Advanced Negative Ion Beam Technology to Improve the System Efficiency of Neutral Beam Injectors

Technologies producing high current density (>20mA/cm2) negative ions at a low operating gas pressure (<0.3Pa) has been established. Reduction of the gas pressure is efficient to improve the acceleration efficiency of neutral beam injectors. Beam optics in a multi-aperture, multi-stage electrostatic accelerator has been studied using a 400keV negative ion source. It was found that the ion beam tends to expand because of beamlet-beamlet interaction. The effect was confirmed by 3D beam trajectory simulation code. Beam steering technology by aperture displacement has been established to compensate the effect and to focus the beam precisely toward the injection port, which improves the geometrical efficiency of neutral beam injectors. A plasma neutralizer producing a high-density plasma (1.7x10 cm) with a high degree of ionization (0.15) has been developed.