D. V. Beloplotov

Dynamics of ionization processes in high-pressure nitrogen, air, and SF6 during a subnanosecond breakdown initiated by runaway electrons

The dynamics of ionization processes in high-pressure nitrogen, air, and SF6 during breakdown of a gap with a nonuniform distribution of the electric field by nanosecond high-voltage pulses was studied experimentally. Measurements of the amplitude and temporal characteristics of a diffuse discharge and its radiation with a subnanosecond time resolution have shown that, at any polarity of the electrode with a small curvature radius, breakdown of the gap occurs via two ionization waves, the first of which is initiated by runaway electrons. For a voltage pulse with an ∼500-ps front, UV radiation from different zones of a diffuse discharge is measured with a subnanosecond time resolution. It is shown that the propagation velocity of the first ionization wave increases after its front has passed one-half of the gap, as well as when the pressure in the discharge chamber is reduced and/or when SF6 is replaced with air or nitrogen. It is found that, at nitrogen pressures of 0.4 and 0.7 MPa and the positive polarity of the high-voltage electrode with a small curvature radius, the ionization wave forms with a larger (∼30 ps) time delay with respect to applying the voltage pulse to the gap than at the negative polarity. The velocity of the second ionization wave propagating from the plane electrode is measured. In a discharge in nitrogen at a pressure of 0.7 MPa, this velocity is found to be ∼10 cm/ns. It is shown that, as the nitrogen pressure increases to 0.7 MPa, the propagation velocity of the front of the first ionization wave at the positive polarity of the electrode with a small curvature radius becomes lower than that at the negative polarity.

Breakdown features of a high-voltage nanosecond discharge initiated with runaway electrons at subnanosecond voltage pulse rise time

In the wide pressure range of the pure nitrogen and sulfur hexafluoride with small admixture of nitrogen (2,5%) the development of the breakdown during the formation of diffuse discharges initiated by runaway electrons and X-Ray was investigated. Nanosecond voltage pulses of both polarities with an amplitude up to ~300 kV and risetime of ~0.5 ns applied across the discharge gap did provide sharply nonuniform electric field distribution. Estimations of average propagation velocity of the ionization wave in the nitrogen and mixture sulfur hexafluoride with nitrogen were performed on the basis of data on dynamics of radiation intensity of the second positive (2+) nitrogen system from various regions along of the longitudinal axis of interelectrode gap. Interrelation between the glow dynamics and the local value of the electric field strength has been defined. The results showed that the breakdown is developed in the form of the ionization wave propagating from the potential electrode with the highest concentration of the electric field to the flat-grounded one. In the regions near the grounded electrode practically simultaneous increasing of radiation intensity is registered, that indicates on a possible change of the breakdown mechanism in this part of the discharge gap.

Spots on electrodes and images of a gap during pulsed discharges in an inhomogeneous electric field at elevated pressures of air, nitrogen and argon

Pulsed discharge in a nonuniform electric field accompanied by the appearance of bright spots due to explosive electron emission on electrodes has been investigated. The experiments were carried out using three experimental setups, a voltage pulse duration at a matched load of 2 ns, 40 ns, and 130 ns, respectively. Data on the formation of electrode spots during diffuse discharges in tube-plate or needle-plate gap configurations filled with gases at elevated pressures (air, nitrogen and argon) were obtained. It was found that in the air and other gases, bright spots arise on the flat electrode, and on the negative polarity of the electrode with a small radius of curvature, during the direction change of the current through the gap and the increase of the voltage pulse duration. It was shown that at the positive polarity of the electrode with a small radius of curvature, bright spots on the flat electrode arise due to the participation of the dynamic displacement current in the gap conductance.

Bent paths of a positive streamer and a cathode-directed spark leader in diffuse discharges preionized by runaway electrons

Diffuse discharges preionized by runaway electrons can produce large-area homogeneous discharges at elevated pressures, which is an intriguing phenomenon in the physics of pulsed discharges. In this paper, runaway-electron-preionized diffuse discharge (REP DD) was obtained in a wide pressure range (0.05–0.25 MPa), and under certain conditions a positive streamer and a cathode-directed spark leader could be observed to propagate at some angles to the applied (background) electric field lines. For a 16-mm gap at an air pressure of 0.08–0.1 MPa, the percentage of pulses in which such propagation is observed is about 5%–50% of their total number, and in the other pulses such bent paths could not be observed because there is even no streamer or cathode-directed spark leader in diffuse discharges. In our opinion, such propagation of the positive streamer and the cathode-directed spark leader at some angle to the background electric field lines owes to different increase rates of the electron density in different regions of the discharge volume under REP DD conditions. Therefore, during the formation of a REP DD, the increase of the electron density is inhomogeneous and nonsimultaneous, resulting in an electron density gradient at the ionization wave front.

