Visualization of H⁻ Dynamics in Extraction Region of Negative-Ion Source by H_α Imaging Spectroscopy

Abstract

We developed a new imaging spectroscopy diagnostic tool for H_α emission and installed it on a negative hydrogen ion (H⁻) source to investigate the H⁻ dynamics in the extraction region. During beam extraction, the H_α emission dropped; the same drop also appeared in the H⁻ density (as measured by cavity ring-down spectroscopy). The reduction in the H_α emission results from the reduction in the excited hydrogen population caused by mutual neutralization processes between H⁺ and H⁻ ions, which in turn are due to a decrease in the H⁻ density. We find a reduction structure in H_α that is observed inside the plasma farther than 20 mm from the plasma grid (PG) surface. The result indicates that H⁻ ions produced at the PG surface accumulate in the extraction region, so we conclude that they flow toward the PG apertures.