Analysis of secondary particle behavior in multiaperture, multigrid accelerator for the ITER neutral beam injector

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

For a neutral beam injector (NBI) of ITER, the negative ion accelerator capable of 1 MeV, 40 A negative ion beam acceleration for up to 3,600 s is required. Suppression of heat load onto acceleration grids is a key issue for the long pulse acceleration of negative ion beams. A major source of the heat load will be from incident of secondary particles, which are neutrals, positive ions and electrons, on the grids. The complicated behavior of secondary particles in the MeV accelerator of JAEA is analyzed using the Electrostatic Accelerator Mote Carlo Code (EAMCC) developed by CEA. Water-cooled new grids were equipped with the MeV accelerator for the long pulse H ion beam acceleration and second acceleration grid (A2G) was removed for simplification. Analytical results for 600 keV H beam acceleration show that the heat load of third acceleration grid (A3G) is inevitably high because of lack of upstream grid, i.e., A2G.