Recent Studies of Hydrogen Negative Ion Source and Beam Production for NBI in Large Helical Device

Abstract

The state of hydrogen plasma in the extraction region in a hydrogen negative-ion (H⁻) source for NBI has been investigated. We clearly observe an improvement of H⁻ density owing to the surface production effect with Cs seeding. H⁻ ions are widely distributed in the extraction region which is obtained by movable cavity ring down (CRD). We confirm a negative ion rich plasma with a few electrons in the extraction region, which state is important for reduction of electron contamination in extracting beam. An extraction area is reached 30 mm from the PG surface, which is measured by a 2D imaging diagnostic for H_α emission. We find the insensitive area for H⁻ extraction at the PG surface between the apertures. Negative ions produced at the surface are considered to have been supplied in the extraction region. The flow velocity of H⁻ ions is obtained by a four-pin Langmuir probe using a photodetachment technique with an Nd:YAG laser. H⁻ ion flows from the plasma grid surface, and its direction drastically changes at 20 mm from the production surface. This flow behavior is considered to be an important characteristic for improving H⁻ density in the extraction region.