Intuitively, the nanosecond repetitively pulsed (NRP) corona and spark regimes are sustained successively after onsets due to the high background electron density and/or the surplus heat. In this paper, the NRP discharge unexpectedly swings among different regimes (corona→glow→spark→corona→…) in one pulse train, which is characterized by the periodical spark quench and reestablishment. We have investigated discharge regime instabilities by applying long-term repetitive high-voltage nanosecond pulses of ~15 ns duration to needle-needle and needle-plane gaps in atmospheric-pressure N2 and N2-O2 mixtures. Pulse-sequence resolved electrical and optical diagnostics have been implemented to capture transition processes. The initial corona gradually grows into glow and then spark “pulse-by-pulse”, however, the spark regime was interrupted after a certain number of voltage pulses until the next reestablishment. Narrow pulse width impedes the discharge instability growth within one pulse, and a certain number of voltage pulses are required for the discharge regime transition. The addition of O2 dramatically boosts the duration length of spark regime. A lower output impedance of the power supply induces a higher deposited energy into a spark, however, not necessarily leads to a longer spark regime duration, although both the energy storage and the average electric field strength are approximate. Polarity effects, conventionally diminished in pulse-periodic discharges, are still evident during the discharge regime transition. The periodical discharge regime transition is qualitatively explained based on the plasma-source coupling and the evolution trajectory along the power transfer curve. Feedback mechanisms and residual-conductivity related screening effect in NRP spark discharges are analysed based on a simplified 0D simulation. The periodical feature is probably caused by the insufficient average deposited energy per unit distance per one pulse cycle. In-depth understandings of “non-binary” regimes (neither corona nor spark) and memory effect mechanisms of NRP discharges could be reached.

Electronegative gas components and gas pressure significantly change residual charge dynamics, which are critical for pulse-periodic streamer discharge behaviors. Evolutions of repetitively pulsed positive streamer discharge and the streamer-to-spark transitions were investigated at high pressures and compared between typical weak (O2) and strong (SF6) electronegative gas mixtures. Pulse-sequence resolved electrical and optical diagnostics were implemented to capture discharge evolutions in long pulse trains. We observe that streamer inception and propagation under subsequent pulses in N2 and N2–O2 mixtures are similar, including the earlier inception of the primary streamer and the accelerated propagation of the secondary streamer. The repetitively pulsed breakdown is extended to the low pulse repetition frequency region with the addition of O2. Discharge evolutions are unexpectedly different in N2–SF6 mixtures. Subsequent discharge channels prefer to propagate around the periphery of the inception cloud region with large radial deviations. Another difference is the precursor channel identified besides multiple streamer channels. Effects of electronegative gas on streamer evolutions under positive repetitive pulses have been qualitatively analyzed. Collisional electron detachment and photo-ionization are crucial in N2–O2 mixtures. With the presence of strong electronegative gas (SF6), the leader formation is probably induced by the earlier corona inception and longer voltage stressing period under following pulses in a pulse train, which are caused by the strong electronegativity of SF6 and the small ion mobility.

Although the nanosecond repetitively pulsed (NRP) discharge normally stabilizes into one of three regimes (corona/glow/spark) in a pulse train, another nonintuitive instability recently proved that it could periodically swing between corona and spark regimes characterized by repeated spark quenches and reestablishments (Zhao Z et al 2022 Plasma Sources Sci. Technol. 31 045005). In this paper, we have further investigated the suitability of NRP discharge regime transitions for different pulsed power supplies and revealed dramatic effects of the gas flow on streamer dynamics that possibly lead to spark quenches. Pulse-sequence and temporally resolved electrical and optical diagnostics were implemented to capture discharge evolutions in long pulse trains. Periodical discharge regime transitions under long-term repetitive nanosecond pulses are prevalent under transmission line transformer pulser and commercially available FID pulser with parameter constraints. A minimum deposited energy per spark is required for the successive spark pattern. The spark channel before its quench statistically prefers to deviate upstream rather than following the straight axis or intuitively bending downstream to search for more remnants. Before spark quenches, the initial streamer already either exhibits a large radial “detour” or propagates with a zig-zag profile along the periphery of previous spark regions. The periodical discharge regime transition and effects of the gas flow are qualitatively explained based on the plasma-source coupling, evolutions of dominant negative ions composition, and 3D streamer simulation. Periodical NRP spark quenches are probably initiated with the streamer “detour” and then accelerated by the thermal-ionization feedback instability. Inhomogeneous residual charge distribution and accumulations of complex negative ions with high electron bound energies may facilitate the following discharge to search for the gas inlet. In-depth understanding of NRP discharge instabilities could be reached, which are fundamentally governed by residual charge transport and energy relaxation.

Effects of the surplus heat and space charges on the evolution of discharge dynamics and the discharge regime transition were investigated by a co-simulation platform consisting of a zero-dimensional (0D) plasma kinetics model and a two-dimensional (2D) Particle-In-Cell/Monte Carlo-Collision (PIC/MCC) model under repetitive nanosecond pulses. The results from the 0D plasma kinetics model show that the evolution could be defined as three stages: (a) initial cloud, (b) corona enhancement, and (c) quasi-stable spark. Surplus heat plays a key role in the transition from corona to spark. However, the evolution behavior under the corona enhancement stage cannot be explained by surplus heat alone. Detailed results from the 2D PIC/MCC model show that considering the effects of space charges, the transition from corona to spark tends to be hindered in the nanosecond repetitively pulsed discharges. A feedback mechanism for discharge evolution considering surplus heat and space charges is proposed in this paper, which provides a qualitative criterion for determining the evolutionary direction of corona discharge under repetitive nanosecond pulses.

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The effects of pulse rise time on the temporal evolution of electron energy and density under repetitive nanosecond pulses in atmospheric nitrogen with 100 ppm oxygen impurities are investigated in this paper by a two-dimensional particle-in-cell/Monte Carlo collision model. It is found that the peak value of mean electron energy increases with decreasing pulse rise time in the single pulsed discharge. However, in the repetitive pulsed discharge approximated by pre-ionization, the peak value of mean electron energy no longer varies with the pulse rise time, showing a saturation trend with decreasing pulse rise time. Whether or not pre-ionization is present, the time required for the mean electron energy to reach its peak is approximately equal to the pulse rise time. It is worth noting that the presence of pre-ionization enhances the tracking ability of the mean electron energy to the pulse waveform during the pulse rise edge. Although after the peak of the pulse, the mean electron energy terminates the tracking process to pulse waveform due to the formation of high-density avalanches and even streamers, its energy decay rate gradually decreases with the increase in the pre-ionization density. Therefore, when the pulse repetitive frequency is greatly increased or the pre-ionization density is increased by other means, it is possible to achieve the complete control of the mean electron energy by pulse waveform modulation.

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