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-α (Hα) 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 Hα 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 Hα 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.