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 deflection magnets, and focus of multi-beamlets at a large-area negative ion source 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 fusion device. The electrostatic lens effect by displacement of the beam extraction aperture of a grounded grid (GG) was utilized for the beam deflection 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 trajectory simulation used for analyzing the beam deflection angle and beam radius as well as theoretical calculations, which are used to calculate the aperture displacement based on the 3-D simulation results. Applicability of the analysis process was examined for a large-area high-current H− ion source of an NBI (0.25 m×1.25 m, 40 A, 180 keV). The analysis process was proved to be useful for adjusting the GG aperture displacement and ion source parameters (acceleration gap length gacc, extraction voltage Vext, etc.) so as to reduce the beam deflection and to focus the multi-beamlets avoiding beam collision with the GG. 3-D simulation results were compared with the experimental results of the H− ion source to confirm validity of the analysis process. The beam deflection angle was reduced to almost zero in the simulation with a displacement equal to the experiment. This simulation result was consistent with the experimental results. The analysis process will allow design work of future negative ion sources to be speeded up, with a minimum of experimental work.