In this study, eight emission lines in the visible wavelength range of neutral helium were used to diagnose the electron density and temperature of the Large Helical Device (LHD) helium plasma instead of the conventional three-line method. The collisional-radiative (CR) model for low-pressure helium plasma was revised to include the optical escape factors for spontaneous transition from the n1P states to the ground state so that the influence of the absorption effect under optically thick conditions could be considered. The developed algorithm was based on fitting the number densities of eight excited states obtained using optical emission spectroscopy (OES). The electron density, electron temperature, ground-state density, and optical escape factors were selected as the fitting parameters. The objective function was set as the summation of the residual errors between the number densities measured in the experiment and those calculated using the revised model. A regularization term was introduced for the optical escape factor and optimized through bias and variance analyses. The results show that the agreement between the number density calculated by the algorithm and its counterpart measured in the experiment was generally improved compared to the method using three lines.

An algorithm for diagnosing the electron density and temperature of helium plasma at atmospheric pressure has been developed based on a revised helium collisional-radiative (CR) model. Atomic collision processes are included and part of the atomic data of electron collision processes in the conventional CR model has been updated to expand its valid pressure. The algorithm uses eight emission lines in the visible-wavelength range as inputs to determine the electron density, electron temperature, and number density of the two metastable states by fitting the number density of the states corresponding to the emission lines. The algorithm has a considerably small theoretical error. In the microwave-discharged low-pressure helium plasma experiment, the results obtained with the algorithm agreed well with the results obtained with the probe method. The electron density and temperature of the atmospheric-pressure helium plasma obtained with the algorithm agreed well with the results of the continuum spectrum analysis.

An inverse model based on a low-pressure helium collisional-radiative model was developed. Parts of the rate coefficients were recalculated from the cross-sections. The dominant processes in the revised model were extracted to simplify the calculation and to develop an inverse model. The model can calculate the electron density and temperature of low-pressure helium plasma by inputting the population densities of levels 3^1 S, 3^3 S, and 3^1 D, which can be measured by optical emission spectroscopy (OES) measurement in the visible wavelength range. The results demonstrate that the electron temperature obtained by the model is extremely close to the original value in the CR model. The output values of the electron density were of the same order and magnitude as the input values. The electron density and temperature measured by OES measurement in the experiment using the developed inverse model are consistent with the results measured by the probe method.