Such plasmas can be produced by means of electrical discharges in which energy is applied in targeted fashion to the free-electrons. Under appropriate conditions, the energized electrons can increase ionization through collisions without significantly heating the gas. This approach was successfully demonstrated in our laboratory in a series of DC discharge experiments with low temperature atmospheric pressure air. Stable diffuse glow discharges were produced with electron number densities in excess of 1012 cm-3, more than six orders of magnitude higher than in thermally heated air. The measured electrical discharge characteristics compare well with predictions based on a two-temperature kinetic model of air plasmas in which the electron temperature is elevated with respect to the gas temperature. The experimental and modeling results provide insight into the mechanisms of ionization of two-temperature air plasmas. They also show that the steady-state electron number density exhibits an S-shaped dependence on the electron temperature, a surprising behavior resulting from the competition between ionization and charge transfer reactions in molecular plasmas.
From a power budget perspective, however, DC discharges are not always practical because more than 30 kW/cm3 are required to sustain 1013 electrons/cm3. We recently showed however that the power budget can be greatly reduced by employing short (1-10 ns) high-voltage pulses. The underlying idea is that inelastic energy losses to nitrogen molecules, per electron created, are several orders of magnitude smaller at say Te=3 ev than at Te=1 ev, and therefore that short high voltage electric pulses producing electron temperatures of 3-5 eV have a much higher ionization efficiency than the typical 1 eV DC discharges. We have devised a repetitively-pulsed discharge scheme using this principle and experiments are in progress in our laboratory with a 100 kHz, 12 kV, 10 ns repetitive pulse generator. We will present preliminary pulsed discharge results that demonstrate that power reductions by over two orders of magnitude can be achieved with these short high voltage pulsed discharges.