Recent Progress in the Negative-Ion-Based Neutral Beam Injectors in Large Helical Device

Abstract

Negative‐ion‐based neutral beam injection (negative‐NBI) system has been operated for 10 years in Large Helical Device (LHD). The injection power has been increased year by year, according to the improvement of the negative ion sources. Up to now, every injector achieves the designed injection energy and power of 180 keV‐5 MW with hydrogen beams, and the total injection power exceeds 16 MW with three injectors. In the multi‐round aperture grounded grid (GG), the diameter of a round aperture has been enlarged for higher GG transparency. Then, the GG heat load is reduced, as well as in the multi‐slotted GG, and the voltage holding ability in the beam acceleration was improved. As a result, the beam energy is raised and the injection power is increased. To improve the anisotropic property of the beamlet convergence condition between the perpendicular and the parallel directions to the slots in the multi‐slotted GG, a round‐shape aperture of the steering grid (SG) has been changed to a racetrack shape. As a r...