Pulsed Streamer Discharges Generated by Sub-Microsecond Pulsed Power in Air

The earlier experimental setup for the study of a multi-streamer single pulse sliding discharge was supplemented by a photoelectric system for recording streamer radiation with a time resolution of 1.5 ns. Measurements of streamer delay and propagation velocity in Ne of atmospheric pressure at two supply voltage polarities were performed. The results showed that during each discharge pulse, regardless of its polarity, streamers of two types propagate across the gap in opposite directions. The first ones, primary streamers, start from a high-voltage electrode and move to a grounded electrode. The other type of streamers, reverse streamers, propagate in the opposite direction from a grounded electrode only after primary streamers have reached it. The experiments were carried out using voltage amplitude of 3 and 3.5 kV applied to plane-parallel electrodes on an alumina ceramic substrate with a rectangular discharge area of 9.5×11.6 cm2 in size. The total delay time range of the streamers was up to 2500–2700 ns and their motion velocity up to (3.5−5.6)×107 cm/s, depending on the discharge voltage and polarity. The dependence of delay time and velocity of the streamers' head motion on their position in the gap is presented. A qualitative explanation of the primary streamer characteristics is proposed, while the obtained experimental data are not sufficient for a similar explanation with respect to reverse streamers. The presented results are significant for studying the physical mechanisms of counterpropagating surface streamer/ionization wave dynamics, while the experimental methodology allows recording their spatiotemporal evolution in the gap.

Summary form given only. Pulsed power technology has been used in many applications such as control of NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> and SO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> from exhaust gases, treatment of dioxins, removal of volatile organic compounds, generation of ozone and excitation of excimer laser. Since pulse duration of applied voltage to discharge reactor has a strong influence on energy efficiency of pollutants removal, the development of short pulse generator is paramount important for practical applications. The discharge observation of the short duration pulse voltage is an essential aspect for understanding plasma physics of this new field. In the present work, the characteristics of the developed nano-seconds (ns) pulse generator, which has the pulse duration of 5 ns, and the observation results of nano-second (ns) pulsed streamer discharges in atmospheric air are presented. The developed ns pulse generator consists of a high pressure spark gap as a lower inductance self-closing switch, a triaxial Blumlein line as a pulse forming line, a coaxial energy transmission line as a connection between the Blumlein and a load, and a pulse charging source. The triaxial Blumlein line is filled up with transformer oil as insulation and dielectric medium. The ns pulse generator outputs the positive voltage, having 2 ns of rise time and 5 ns of pulse duration. The maximum output voltage reaches up to 100 kV. The propagations of the positive and the negative ns pulsed streamer discharges in a coaxial electrode were observed using a high speed gated ICCD camera and a high dynamic range streak camera. During the both polarities of ns pulsed streamer discharges, the primary and the secondary streamers propagated from inner wire to outer cylinder electrodes. This phenomenon has good agreement to the previous works which are observed in the cases of sub-frac14s pulsed streamer discharges. However, the propagation velocity of the primary streamers in the present work is approximately 6 times faster than the previous one. This is because that the velocity of primary streamers dependent on applied voltage to electrode and ns pulse generator is able to apply the higher voltage to electrode than sub-frac14s pulse generator.

Generation and propagation of the primary and secondary streamers, initiated by a positive pulsed surface dielectric barrier discharge under different pulse amplitude and different dielectric permittivity, are studied via experimental and numerical simulation. Species (positive ions, negative ions and oxygen atoms) generation and distribution in the primary and secondary streamers are discussed. Experimental and numerical results are qualitatively coincident in discharge morphology, voltage–current characteristics and deposited energy. From visual forms of the discharge, both the primary and secondary streamers are initiated from the edge of the high-voltage (HV) electrode and propagate along the dielectric barrier surface. However, the numerical results show that the primary streamer is not completely attached to the dielectric barrier surface, and the distance between the primary streamer head and the surface is the largest, which is about several microns. Moreover, the secondary streamer starts about tens of microns away from the HV electrode edge. Furthermore, when the secondary streamer occurs, the trailing phenomenon of the primary streamer can still be observed, which is sustained by the accumulated positive charge.

