Negative Streamers Research Articles

Pre-Breakdown and Breakdown Phenomena in Liquid Nitrogen With a Tape–Plane Electrode Geometry

Pre-breakdown and breakdown phenomena are investigated in liquid Nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) with a tape-plane electrode geometry. Different voltage waveforms are used: lightning and switching standard impulses, step impulse, and DC ramp. The influence of temperature is also investigated, by sub-cooling LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> down to 65 K. Breakdown voltage depends on polarity, gap distance, temperature, and HV waveform: the longer waveform, the lower breakdown voltage. Positive pre-breakdown streamers are about 3 decades faster than negative ones. With short duration lightning impulses, the propagation of slow negative streamers is quenched, resulting in higher breakdown voltage. In boiling conditions, slow negative streamers may propagate for long durations. Sub-cooling LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> induces a slight increase of streamer initiation voltage, and impedes the propagation of slow streamers due to condensation.

Breakdown Phenomena in Liquid Nitrogen Under Synchronized Transient Boiling and Impulse Voltage

Resistive superconducting fault current limiters constitute a promising solution for high voltage networks. During the resistive current limitation, transient overvoltages and heat dissipation occur on superconducting tapes, inducing fast boiling of surrounding liquid nitrogen (LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). An experimental system reproducing the fast transient heating of a tape electrode, synchronized with a high voltage impulse is described. Breakdown voltages measurements in both polarities are carried out versus synchronization delay, pressure (0.10 to 0.40 MPa), temperature (77 K and 65 K). Breakdown voltages show complex variations with these parameters. More insight is obtained from the recording of pre-breakdown phenomena (streamer). The size of vapor bubbles has a large and counter-intuitive influence on streamer initiation: the smaller bubbles, the lower inception voltage. Subcooling LN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (by decreasing temperature and/or increasing pressure) sometimes has a beneficial influence (quenching of negative streamer propagation), but deleterious effects can be as well observed in other circumstances (by favouring positive streamer initiation).

Streamer and Breakdown Characteristics of Transformer Liquids in Moderately Uniform Fields Under Negative Lightning Impulse

The identification of transition field factor boundary between streamer initiation dominated breakdown and streamer propagation dominated breakdown is important for understanding the breakdown mechanisms of transformer liquids and for the transformer insulation design in practice. Most previous studies focused on either very divergent field or nearly uniform field. This paper reports on breakdown voltage tests with three transformer liquids in moderately-uniform fields. The results show that the transition field factor boundary between streamer initiation dominated breakdown and streamer propagation dominated breakdown under negative lightning impulse is liquid dependent. It is 9.6 for a mineral oil, 7.3 for a GTL oil, and 42.0 for a synthetic ester. The difference of the transition field factor boundary helps explain the observed breakdown voltage distinction among the three transformer liquids. In addition, the characteristics of negative streamers leading to breakdown were also investigated in the moderately-uniform fields. Streamer mode transitions with gap distance were observed and clarified, which is from 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> mode streamer to 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> mode streamer in both the mineral oil and GTL oil, while it is the 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> mode streamer to fast streamer in the synthetic ester.

On propagation of positive and negative streamers in air in uniform electric fields

Recently published results of numerical simulations of positive and negative streamers propagating in uniform applied electric fields in air are analyzed in the framework of an analytical approach. It is shown that obtained approximate relations between the streamer radius, velocity and length, depending on the value of applied electric field, are in reasonable agreement with the computational data.

Comparison of negative streamer development in synthetic ester and mineral oil in a point-sphere gap divided by a pressboard barrier

This article presents the results of the studies on development of negative streamers in synthetic ester and mineral oil with electrode system using a pressboard barrier. The tests were performed at standard lightning impulse voltage in point-sphere electrode configuration as reference system and a 5 mm thick pressboard barrier placed in different locations between the point and the sphere. Introducing pressboard between the electrodes allowed to relate the observations to the real insulating systems where long oil gaps are divided on shorter by insulating barriers. It was done to show whether pressboard barrier changes the streamers behavior in the considered electrode gap distance what is important from the viewpoint of assessment of dielectric properties of alternative liquids versus mineral oils. The results showed that the pressboard barrier for both liquids tested prevents breakdown. In addition, the barrier causes reduction of the propagation velocity of the streamers in synthetic ester, which is benefit in relation to the results concerning free liquid gaps.

