NONEQUILIBRIUM EFFECTS IN THERMAL PLASMAS

An assessment is made of the occurrence, nature, and effects of nonequilibrium in "thermal" plasmas at atmospheric pressure, ranging from high-temperature highly ionized plasmas to lower-temperature molecular gases. Particular attention is directed to recombining plasmas, which occur in many plasma-chemistry applications, where the electron density significantly exceeds its equilibrium value with respect to the ground state. For an atomic argon plasma, experiments with a 50kW inductively coupled plasma and optical diagnostics support a partial-equilibrium hypothesis where the bound and free electrons are mutually equilibrated, but elevated with respect to the ground state, for a range of moderate-temperature conditions of interest. Under those circumstances, radiation from the plasma can be substantially enhanced from its equilibrium value and spectroscopic diagnostics must be interpreted with considerable care. When a molecular diluent (N2,H2) is added to the recombining argon plasma, excited state populations are observed to be "quenched", the partial-equilibrium approximation breaks down, and the Boltzmann temperature based on ratios of excited state number densities is no longer useful. However, elevated electron densities and enhanced radiation can still be present. This quenching is interpreted in terms of a two-species collisional/radiative model which allows for near-resonant transfer of bound electronic energy between the species. Under such circumstances, which are of considerable practical importance, direct temperature measurements are recommended. To this end, the wavelength dependence of two-body recombination radiation in the UV is used as a direct measurement of the electron temperature.