In pre-ionized pulsed streamer discharge, regulating the seed-electron density and its spatial distribution during the pre-ionization stage provides an effective approach to control streamer development and plasma chemical activity. Using electrical measurements, optical diagnostics, and a two-dimensional axisymmetric fluid model, this study systematically analyzes the behavior of pulsed streamer discharges under seed-electron pre-ionization and quantitatively clarifies how seed-electron density and spatial distribution govern the evolution of the primary streamer (PS), the secondary streamer (SS), as well as the generation of reactive species. The results show that UV pre-ionization enhances the discharge intensity and promotes streamer development. Under the same experimental conditions, simulations indicate that the minimum seed-electron densities required to initiate the development of PS and SS are 10 10 m -3 and 10 12 m -3 , respectively. When the seed-electron density exceeds 10 10 m -3 , PS development is enhanced, leading to increased formation of N 2 (C), N 2 + and O 2 + . Once the density exceeds 10 12 m -3 , SS development is accelerated, and the generation of N and O increases markedly. The spatial distribution of seed electrons is also critical to discharge development. Placing electrons near the anode region accelerates breakdown and promotes PS development, whereas electrons near the cathode region favor SS development, intensifies the radial electric field and electron density near the cathode region, and ultimately leads to a large discharge volume. These findings clarify how seed electrons regulate pulsed streamers and provide guidance for tailoring pre-ionization strategies to optimize plasma reactivity.

The NOx treatment using streamer discharges generated by nanosecond pulsed powers was studied. The streamer discharges were produced in the treatment reactor by adopting nanosecond pulsed powers. In the previous study, temporal decrease in NO removal ratio was observed as pulse repetition rate increased. This was because the inner wire electrode of coaxial treatment reactor curved during pulse application and spark discharges occurred between the electrodes. A tensioned inner wire electrode was introduced in the coaxial reactor by using a spring system. As a result, the temporal decrease in removal ratio was not observed and the removal ratio was righted. To utilize denser streamer discharges near the inner wire electrode, the diameter of external cylindrical electrode was made smaller. The thinning of reactor improved the NO removal ratio with the tensioned inner wire electrode. When the initial NO concentration was 100 ppm, the removal ratio reached up to approximately 100 %. However, the NO removal ratio decreased when the inner diameter of the external cylindrical electrode was decreased from 14 mm to 10 mm. This might be because spark discharges would occur in the reactor by excessively shortened electrode separation. In addition, the energy deposited in reactor might decrease with decrease of reactor volume.

The presence of water molecules in air introduces complexities to residual charge transports and energy relaxations that may provoke streamer discharge instabilities under repetitive pulses. Evolutions of pulse-periodic positive streamer dynamics were investigated in humid air. Pulse-sequence and temporally resolved diagnostics were implemented to capture discharge evolutions. The streamer development and evolutions of charged species in humid air are qualitatively analyzed based on a 2D–0D combined simulation. Evolution features of streamer behaviors in humid air include the faster filamentation of the primary streamer, pronounced propagation selectivity to previous secondary streamers, more branches of both primary and secondary streamers, and accelerated secondary streamer into stagnation. The repetitively pulsed breakdown is proceeded by the progressive axial prolongation of secondary streamers with bright heads and faint tails. Nonintuitively, the withstanding capability does not illustrate dramatic differences in dry air and humid air especially at high pulse repetition frequency. High-density residing hydrated ions with high electron bound energies (impeding streamer propagation) and the enhanced thermal release to cause higher reduced electric field (facilitating streamer propagation) may contrarily affect evolutions and discharge instability developments of positive streamer in humid air.

Pulsed streamer discharges have been extensively used in many applications such as control of NO/sub X/ and SO/sub 2/ from exhaust gases, treatment of dioxins, removal of volatile organic compounds, generation of ozone, and laser excitation. An operation with a high energy efficiency is necessary for practical applications. It is very important to know the propagation mechanism of streamer discharges in order to improve the energy efficiency of pulsed discharge systems. In this paper, the emission from pulsed streamer discharges in a coaxial electrode system in air at 0.1 MPa was observed using a high-speed gated intensified charge-coupled display camera. A concentric wire-cylinder electrodes configuration was used. A positive pulsed voltage having a width of about 100 ns was applied to the central electrode. The streamer discharges were initiated at the inner electrode and terminated at the outer electrode. The propagation velocity of the streamer discharges was 1.8-3.3 mm/ns.

