Design and experimental study of a large vacuum-immersed negative-ion source

A 1 3-scale source was designed on the basis of previous multi-cusp H^- sources developed at the National Institute for Fusion Science (NIFS). It has been developed for the negative-ion-based neutral beam injection system on the test stand in the large helical device (LHD). The ion source was immersed in a vacuum vessel, making the ion source simple and compact. A reduction in the stripping loss of H^- ions is also expected. Three studies on the 1 3-scale vacuum-immersed H^- ion source (aiming at a 15 A × 125 kV beam) are described: design features for the vacuum-immersed source at high voltage, a Monte Carlo simulation for pumping gases in the accelerator to estimate the stripping loss of H^- ions, and the experimental results of testing the plasma source and beam extraction and acceleration during beam conditioning. As a result of the simulation, the stripping loss was calculated to be about 26% for a reference design. A guiding principle for accelerator design was obtained from the viewpoint of enhanced pumping on transverse gas conductance and resulting reduction in the stripping loss. In experiments, a plasma density of more than 2.5 × 10¹²cm^-3 was produced in the first chamber of the plasma source. During beam extraction, it was found that a beam-induced low density plasma (i.e. 10⁵-10⁷ cm^-3) exists in the vacuum vessel. In order to produce a long pulse beam, this plasma must be shielded. Beam conditioning has been completed up to 1 A with beam energy of up to 85 keV for 0.1 s pulse duration. H^- current scaling and engineering problems related to full scale performance are discussed.