Pulsed breakdown at high overvoltage: development, propagation andenergy branching

Abstract

This paper presents a review of investigations of detailed spatialand temporal structures of high-voltage pulsed nanosecond discharges in theform of fast ionization waves. The most distinctive features of this type ofdischarge are a high propagation velocity (109-1010 cm s-1),good reproducibility of the discharge parameters at a moderate (tens of hertz)repetition rate and spatial homogeneity over a large gas volume. The dischargewas initiated by voltage pulses of negative polarity with an amplitude of10-15 kV, a duration at half maximum of 25 ns and a rise time of the front of3-5 ns. The behaviour of the electric field and electron andexcited-stateconcentrations were analysed on the basis of experimental data within theframe of the unified kinetic approach. It was found that the longitudinalcomponent of the electric field has a sharp (2-3 ns) maximum and that theelectrons and excited particles are produced preferentially behind the frontin relatively weak electric fields. The peak field value was close to or evenstronger than the threshold for the generation of runaway electrons in asteady-state uniform electric field. An analysis based on absolutetime-resolved measurements of the spectrum of two molecular bands showed that,behind the breakdown front, the EEDF should be substantially overpopulatedwith high-energy electrons. Energy branching in the discharge was analysed.Possibilities of application of the fast ionization wave as a source of auniform pulsed plasma were suggested and justified.