On the parameters of runaway electron beams and on electrons with an “anomalous” energy at a subnanosecond breakdown of gases at atmospheric pressure

The generation of runaway electron beams in gases at atmospheric pressure has been studied with a real picosecond accuracy. Their main parameters have been determined. It has been found that three groups of electrons can be separated at a subnanosecond voltage pulse in a runaway electron beam generated in air at atmospheric pressure. It has been proven that the duration of a beam pulse in air at atmospheric pressure behind an anode foil is ~100 ps.

Formation of ball streamers at a subnanosecond breakdown of gases at a high pressure in a nonuniform electric field

The formation of a diffuse discharge plasma at a subnanosecond breakdown of a “cone–plane” gap filled with air, nitrogen, methane, hydrogen, argon, neon, and helium at various pressures has been studied. Nanosecond negative and positive voltage pulses have been applied to the conical electrode. The experimental data on the dynamics of plasma glow at the stage of formation and propagation of a streamer have been obtained with intensified charge-coupled device and streak cameras. It has been found that the formation of ball streamers is observed in all gases and at both polarities. A supershort avalanche electron beam has been detected behind the flat foil electrode in a wide range of pressures in the case of a negatively charged conical electrode. A mechanism of the formation of streamers at breakdown of various gases at high overvoltages has been discussed.

Inverted Polarity Effect at the Subnanosecond High-Voltage Breakdown of Air

Inverted polarity effect was studied at the breakdown of point-to-plane discharge gap filled with air at a pressure range of 0.013-0.3 MPa. High-voltage nanosecond pulses were applied to the electrode with a small radius of curvature. Pulser was operated at both negative and positive polarities in the single-pulse mode. Waveforms of voltage pulse, discharge current, and intensity of the discharge plasma radiation near the electrode with a small radius of curvature were registered. It was obtained that at the breakdown of point-to-plane discharge gap filled with air, the breakdown voltage at positive polarity of the pointed electrode was higher than that at negative one. The inverted polarity effect was shown to come to naught with an increasing of air pressure up to 0.3 MPa. It is suggested that the inverted polarity effect is related to the impeded formation of the explosive emission centers on the flat electrode (cathode) after arrival of the ionization wave front to the one. In addition, both the inverted and the ordinary polarity effects were not observed at the breakdown of nitrogen at pressures of 0.4 and 0.7 MPa.

Amplitude−temporal characteristics of a supershort avalanche electron beam generated during subnanosecond breakdown in air and nitrogen

The amplitude−temporal characteristics of a supershort avalanche electron beam (SAEB) with an amplitude of up to 100 A, as well as of the breakdown voltage and discharge current, are studied experimentally with a picosecond time resolution. The waveforms of discharge and SAEB currents are synchronized with those of the voltage pulses. It is shown that the amplitude−temporal characteristics of the SAEB depend on the gap length and the designs of the gas diode and cathode. The mechanism for the generation of runaway electron beams in atmospheric-pressure gases is analyzed on the basis of the obtained experimental data.

Luminescence of Polymethyl Methacrylate Excited by a Runaway Electron Beam and by a KrCl Excilamp

This paper reports an experimental study of luminescence excited in polymethyl methacrylate (PMMA) by a runaway electron beam and by a KrCl excilamp with a wavelength of 222 nm. It is shown that the major contributor to the luminescence of PMMA in both cases is a luminescence band with a maximum intensity at ~490 nm. Based on experiments with the excilamp, it is supposed that Cherenkov radiation with a wavelength shorter than 300 nm is bound to increase the intensity of this band. The luminescence intensity at ~490 nm varies proportionally with the number of beam electrons, allowing the use of this radiation to determine the number of high-energy electrons in electron beams. In PMMA with high absorption at 300–400 nm, one more luminescence band with a maximum intensity at ~400 nm is observed. It is confirmed that in gas diodes at a pressure of 760 torr, the intensity of Cherenkov radiation against the background of luminescence is low and undetectable.

Analogue of bead lightning in a pulse discharge initiated by runaway electrons in atmospheric pressure air

A breakdown initiated by runaway electrons in an inhomogeneous electric field in atmospheric pressure air was studied. Bright plasma channels (spark filaments) in the center of the discharge gap were observed on the background of a diffuse discharge by applying high-voltage pulses with an amplitude of about 200 kV across the discharge gap with an interelectrode distance of 18 mm. Their occurrence probability is no more than 10%. The glow of the bright plasma channels is similar to that of a spark channel. It was found that the bright plasma channels appear with a decrease in the discharge current. Its radiation intensity is an order of magnitude lower than that of the diffuse discharge. Such bright plasma channels were observed in pulseperiodic discharges, where nanosecond voltage pulses with amplitudes of 10–15 kV were applied across the discharge gap with an interelectrode distance of 6 mm. The pulse repetition rate was about 400 Hz. Occurrence of the bright plasma channels on the background of diffuse discharge is caused by generation of runaway electrons and the discharge current redistribution over the discharge plasma cross-section during a decrease in the discharge current. It is assumed that the discharge recorded is an analogue of bead lightning observed in the Earth’s atmosphere.