The streamer formation in a point-to-plane gap filled with atmospheric-pressure air has been experimentally studied using a streak camera and a four-channel intensified charge-coupled device camera with simultaneously recording waveforms of voltage and discharge current pulses. A large diameter streamer was observed at various amplitudes of nanosecond voltage pulses. The instantaneous streamer velocity was measured using the streak camera. It was found that the streamer has a high velocity at the initial stage of development, but it rapidly decreases. The minimum streamer velocity corresponds to the maximum diameter. The streamer velocity increases again by an order of magnitude when it approaches the opposite electrode. It was found that the streamer velocity correlates with the value of a displacement current induced by its propagation. At the initial stage of the streamer development during subnanosecond breakdown, the displacement current can reach several kiloamperes; this is comparable to the conduction current after the breakdown.

Results of experimental studies of the amplitude–temporal characteristics of a runaway electron (RE) beam, as well as breakdown voltage and discharge current with a picosecond time resolution are presented. The maximum pressure, at which a RE beam is detectable, decreases with increasing the voltage rise time. The waveforms of the discharge and RE beam currents are synchronized with those of the voltage pulses. It is shown that the amplitude–temporal characteristics of the RE beam depend on the designs of the gas-filled diode and cathode, as well as the gap length. The mechanism for the generation of REs in atmospheric-pressure gases is analyzed on the basis of the obtained experimental data.

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This paper presents experimental data on the dynamics of positive streamers formation in a highly inhomogeneous electric field under threshold for the breakdown of the gap conditions as well as at high voltage. The glow of the streamer was registered with a four-channel intensified charge-coupled device camera with simultaneous recording of voltage and current characteristics per pulse. It was shown that the streamer appearance is accompanied by a displacement current pulse induced by a redistribution of the electric field strength in the gap. We call this current the dynamic displacement current (DDC). The DDC value changes during the streamer propagation in the gap. It reaches the maximum values when the streamer starts and when it is close to bridge the gap. The DDC was also registered with a collector placed behind the grid plane electrode which is usually applied for the registration of runaway electrons when a negative streamer is formed. It was shown that the DDC allows to study the features of the streamer formation.