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The results of the investigations of the prebreakdown stage of the self-sustained subnanosecond discharge in nitrogen at pressures between 5 and 40 atm are presented. A high voltage pulse with a front of approximately 250 ps at the level of 0.1–0.9 in amplitude (full duration of the pulse front was 500 ps) was applied to the studied gas gap. In this case, the voltage rise rate in the discharge gap at the prebreakdown stage reached up to 7 × 1014 V/s. Breakdown occurs at the front of the voltage pulse. During these experiments, the parameters of the voltage pulse at the output of the pulse generator were not modified. In this study, it was discovered that increasing of the pressure from 5 atm to 40 atm leads to a significant decrease in the overvoltage in the discharge gap. It is shown that at pressures above 10 atm, the delay time of breakdown is less than the time of growth for electron avalanches to reach a critical size. The critical length of avalanche is approximately one order of magnitude less than the length of the discharge gap. Hence, the avalanche-streamer model is inapplicable in this situation. A mechanism of subnanosecond breakdown initiation with a help of runaway electrons at pressures above 10 atm has been suggested.

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The initiation and the development of a breakdown of highly overvoltaged high-pressure (from 4 to 40 atm) gas gaps by voltage pulses having the risetime of 1 ns or shorter are studied experimentally and in theoretical terms. The study revealed that ionization processes leading to the breakdown start in the gas volume and not from the surface of the electrodes. The gap flashover is followed by ionization wave processes initiating in the gas volume and playing the decisive role at the first phase of the breakdown. The dynamics of the ionization waves strongly depends on the initial distribution of free electrons over the gas gap. The distribution of ionization waves is analysed when the initial electrons are distributed uniformly and nonuniformly over the gap. The calculation results are in qualitative agreement with the relevant experimental data. It is shown that the propagation of the ionization waves at the initial stage of subnanosecond pulsed electrical breakdown of gas leads to a redistribution of the electric field in the discharge gap and a region of a strong field, whose intensity is sufficient for the onset of emission processes and the generation of a short beam of fast electrons near the cathode, is formed at the cathode for a very short (up to 100 ps) time.

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