VUV Emission Spectroscopy for Evaluation of Optical Thickness in He Cascade Arc Plasmas

Abstract

We have generated a cascade arc plasma with a channel diameter of 8 mm for application to plasma window. Helium arc plasma up to a discharge current of 100 A were generated. We investigated the radiation trapping effect in the high-density He plasmas under various ambient gas pressures. In order to investigate the radiation trapping effect (optical thickness) in the high-density plasmas, we conducted vacuum UV (VUV) emission spectroscopy associated with He I resonance line. By observing the VUV spectrum attributed to He I resonance (1 1 S-2 1 P) transitions, we compared the emission intensities for different gas pressures. Lyman α intensity was increased by 3 times with a magnetic field due to well confinement of He plasma. The intensity decreased rapidly at high-ambient pressures due to the strong self-absorption. Therefore, the high-pressure arc plasma generated was classified into the optically thick one, and the self-absorption process of the resonance lines drastically influenced the population kinetics and energy transport.