Collisional-radiative calculations of He line emission in low-temperature plasmas

We present spectral modeling results for neutral helium. Our underlying atomic data contains radiative transition rates that are generated from atomic structure calculations and electron-impact excitation rates, that are determined from both the standard $R$-matrix method and the $R$-matrix with pseudostates (RMPS) method. In this paper, we focus on transitions of particular importance to diagnostic line ratios. For example, our calculated rate coefficient for the electron-impact transition $1s3s{}^{1}S\to 1s3p{}^{1}P$, which has a pronounced effect on the $728.1\mathrm{nm}$ diagnostic spectral line, is found to be in good agreement with previous experimental measurements. We also consider transitions from the $1s^2{}^{1}S$ ground and $1s2s{}^{3}S$ terms to terms of the $n=4$ shell. They are found to be affected significantly by coupling of the bound states to the target continuum (continuum coupling), which is included in our RMPS calculation, but not in our standard $R$-matrix calculation. We perform collisional-radiative calculations to determine spectral line intensity ratios for three ratios of particular interest, namely the $504.8\mathrm{nm}∕471.3\mathrm{nm}$, $492.2\mathrm{nm}∕471.3\mathrm{nm}$, and $492.2\mathrm{nm}∕504.8\mathrm{nm}$ line ratios. Comparing our results determined from the RMPS excitation rates with those from the standard $R$-matrix excitation rates, we find that continuum coupling affects the rate coefficients significantly, leading to different values for all three line ratios. We also compare our modeling results with spectral measurements taken recently on the Auburn Helicon plasma device, finding that the ground and metastable populations are not in equilibrium, and that the experimental measurements are more consistent with the $1s2s{}^{3}S$ metastable term populations being short lived in the plasma.