Streamer properties such as their velocity, diameter, intensity and density, can be obtained by analysis of temporal and spatial resolved ICCD imaging. In this paper, experimental results on streamer generation and propagation as a function of several high-voltage pulse and reactor parameters are described. Experiments were performed on a large scale wire–plate reactor in ambient air. The set-up allows for independent variation of the parameters over wide ranges. The minimum gate time of the ICCD camera is 5 ns, allowing for a high temporal resolution. The camera can be triggered with a precision of 1 ns. Both negative and positive polarity pulses are investigated. The most important conclusions are as follows. (1) The streamer velocity ((0.5–2.5) × 106 m s−1) increases if the applied electric field and/or the voltage rise rate is increased. (2) The same is true regarding the velocity ((0.2–1.2) × 105 m s−1) with which the streamer diameter (0.7–3.0 mm) increases during propagation. (3) Typical properties (velocity, diameter, etc) of negative and positive polarity streamers vary less than 25%, especially when the applied electric field is high. (4) As long as the dc bias voltage is below the dc corona onset value it does not have a separate effect on the visual streamer properties. Only the total voltage (peak voltage + dc) is of importance. (5) A simple model was used to determine the electric field in the secondary streamer channel. It was found that in the light emitting part of the secondary streamer the electric field is approximately 21.5 kV cm−1. In the remainder (dark part) of the channel the electric field is around 6.5 kV cm−1. This paper shows mainly experimental findings. Not all observed relations and phenomena could be explained. This is partly caused by the fact that current theoretical and numerical models are not yet able to describe the experimental situation as used during this study.

Streamer discharge plays a central role in the electrical breakdown of matter in high electric fields, both in nature and in applied technology. This tutorial provides an intuitive introduction to the physics of streamer discharges using a fluid modeling approach. The fundamental mechanisms governing streamer inception, propagation, and interaction are demonstrated under representative electrode configurations and electric field conditions. Both positive and negative streamers are examined under uniform and nonuniform background fields. Temporal evolutions of key parameters, such as charge density, electric field, and propagation velocity, are analyzed to build a physical understanding of streamer dynamics. The relationships among streamer velocity, electron drift velocity, and diffusion velocity are analyzed to elucidate the self-consistent dynamics of the discharge front. The influence of humidity is also explored to demonstrate the environmental effects on discharge behavior. Under nonuniform electric fields, the development of the conduction and displacement currents and the morphological differences between positive and negative streamers are clarified. Finally, the interaction between oppositely charged streamers is analyzed, revealing how the electric field distribution at their heads determines the streamer interaction behavior. This tutorial integrates physical reasoning and numerical visualization to build an accessible foundation for understanding streamer discharge physics.

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.

Axisymmetric two-dimensional (2D) simulation is often used in case of streamer discharge in atmospheric-pressure air. To validate the 2D simulation of streamer discharge, the simulation has to be compared with the measurement results. However, there is difficulty in performing the comparison because the streamer is not a 2D phenomenon but a three-dimensional phenomenon. In this study, a pseudo-2D axisymmetric single-filament pulsed positive streamer discharge was generated in atmospheric-pressure air and compared with our 2D simulation. The propagation, emission intensity, shape, and cathode current of the streamer were compared. The ozone density was also measured using laser spectroscopy and compared with the simulation because our simulation intends to simulate the dynamics of reactive species. The measurements and simulations exhibited some agreements and disagreements. The measurement also showed a cathode fall and other cathode phenomena, which were not considered in the simulation.

