Identification of the extraction structure of H- ions by Hα imaging spectroscopy

Extraction structure of negative hydrogen ions (H-) (i. e. the regions where H- ions are effectively extracted from the plasma) was obtained using hydrogen Balmer-alpha (H alpha) imaging spectroscopy and cavity ring- down spectroscopy. The H- ion density increases after caesium (Cs) seeding through a surface conversion process on a Cs- covered plasma- grid (PG) surface. We found a reduction in the H- density during beam extraction after Cs conditioning, and the same signal reduction appeared in the H ff intensity caused by the reduction in the excited hydrogen (n = 3) population, which in turn is caused by the decrease in the mutual neutralization process between positive and negative hydrogen ions. We clearly observed the reduction structure of the H ff emissions in the extraction region; the structure expands at optimal Cs conditioning. From this result, the H- ions, which are produced at the PG surface, release to the extraction region and widely distribute during arc discharge. We conclude that the reduction of the H- density is caused by the particle loss due to beam extraction from the PG apertures.