Displacement current during the formation of positive streamers in atmospheric pressure air with a highly inhomogeneous electric field

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

Blue and green jets in laboratory discharges initiated by runaway electrons

Spectral and amplitude-temporal characteristics of plasma radiation of nanosecond pulse-periodic discharge in air, nitrogen and argon in pressure range of 30-760 Torr were investigated. Discharge gap geometry was a "point-to-plane". Voltage pulses of negative polarity (amplitude, FWHM and risetime was 13 kV, 10 ns and 4 ns, respectively) were applied to a pointed cathode made of different metals (stainless steel, aluminum and copper). Jets of different colour were observed near a cathode tip. They are formed due to explosive emission. Colour of jets depends on the cathode material. Intense lines of the atoms and ions of iron in the wavelength range of 200-600 nm, aluminum with λ = 394.4, 396.15 nm, including multiply charged ion Al VI with λ = 360.39 and 361.65 nm, copper with λ = 324.8, 327.3, 510.6, 515.3, 521.8, 522 nm were registered. The resonance energy transfer from metastable A3Π+u level of nitrogen molecule to 3d104p level of Cu I was found. As a result a luminescence duration of Cu I was about 1.5 μs at duration of discharge current of 1.5 ps. During constriction of the diffuse discharge the sputtering of material occurs in the direction perpendicular to the longitudinal axis of the discharge gap

Features of streamer formation in a sharply non-uniform electric field

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

ICCD-imaging of a plasma glow during the prebreakdown stage of nanosecond discharges at both polarities in nitrogen, air and argon

Dynamics of a plasma glow during the prebreakdown stage of a nanosecond discharge in the «cone-to-plane» gap with length of d = 3 mm was investigated with a HSFC PRO 12-bit four-channel ICCD camera. The gap was filled with nitrogen, air, argon. Gas pressure was ranged from 12,5 to 400 kPa. Voltage pulses of negative (U = 25 kV, τ0,5 = 3 ns, τ0,1–0,9 = 0,7 ns) and positive (U = 25 kV, τ0,5 = 10 ns, τ0,1–0,9 = 3 ns) polarities were applied across the gap. Images of the plasma glow at different stages of streamer formation are presented. It was established that a diffuse discharge is formed due to formation of a large streamer. It was found that plasma appears at a certain distance from the conical electrode at both polarities. These and other features of streamer formation are discussed

On the nature of radiation of blue and green jets in laboratory discharges initiated by runaway electrons

Spectral and amplitude-temporal parameters of radiation from different regions of discharges initiated by runaway electrons have been studied. The pulse-periodic mode of discharge formation was used. It is shown that the color of a part of jets observed during laboratory discharges is determined by radiation of electrode metal vapors. It is found that blue mini jets from an electrode with a small radius of curvature appear in the cases of stainless steel and aluminum electrodes and are caused by emissions of atomic transitions of these metals. Green mini jets observed near copper electrodes are mainly caused by CuI atomic transitions mainly at wavelengths of 521.8 and 522 nm. It is confirmed that jets of different colors appear during formation of bright spots on electrodes, as well as sparks in the discharge

Parameters of REP DDs plasma formed during the pulse and pulse-periodic modes in dense gases

Main parameters of plasma formed during the pulse and pulse-periodic runaway electron preionized diffuse discharge (REP DD) in argon, nitrogen and air at high pressure were measured. An electron concentration in the plasma of pulse and pulse-periodic REP DD in the elevated pressure argon was determined. Average for pulse value of electron density in the argon plasma of pulse REP DD was ~ 3·1015 cm-3. Dynamics of electron density in the atmospheric-pressure plasma of the argon during the REP DD was determined. Measured average values of an electron concentration in the plasma of the pulse-periodic REP DD in atmospheric-pressure air and nitrogen were ~ 3·1014 and ~ 4·1014 cm-3, respectively. In addition, for the plasma formed during the pulse-periodic REP DD in atmospheric-pressure nitrogen and air average values of an electron temperature and reduced electric field, as well their dynamics were determined. Average value of an electron temperature during the pulse duration for nitrogen and air plasmas was ~ 2 eV. Dynamics of an electron temperature and reduced electric field strength was registered. Data on rotational and gas temperatures in the discharge plasma of atmospheric-pressure nitrogen formed in pulse (Tr ≈ 350 K, Tg ≈ 380 K) and pulse-periodic (Tr ≈ 750 K, Tg ≈ 820 K) modes were obtained. In addition, measured value of vibrational temperature in REP DD’s plasma formed in pulse mode in nitrogen at pressure of 1 bar was Tv ≈ 3000 K. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

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