In helium discharge plasmas, the relative emission intensities of the fine-structure transitions belonging to the HeI 2 3 S−2 3 P transition can be affected by radiation reabsorption. Since the magnitude of the reabsorption depends on the density and temperature of the 2 3 S metastable atoms, their density can be determined by measuring the 2 3 S−2 3 P emission line shape using a high wavelength-resolution spectrometer. In this study, the applicable conditions of the method in terms of the opacity and line broadening are revealed, and possible causes of errors in the measurement, i.e., spatial distributions of the density and temperature and the effects of external magnetic and electric fields, are investigated. The effect of reabsorption under an external magnetic field is experimentally confirmed using a glow discharge plasma installed in a superconducting magnet.

The helium 23S metastable atom densities are experimentally evaluated by self-absorption spectroscopy of the HeI 23S–23P transition spectra in two kinds of cylindrical glow discharge plasmas, which have different radii and are operated under different pressures of 300 and 20 Pa. The spectra are measured by using an interference spectroscopy system with a wavelength resolution of about 60 pm, and the relative intensities of the fine structure transitions are analyzed. It is found that the method is in principle applicable to plasmas with the pressure up to about the atmospheric pressure and electron density on the order of up to 1022 m−3. For a plasma with an absorption length of 10 mm and a spatially uniform temperature of 300 K, the method is sensitive to the metastable atom density roughly from 1016 to 1019 m−3.

In magnetically confined torus plasmas, the local emission intensity, temperature, and flow velocity of atoms in the inboard and outboard scrape-off layers can be separately measured by a passive emission spectroscopy assisted by observation of the Zeeman splitting in their spectral line shape. To utilize this technique, a near-infrared interference spectrometer optimized for the observation of the helium 2(3)S-2(3)P transition spectral line (wavelength 1083 nm) has been developed. The applicability of the technique to actual torus devices is elucidated by calculating the spectral line shapes expected to be observed in LHD and QUEST (Q-shu University Experiment with Steady State Spherical Tokamak). In addition, the Zeeman effect on the spectral line shape is measured using a glow-discharge tube installed in a superconducting magnet.