Nonequilibrium effects in two-temperature atmospheric pressure air and nitrogen plasmas

There has been considerable interest in methods for producing atmospheric pressure air plasmas at gas temperatures below 2000 K with free electron densities in excess of 1012 cm−3. These active plasmas are of interest for practical applications ranging from biodecontamination to electromagnetic wave shielding. One approach is to apply energy addition to the free electrons by means of an electrical discharge.; Investigations were conducted to understand and validate the mechanisms of ionization in two-temperature atmospheric pressure air and nitrogen plasmas in which the electron temperature is elevated with respect to the fixed gas temperature. With a new two-temperature chemical kinetic model for air and nitrogen plasmas, we found that, for a given gas temperature, the steady state electron number density exhibits an S-shaped dependence on the electron temperature. This S-shaped behavior is the result of competition between ionization and charge transfer reactions and is characteristic of molecular plasmas. The predicted steady-state electron number density n_e vs. electron temperature T_e curves were interpreted in terms of macroscopic discharge parameters, namely current density j and the electric field E, by means of Ohm's law and the electron energy equation.; The possible effects of a non-Maxwellian electron energy distribution function of electrons on the chemistry of the plasma were examined by means of a simplified collisional-radiative model coupled with a Boltzmann solver for nitrogen plasmas. It was found that non-Maxwellian effects do not appreciably affect the discharge characteristics obtained under the assumption of a Maxwellian electron energy distribution function.; DC glow discharge experiments have been conducted in flowing atmospheric pressure air and nitrogen plasmas with either room temperature or preheated (around 2000K) working gas. Electrical discharge characteristics were measured and the thermodynamic parameters of the discharge were obtained by emission spectroscopy measurements. The measured discharge characteristics agree well with the predicted S-shaped characteristic curves. The experiments showed that it is feasible to obtain stable diffuse glow discharges with highly nonequilibrium electron number densities (above 1012 cm−3) in atmospheric pressure air and (above 1013 cm−3 ) in nitrogen plasmas.