Threshold effects in the development of highly ionized plasma channels in a pulsed nanosecond discharge in air at pressures of 100–760 Torr

The formation processes of highly ionized plasma channels (filaments), with an electron concentration of N_{e}∼10^{18}-10^{19}cm^{-3}, in a pulsed nanosecond discharge in air at a pressure of 100-760 Torr are investigated. The studies are carried out for millimeter-sized gaps with a point cathode and flat anode. It is established that explosive processes at the cathode give rise to the emergence of an ionization wave propagating toward the anode with a velocity of ∼10^{6}-10^{7} cm/s and initiating the development of highly ionized filaments. The wave's propagation velocity is practically independent of the air pressure. At pressures 400-760 Torr, the ionization wave front is unstable and splits into numerous microchannels with typical diameters of ∼20µm and electron concentrations N_{e}=(1-5)×10^{19}cm^{-3}. The microchannel formation ceases to be observed when the air pressure decreases from 400 to 300 Torr, whereas the discharge development occurs mainly through the development of a uniform highly ionized plasma filament. A decrease in pressure entails the drop in the maximum values of electron concentrations, which, however, are close to the values corresponding to complete ionization. Theoretical estimates of characteristic filamentation times are presented together with the analysis of the possible mechanism responsible for generating plasma with a high degree of ionization. The findings can be helpful in refining the models of plasma-chemical kinetics and the formation mechanisms of a system of highly ionized microchannels in high-pressure discharges at nanosecond times.