Air plasma production by high-voltage nanosecond gas discharge

This paper represents our experimental and numerical investigations of plasma created at high overvoltage in air. Two different types of discharge are considered: streamer at high pressures (1 atm) and so-called fast ionization wave at low pressures (0. 1-30 Torr). The streamer was initiated by voltage pulse of positive polarity, 9 kV amplitude, duration of 75 ns, the rise time of 25 ns, and the repetition rate of 1. 2 kHz. The absolute densities of molecules in the N₂(C³Π_u, v; = 0), N₂⁺(B₂∑_u⁺, v = 0), and NO(A²∑⁺, v = 0) states across the discharge gap were determined, and the reduced electric field was estimated. It was shown that the increase in the average electric field in the gap substantially intensifies the production of active particles and makes the plasma more homogeneous and that the active particles are preferably produced in the streamer head. The fast ionization wave was initiated by voltage pulses of 10-15 kV amplitude, a duration of 25 ns, and a rise time of 2-8 ns. The most distinctive features of this discharge are high velocity (10⁹-10¹⁰ cm/s), good reproducibility at a low repetition rate, and spatial homogeneity. Behaviour of electric field, electron and excited state concentrations were analyzed experimentally. It was found that the longitudinal component of the electric field has a sharp (a few ns) maximum and that the active particles, in contrast to the streamer case, are produced behind the front of the discharge in relatively weak fields.