Abstract
We developed a new imaging spectroscopy diagnostic tool for Hα emission and installed it on a negative hydrogen ion (H−) source to investigate the H− dynamics in the extraction region. During beam extraction, the Hα emission dropped; the same drop also appeared in the H− density (as measured by cavity ring-down spectroscopy). The reduction in the Hα 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 Hα 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.
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