Nonequilibrium state of highly ionized helium plasma at atmospheric pressure

A spectral study of arc helium plasma of atmospheric pressure with the electron density $n_e\leqslant10^{17}\text{ cm}^{-3}$ and the ionization degree of $\sim0.25$ is performed. The character of the population of $\mathrm{HeI}$ levels with the excitation energy of $20.96$–$24.04$ eV, which indicates infeasibility of the Boltzmann law with the electron temperature $T_e$ for describing the relative population of excited $\mathrm{HeI}$ states, is experimentally determined. Due to the experimentally revealed unfitness of the local thermodynamic equilibrium (LTE) model for estimating $T_e$, mixed contours of the $\mathrm{HeI}$ $1083$ line were divided into the Lorentz and Gaussian components. As a result, both the Stark effect constant that is poorly studied for this line and the temperature of heavy particles $T_g$ in the plasma were estimated based on the Doppler component of its Gaussian half-width. As the arc current varied from $200$ to $400$ A, $T_g$ increased from $20000$ to $25 500$ K, while the increase in the ionization temperature was, in this case, negligible (from $20000$ to $21000$ K).