The influence of humidity and applied voltage on negative streamer discharge in air gaps exhibits a nonlinear synergistic effect. Numerical simulation and experimental investigation were conducted on negative streamer discharges in a 3 mm needle–plane air gap. A fluid model was established that simultaneously accounts for both humidity and the applied voltage. The validity of the model was confirmed by comparison with experimental data. The electron density and electric field at the streamer head increase with increasing humidity, and both quantities continue to intensify as the streamer propagates toward the plane electrode. The magnitude of the applied negative DC voltage and its influence become more pronounced in the vicinity of the plane electrode. The applied voltage has almost no effect on the streamer channel radius, whereas the channel radius increases with increasing humidity. When the applied voltage amplitude is below 10 kV, an increase of 30% in relative humidity leads to approximately a 10% reduction in streamer propagation velocity, while the discharge current exhibits only minor variation. In contrast, when the applied voltage amplitude exceeds 10 kV, every 30% increase in relative humidity causes the electric field at the streamer head, the streamer channel radius, and the streamer propagation velocity to increase by 7.8%, 10%, and 50%, respectively, and the discharge current nearly doubles. At constant relative humidity, every additional 1 kV in applied voltage increases the electric field at the streamer head, the streamer propagation velocity, and the discharge current by approximately 5%, 25%, and 30%, respectively. Moreover, these increasing trends become more pronounced at higher relative humidity. However, the increase in applied voltage has no significant effect on the streamer channel radius.

We present a computational simulation study of non-equilibrium streamer discharges in a coaxial electrode and a corona geometry for automotive combustion ignition applications. The streamers propagate in combustible fuel-air mixtures at high pressures representative of internal combustion engine conditions. The study was performed using a self-consistent, two-temperature plasma model with finite-rate plasma chemical kinetics. Positive high voltage pulses of order tens of kV and duration of tens of nanoseconds were applied to the powered inner cylindrical electrode which resulted in the formation and propagation of a cathode-directed streamer. The resulting spatial and temporal production of active radical species such as O, H, and singlet delta oxygen is quantified and compared for lean and stoichiometric fuel-air mixtures. For the coaxial electrode geometry, the discharge is characterized by a primary streamer that bridges the inter-electrode gap and a secondary streamer that develops in the wake of the primary streamer. Most of the radicals are produced in the secondary streamer. For the corona geometry, only the primary streamer is observed and the radicals are produced throughout the length of the primary streamer column. The stoichiometry of the mixture was observed to have a relatively small effect on both the plasma discharge structure and the resulting yield of radical species.

In an underwater streamer discharge, plasma channels are created and propagate in water. After the discharge, the ionized gas channels gradually cool down and decay. In this paper, a time-resolved study on the decay process of the gas channels induced by pulsed positive streamer discharge in water is performed by using a high-speed camera system. The gas channels created by the streamer discharge in water have many branches. Some of them grow from the tip of the point anode, forming main branches. From the main branch, some subbranches develop. The subbranch dilapidates rapidly into microbubbles, while the main branch abbreviates toward the root of the branch. A bubble that formed at the tip of the anode connects with the main branch. The main branch does not completely disappear until 80 μs after the discharge. When the main branch disappeared, the bubble at the tip of the anode becomes a perfect sphere and approximately reaches its maximum radius. Then, the bubble starts to collapse, holding its spherical shape, and reaches its minimum radius at about 170 μs after the discharge.

In this paper we present our solid-state nanosecond pulse source (the solid-state impedance-matched Marx generator) which can generate arbitrary waveforms and which can be used for pulsed discharge generation. The purpose of the development of such a generator is twofold: by being able to change the waveform at will, we aim to control the discharge generated by such pulses very precisely which can be very useful for plasma applications, but also for more fundamental studies. In the presented study, we applied the arbitrary-waveform pulse source for streamer discharge generation in a cylinder-wire-like arrangement and used the arbitrary-waveform capability to change the rise time (in our experiments we used 6.8–26.2 ns) of unipolar positive pulses with 6-10 kV amplitude and 80 ns duration. Additionally, we introduced variations of a step in the rising edge of the waveform. We performed measurements both in air and nitrogen to electrically characterize the discharge while analyzing the streamer propagation in the plasma reactor with intensified charge-coupled device imaging and measured ozone generation (in air). The results show that we can indeed control the propagation of the streamer discharge with the stepped waveform, but that the rise-time variation has little effect on the streamer propagation in our system. However, the streamer velocity and structure differs significantly comparing discharges in nitrogen and air for the same applied voltage waveform. Additionally, for some of the stepped waveforms we found a slight increase of the ozone yield for air at low overall energy densities.