Studies of emission spectra in helium plasmas at atmospheric pressure and local thermodynamical equilibrium

A collisional-radiative model coupled with the electron Boltzmann equation is used to perform a parametric study of the excitation spectrum in microwave-induced helium plasmas at atmospheric pressure. It is shown that the populations of the helium excited levels are much higher than predicted by the Saha equation. The excitation spectrum does not follow a Boltzmann distribution and is typically characterized by two excitation temperatures independent of the electron temperature. At sufficiently large electron density, the levels very close to the continuum limit come in partial Saha equilibrium. When nitrogen impurities are introduced into helium, the populations of the helium excited levels strongly decrease and drastically change the behavior of the plasma with respect to Saha equilibrium.