The results of studies of the microstructure in the initial phase of the discharge in air in the gap between the pin and a plane 1.5 mm long are presented. Measurements show that within 15 ns after breakdown, the channel is a bundle of a large number of microchannels, the current in the channel grows almost linearly up to 1 kA, and the electron concentration reaches 2 × 1019 cm−3. Taking into account the experimental data, the electron temperature dynamics in a separate microchannel was calculated. It was found that the average electron temperature is from 4 to 8 eV, the electric field strength is ∼300 kV/cm, and the electrical conductivity is ∼10 Ω−1 cm−1. The obtained results indicate that it is the microstructure of the discharge that determines the relatively high values of the average temperature of electrons in combination with a sufficiently high degree of ionization.

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Observations of the early stage of a spark discharge in air at atmospheric pressure in the pin-to-plate geometry under a single high-voltage pulse, including an initial stage of the channel expansion, are presented. It was shown that an interelectrode gap is closed by a large number of microchannels at 20-30 ns. The start of a radial channel expansion begins at the moment when the electron concentration reaches the maximum value of 2 × 10 19 cm −3 . Theoretical analysis based on the proposed model takes the following plasma-channel properties: electric field strength is about several kV cm −1 , electron temperature is close to 2-3 eV, plasma conductivity is in the order of ∼200 Ω −1 •cm −1 , speed of the radial spark channel expansion is ∼5 km s −1 .

Experimental and theoretical investigations of the initial stage of the formation of a pulsed volume discharge between two plane electrodes in argon at atmospheric pressure under conditions of inhomogeneous preionization are reported. The experimental results of the propagation velocity of an ionization wave are compared with the obtained value from the numerical simulation. The features of the development of ionization waves in atmospheric-pressure argon are studied.