Helium atom line-intensity ratios as an integrated diagnostic tool for low-pressure and low-density plasmas

Abstract

The intensity ratios between specific pairs of helium atom (HeI) emission lines are functions of the electron temperature (Te) and density (ne), and these functions have been used for the analysis of Te and ne in various types of discharge plasma. We applied this method to a low-density (ne < 10¹⁸ m⁻³) plasma, where the procedure of the analysis is markedly different from that of higher-density plasmas. The 2¹S and 2³S metastable atom densities are affected by transport, making it practically necessary to set Te, ne, the metastable atom densities, and the optical escape factors, which represent the effect of photoexcitation, as unknown variables and determine them simultaneously. Conversely, the transport of metastable atoms can be evaluated from the analysis.