Investigation of beam deflection reduction and multi-beamlet focus at a large-area negative ion source for a neutral beam injector with 3-D beam trajectory simulation

We investigated the reduction of ion beam deflection caused by electron def lection magnets, and focus of multi-beamlets at a large-area negative ion s ource of a neutral beam injector (NBI) in order to reduce beam loss during long-distance beam transport ( > 10 m) and beam injection into a nuclear fu sion device. The electrostatic lens effect by displacement of the beam extr action aperture of a grounded grid (GG) was utilized for the beam deflectio n reduction and the multi-beamlet focus. We proposed an analysis process to adjust: the aperture displacement which avoids beam collision with the GG by too much displacement. The analysis process includes a 3-D beam trajecto ry simulation used for analyzing the beam deflection angle and beam radius as well as theoretical calculations, which are used to calculate the apertu re displacement based on the 3-D simulation results. Applicability of the a nalysis process was examined for a large-area high-current H- ion source of an NBI (0.25 m x 1.25 m, 40 A, 180 keV). The analysis process was proved t o be useful for adjusting the GG aperture displacement and ion source param eters (acceleration gap length y(acc), extraction voltage V-ext, etc.) so a s to reduce the beam deflection and to focus the multi-beamlets avoiding be am collision with the GG, 3-D simulation results were compared with the exp erimental results of the H- ion source to confirm validity of the analysis process. The beam deflection angle was reduced to almost zero in the simula tion with a displacement equal to the experiment. This simulation result wa s consistent with the experimental results. The analysis process will allow design work of future negative ion sources to be speeded up, with a minimu m of experimental work. (C) 2000 Published by Elsevier Science B.V. All rig hts reserved.