High efficiency nonequilibrium air plasmas sustained by high energy electrons

The Air Plasma Ramparts MURI goal is to sustain a volume-filling, nonequilibrium plasma with an electron number density of 10/sup 13/ electrons/cm/sup 3/ in atmospheric pressure air at temperatures at or below 2000 K, with the minimum expenditure of power. That minimum depends directly on the energy cost of ionization. In this paper, approaches to minimize the cost of ionization are examined. It is well understood that the most practical ionization mechanism is through collisions of electrons with neutral species. The cost of ionization as a function of the electron energy indicates that there is more than a two order of magnitude reduction in power required if the plasma is sustained by high energy electrons. These electrons can be supplied from outside the plasma by the use of electron beams, or from within the plasma by short, repetitively-pulsed, very high voltage acceleration of thermal electrons between collisions with neutral gas molecules. Electron beams offer the possibility of sustaining single electrode plasma "fountains" or plasma "thunderstorms," and generate stable, non-filamentary, volume-filling plasmas. Electron beams are approximately a factor of two more efficient than the best that can be achieved with pulse sources, but they must enter the plasma through windows whose losses can easily offset that advantage. High voltage, subnanosecond length pulses at repetition rates of more than 100 KHz avoid the window problem.