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

Abstract

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- α (H _α ) 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 _α 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 _α 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.