A numerical investigation on electron runaway threshold at the initial stage of atmospheric streamer development

Pre-ionization caused by runaway electrons is an important mechanism for negative streamer development. The aim of this paper is to investigate the runaway criteria and overvoltage threshold of electrons at the initial stage of streamer development in air, with the self-developed 3D particle-in-cell with Monte Carlo Collision code. First, numerical simulations are performed with fixed number of electrons to study the runaway criteria in nonrelativistic cases. This method takes the stochastic fluctuations of collisions into account and solves the major shortcomings of theoretical approach. The simulated critical electric field is less than that of the theoretical approach, and the amplitude of the difference increases with electron energy, due to the “tunneling effect” caused by the stochastic fluctuations of collisions. Then, simulations of negative streamers at various applied voltages are performed to investigate the overvoltage threshold. A more intuitive method, searching energetic electrons in front of the negative streamer head, which corresponds to the nature of runaway electrons, is applied to determine the generation of runaway electrons. Electrons that escaped a certain distance ahead of the streamer can be observed at 30 kV. Thus, the overvoltage threshold for runaway electrons can be roughly estimated as 3.3 in our simulations, which is about three times less than the previously published one. At last, with the redefined overvoltage threshold, the figure of regions of breakdown development for various mechanisms depending upon the overvoltage in air is updated.

Lightning Discharge as a Self-Organizing Transport Network. Part 1. The Concept of an Asymmetric Discharge Tree

Lightning is a self-developing transport system of plasma channels demonstrating the ability to self-regulation due to self-consistent maintenance of zero total charge of the entire branched discharge network. The lightning structural evolution is accompanied by significant morphological changes: new plasma channels appear, and some of old ones disappear. The accomplished work was in general aimed at studying intracloud lightning and cloud-to-ground discharges up to the moment the latter come in contact with the ground. It has been shown, as part of the study, that the ability of lightning, as an open system, to maintain its structural “homeostasis” is closely linked with macroscale asymmetry: the lightning morphological and transport properties are caused by loss of electric discharge tree structural symmetry when the discharge polarity changes for the opposite. Indeed, the asymmetry of electric field threshold strength levels needed to support the growth of positive and negative streamers leads to pronounced macroscale effects in the physics of lightning. Ground discharges of negative polarity consist most often of a series of strokes passing through the same channel, while positive flashes are as a rule limited to a single stroke. Negative and positive lightning leaders have significant morphological differences: the growth of positive leaders is accompanied by development of negative recoil leaders, whereas no one has observed positive recoil leaders, if they exist at all. The results of the study are presented in two parts. This article contains an introductory part that sets out the narration general context, and, by analogy with the hierarchical Horton-Strahler scheme used for river systems, analyzes the asymmetry of the spatial distribution of capacitance in different parts of the lightning discharge tree. The concept of a point of zero induced charge or a reversing point is discussed.

Numerical simulation on the behavior of a negative streamer encountered with a cloud of positive ions in atmospheric pressure plasma jet

Plasma jet normally operates in a regime of positive or negative streamers. Different propagating behaviors have been investigated for positive streamers. However, there are few studies on negative streamers. In this paper, a two-dimensional fluid model is used to simulate the behavior of a negative streamer encountered with a cloud of positive ions. Simulation results indicate that the negative streamer behaves like a free streamer when the ion cloud has a low charge density. After initiation near the needle tip, the streamer propagates along the axis, passes through the ion cloud, and finally reaches the anode vicinity along a straight line. When the streamer reaches the ion cloud, its head radius (R0) is investigated as a function of parameters, including the voltage amplitude and the gap width, as well as the charge density, axial position, and spatial size of the ion cloud. At a given moment, electric field strength and electron density near the streamer head increase with increasing charge density of the ion cloud. In the case of a high charge-density ion cloud, after a primary streamer propagates some distance along the axis, a positive secondary streamer, and a negative secondary streamer start to propagate from the cloud to the cathode and the anode, respectively.

Numerical modeling of different streamer breakdown modes of gas switch under the external circuit

Gas spark switches had been applied extensively in the field of pulse power technology on account of the advantages of a simple structure, short breakdown time, and high through-current capability. This paper investigates the streamer breakdown process in a two-electrode switch in a nitrogen environment under atmospheric pressure using a numerical method. First, a two-dimensional fluid simulation was carried out for the streamer generated under the negative and positive trigger voltages, and the difference in the breakdown mechanism in the two modes was analyzed. The study showed that when a negative trigger voltage is applied to the rod electrode, a negative streamer from the cathode to the anode will first be formed, and then a positive streamer from the anode to the cathode will be formed. When a positive trigger voltage is applied to the rod electrode, the streamer propagation mechanism is significantly different from that obtained when a negative voltage is applied. In this case, the breakdown mode is a cathode-directed streamer. Finally, we investigated the effect of circuit parameters on the breakdown process. The time scale of breakdown varies greatly with the capacitance. The significant difference in the breakdown process may come from the charging process of the capacitor. The larger the capacitance is, the slower the charging process will be.

