Experimental investigation of ionizational nonequilibrium in atmospheric pressure air plasmas

Summary form only given. In air plasmas, the presence of molecular ions, electronegative species, dissociative recombination, charge exchange, and finite-rate heavy-particle chemistry produces a complex situation in which the balance between these various effects is not currently well understood, mainly because of a lack of understanding of the dominant mechanisms and the large differences between the reaction rates proposed in the literature. To understand the ionization/recombination mechanisms and to assess the rates of the controlling reactions, experiments have been conducted in our laboratory with atmospheric pressure plasmas of either pure air or 10% air in argon. In these experiments, electron recombination was measured as a function of residence time as the plasmas flowed through water-cooled test-sections mounted on the exit nozzle of a 50 kW RF plasma torch. In the case of the recombining pure air plasma, electron number densities were found to remain close to equilibrium. In contrast, large electron over populations appeared in the air/argon plasma as it cooled from 7900 K to 2500 K within approximately 1.3 ms: at 2500 K, the electron density was measured to be -2/spl times/10/sup -3/, which is /spl sim/500 times larger than the equilibrium density. Results from these experiments and from kinetic analyses indicate that electrons recombine primarily via the fast two-body dissociative recombination reaction NO/sup +/+e/spl hArr/N+0, and that the slow subsequent three-body recombination reaction N+O+M/spl hArr/NO+M ultimately controls the degree of ionizational nonequilibrium. Thus, ionizational nonequilibrium results from and is controlled by the recombination of neutrals. Inhibiting the three-body recombination of NO may then be one key to sustaining elevated electron number densities in low temperature air plasmas.