Pulsed breakdown at high overvoltage: Development, propagation and energy branching

This paper presents a review of investigations of detailed spatial and temporal structures of high-voltage pulsed nanosecond discharges in the form of fast ionization waves. The most distinctive features of this type of discharge 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 discharge was initiated by voltage pulses of negative polarity with an amplitude of 10-15 kV, a duration at half maximum of 25 ns and a rise time of the front of 3-5 ns. The behaviour of the electric field and electron and excited-state concentrations were analysed on the basis of experimental data within the frame of the unified kinetic approach. It was found that the longitudinal component of the electric field has a sharp (2-3 ns) maximum and that the electrons and excited particles are produced preferentially behind the front in relatively weak electric fields. The peak field value was close to or even stronger than the threshold for the generation of runaway electrons in a steady-state uniform electric field. An analysis based on absolute time-resolved measurements of the spectrum of two molecular bands showed that, behind the breakdown front, the EEDF should be substantially overpopulated with high-energy electrons. Energy branching in the discharge was analysed. Possibilities of application of the fast ionization wave as a source of a uniform pulsed plasma were suggested and justified.