for 3600 s is required for the ITER neutral beam injector (NBI). To realize such a high power accelerator, numerical studies have been carried out in parallel to acceleration tests in JAEA. After long pulse acceleration tests up to 30 s, it was found that parts of grid were melt around the grid apertures. In order to investigate how the grids melted, a three dimensional beam analysis was carried out in combination with 2D beam analysis code and gas flow code. The analysis clarified that the beamlet was deflected due to their own space charge repulsion and magnetic field at extraction grid, which led to direct interceptions of the beamlets at the grids. The power loads at the grid by these deflected beamlets were found to be more than 20 kW/cm. For next long pulse tests, a new extraction grid with aperture offset and field shaping plate has been designed so as to compensate the beamlet deflections.