Measurement of Paα line from pellet ablation cloud in Heliotron J

The Pa-alpha line (1875.13 nm) in the near-infrared (NIR) region was evaluated to apply Stark broadening of the line spectrum to the electron density measurement of the small-pellet ablation cloud in Heliotron J, a medium-sized helical-axis heliotron device. Pa-alpha is three-to-four times broader than the visible H-beta line (486.13 nm) for the same electron density. Using a portable NIR spectrometer, preliminary proof-of-concept experiments determined the marginal density, below which the broadening was undetectable. The lower detection density limit can be decreased using a narrower entrance slit or a denser grating.

Automated summary

This study evaluated the application of the Pa-alpha line in the near-infrared region for measuring the electron density of small-pellet ablation cloud in Heliotron J. The results showed that the Pa-alpha line is three-to-four times broader than the visible H-beta line for the same electron density. Preliminary experiments using a portable NIR spectrometer determined the marginal density below which the broadening was undetectable, and suggested methods to decrease the lower detection density limit.