High‐energy and high‐current hydrogen negative‐ion beam production with an external‐filter‐type large negative‐ion source

A large hydrogen negative‐ion source with an external magnetic filter has been developed for a neutral beam injection (NBI) system in the Large Helical Device (LHD), and a high‐energy and high‐current H− ion beam has been produced. The ion source is operated at a high arc efficiency of 0.1 A/kW at an operational gas pressure of less than 3.5 mTorr, and produces 47 keV–16.2 A of a H− ion beam from a grid area of 25 cm×50 cm. With two‐stage acceleration, 13.6 A of a H− ion beam has been successfully accelerated to 125 keV. Multibeamlet focusing by the aperture displacement technique has been achieved 11.2 m downstream with a gross divergence angle of 9 mrad. The alternate beamlet deflection by the magnetic field at the extraction grid, which results in beam broadening in the deflection direction, was well compensated also by the aperture displacement technique. These results satisfy the specification of the negative‐ion‐based LHD–NBI system.