Highly ionized arc $\rm He$ plasma: Nonequilibrium, nonideality, and kinetics

We have experimentally established that a strongly ionized, arc He plasma of atmospheric pressure does not exist in the state of the local thermodynamic equilibrium (LTE) expected for a plasma with an electron concentration above $10^{16}$ cm$^{-3}$. We discover that this is caused by transverse plasma nonuniformity and density effects. The plasma quasistatic microfield destroys the upper electron levels of He atoms, and the radial ambipolar diffusion flow transports the charged particles from the narrow $(1$–$2$ mm$)$ plasma channel on the chamber wall. In the distribution of atoms and the ions over the excited levels, we observe strong ionization nonequilibrium and a large gap of the ground state; these factors cause large difficulties in the spectral diagnostics of the plasma parameters due to the inapplicability of the relations respecting to the equilibrium conditions. We propose a method to measure the concentration of double-charged ions $\rm He^{++}$ ($\alpha$-particle), which is inaccessible by direct spectral registration, as well as a simple kinetic model of strongly ionized He plasma with the use of a modified diffusion approximation; the model adequately describes the plasma parameters.