Physics-based investigation of negative ion behavior in a negative-ion-rich plasma using integrated diagnostics

Abstract

Total power of 16 MW has been successfully delivered to the plasma confined in the Large Helical Device (LHD) from three Neutral Beam Injectors (NBIs) equipped with negative hydrogen (H−) ion sources. However, the detailed mechanisms from production through extraction of H− ions are still yet to be clarified and a similar size ion source on an independent acceleration test bench called Research and development Negative Ion Source (RNIS) serves as the facility to study physics related to H− production and transport for further improvement of NBI. The production of negative-ion-rich plasma and the H− ions behavior in the beam extraction region in RNIS is being investigated by employing an integrated diagnostic system. Flow patterns of electrons, positive ions and H− ions in the extraction region are described in a two-dimensional map. The measured flow patterns indicate the existence a stagnation region, where the H− flow changes the direction at a distance about 20 mm from the plasma grid. The pattern also suggested the H- flow originated from plasma grid (PG) surface that turned back toward extraction apertures. The turning region seems formed by a layer of combined magnetic field produced by the magnetic filter field and the Electron-Deflection Magnetic (EDM) field created by magnets installed in the extraction electrode.