Investigation of plasma properties in the phase of the radial expansion of a spark channel in the ‘pin-to-plate’ geometry

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 of a large number of microchannels at 20-30 ns. Start of a radial channel expansion is beginning at the moment when the electron concentration reaches the maximum value of 2*10^19 cm-3. Theoretical analysis based on proposed model takes the following plasma-channel properties: electric field strength is about several kV/cm, 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 about ~5 km/s.