Nonequilibrium discharges in atmospheric pressure air

Summary form only given, as follows. There is considerable interest in finding methods to efficiently generate large volumes of atmospheric pressure air plasmas at temperatures below 2000 K with electron number densities of the order of 10/sup 13/ cm/sup -3/. Such plasmas can be produced by electrical discharges in which energy is applied to the free-electrons in a targeted fashion. Under appropriate conditions, the energized electrons can increase ionization through collisions without significantly heating the gas. This approach was successfully demonstrated in DC discharge experiments with low temperature atmospheric pressure air. Stable diffuse glow discharges were produced with electron number densities in excess of 10/sup 12/ cm, more than six orders of magnitude higher than in thermally heated 2000 K air. The measured discharge characteristics compare well with the predictions of a two-temperature kinetic model. The experimental and modeling results provide insight into the mechanisms of ionization of two-temperature air plasmas and show that the steady-state electron number density exhibits an S-shaped dependence on the electron temperature, a behavior resulting from competition between ionization and charge transfer reactions. DC discharges are not always practical because of their large power requirements. We recently showed that the power budget can be reduced by several orders of magnitude using short repetitive high-voltage pulses. Because inelastic energy losses, per electron created, are orders of magnitude smaller at Te=3-5 eV than at 1 eV, short pulses producing electron temperatures of 3-5 eV have much higher ionization efficiencies than typical 1 eV DC discharges. Between consecutive pulses, the plasma is sustained owing to the finite rate of electron recombination. A repetitively-pulsed discharge scheme was devised using these ideas and experiments are in progress in our laboratory with a 100 kHz, 12 kV, 10 ns pulse generator. Experimental and mo- eling results of these repetitively pulsed discharges will be presented.