Conditions for Inception of Relativistic Runaway Discharges in Air

AbstractTerrestrial gamma‐ray flashes are linked to growth of long bidirectional lightning leader system consisting of positive and stepping negative leaders. The spatial extent of streamer zones of a typical lightning leader with tip potential exceeding several tens of megavolts is on the order of 10–100 m. The photoelectric absorption of bremsstrahlung radiation generated by avalanching relativistic runaway electrons occurs efficiently on the same spatial scales. The intense multiplication of these electrons is triggered when the size of the negative leader streamer zone crosses a threshold of approximately 100 m (for sea‐level air pressure conditions) allowing self‐replication of these avalanches due to the upstream relativistic electron seeds generated by the photoelectric absorption. The model results also highlight importance of electrode effects in interpretation of X‐ray emissions from centimeter to meter long laboratory discharges, in particular, a similar feedback effect produced by generation of runaway electrons from the cathode material.

Impact of surface charges on energy deposition in surface dielectric barrier discharge: a modeling investigation

Surface charges have significant impact on the evolution of surface dielectric barrier discharge (SDBD). In this work, the role of residual surface charges on repetitively nanosecond pulsed SDBD in atmospheric air is investigated using a two-dimensional fluid model, based on the assumption of preserving the distribution of surface charges at the end of the previous high voltage (HV) pulse. In the bipolar mode when the polarity of residual surface charges is opposite to that of the current HV pulse, a lower breakdown voltage and more deposited energy can be observed, showing an obvious enhancement of SDBD. In the unipolar mode, residual surface charges suppress the development of discharges and energy deposition. It is found that more residual surface charges are accumulated during the negative pulsed discharge, which have a more pronounced effect on the subsequent positive pulsed one. This is explained by the fact that the negative surface streamers directly contact the dielectric and charge it, while the positive surface streamers float above the dielectric, forming a ion-rich region near the surface. The results in this work demonstrate the mechanism of how residual surface charges affect discharge dynamics, which can be utilized to regulate energy deposition in SDBDs.

A computational study on the energy efficiency of species production by single-pulse streamers in air

We study the energy efficiency of species production by streamer discharges with a single voltage pulse in atmospheric dry air, using a 2D axisymmetric fluid model. Sixty different positive streamers are simulated by varying the electrode geometry, the pulse duration and the applied voltage. Between these cases, the streamer radius and velocity vary by about an order of magnitude, but the variation in the maximal electric field is significantly smaller, about 30%. We find that G-values for the production of N(4S), O(3P), NO and N2O, which have relatively high activation energies, vary by about 30% to 60%. This variation is mainly caused by two factors: differences in the fraction of energy deposited in the streamer head region, and differences in the maximal electric field at the streamer head. When accounting for both factors, our computed G-values are in good agreement with an analytic estimate proposed by Naidis (2012 Plasma Sources Sci. Technol. 21 042001). We also simulate negative streamers and find that their production of N(4S), O(3P) and NO is less energy efficient. The results suggest that energy efficiency can be increased by reducing Joule heating in the streamer channel and by increasing the maximal electric field at the streamer head, for example by using short voltage pulses with a high applied voltage.

Discharge characteristics of positive and negative polarity streamers with submillimeter gap based on fractal theory

In order to study the characteristics of streamer discharge in sub-millimeter gap, an experimental system and an image acquisition system of positive and negative polarity streamer discharge in the gap of 100–500 μm at atmospheric pressure are built, and the initial breakdown voltages and breakdown images of positive and negative polarity streamer discharge in a gap range of 100–500 μm are collected, and the fractal dimension of the experimental discharge images is calculated by Matlab. The experimental analysis shows that the air breakdown discharge in the gap range of 100–500 μm still conforms to the stream theory, and the polarity effect between the breakdown voltage of positive and negative streamers and the fractal dimension of the discharge image is obvious. The influence of space charge and photoionization on the meandering degree and coverage of the discharge channel in stream theory increase with the increase of the gap, and jump at 300 μm. The fractal simulation of positive and negative streamer discharge with a gap of 500 μm is established by using WZ (Wiesmann-Zeller) model based on fractal theory in Matlab. It is found that the development probability index &lt;i&gt;η&lt;/i&gt; is inversely proportional to the fractal dimension, and that the &lt;i&gt;η&lt;/i&gt; is smaller in positive streamer than in negative streamer under the same discharge condition.

Production of runaway electrons and x-rays during streamer inception phase

Streamers play a key role in the formation and propagation of lightning channels. In nature streamers rarely appear alone. Their ensemble behavior is very complex and challenging to describe. For instance, the intricate dynamics within the streamer zone of negative lightning leaders give rise to space stems, which help advance the stepped-leader. Another example is how the increasing morphological complexity of sprites can lead to higher sprite current and greater energy deposition in the mesosphere. Insights into the complex dynamics of a streamer corona can be obtained from laboratory experiments that allow us to control the conditions of streamer formation. Based on simultaneous nanosecond-temporal-resolution photography, and measurements of voltage, current, and x-ray emissions, we report the characteristics of negative laboratory streamers in 88 kPa of atmosphere. The streamers are produced at peak voltages of 62.2 ± 3.8 kV in a point-to-plane discharge gap of 6 cm. While all discharges were driven to the same peak voltage, the discharges occurred at different stages of the relatively slow voltage rise (177 ns), allowing us to study discharge properties as a function of onset voltage. The onset voltage ranged between 24 and 67 kV, but x-ray emissions were observed to only occur above 53 kV, with x-ray burst energies scaling quadratically with voltage. The average delay between the current pulse and x-ray emission was found to be 3.5 ± 0.5 ns, indicating that runaway electrons are produced during the streamer inception phase or no later than the transition stage, when the inception cloud is breaking into streamer filaments. During this short time span, runaway electrons can traverse the gap, hit the ground plate and produce bremsstrahlung x-ray photons. However, streamers themselves cannot traverse more than 3.5 mm across the gap, which supports the idea that runaway electron production is not associated to streamer connection to the ground electrode.

Positive and negative streamers in air and nitrogen in a sharply inhomogeneous electric field under conditions of runaway electron generation

AbstractThe development of positive and negative streamers in a point‐to‐plane gap filled with air and nitrogen at various pressures (50–200 kPa) and voltages (8–25 kV) was studied. A four‐channel intensified charge‐coupled device (ICCD) and a streak‐camera were used. Electrical parameters were measured with high resolution (10 GHz). An original method of measuring a displacement current caused by a streamer was applied. As was expected, positive streamer branches in nitrogen at low voltages and/or elevated pressures while a large‐diameter streamer is formed in air. However, at high voltages or negative polarity, the large‐diameter streamer is formed both in nitrogen and air in the entire pressure range. It was found that runaway electrons (REs) are generated in the very first picoseconds of gas ionisation near the pointed cathode. It was assumed that REs can be generated near the pointed anode and produce bremsstrahlung radiation due to a sharply inhomogeneous distribution of electrical potential. It was found that in the final stage of negative streamer development in air and nitrogen, the gas between the streamer front and the opposite electrode is ionised almost simultaneously in the entire volume when the streamer diameter is almost equal to the interelectrode gap.

Studying the Features of Streamer Formation and Development in Air by Using Computation Experiments

The aim of the work is to estimate the relevance and current state of the problem of mathematical modeling of streamer discharges in air, as well as to identify the features pertinent to formation and initial development stage of two-head streamers in air by using computational experiments. The streamer discharge mathematical model developed at the National Research University Moscow Power Engineering Institute Department of High Voltage Engineering and Electrophysics was used as the research tool. It is shown that mathematical modeling of a streamer discharge can be effectively used in its studies and in training specialists in the field of electric power engineering and high-voltage electrical technologies. The article presents the results of computational experiments on studying the occurrence of streamers in a discharge gap filled with dry air with a uniform electric field (EF) under standard atmospheric conditions. The initial conditions were chosen so that a single-avalanche-streamer formation mechanism was realized. It is shown that in the considered range of initial conditions, the distance between the anode and initial inhomogeneity has a weak, within 1%, influence on the minimum and maximum values of electric field strength in the streamer. A different picture is observed on the anode surface. When a two-head streamer is formed in the discharge gap depth, the field strength at the anode remains close to the average strength in it. As the negative streamer head approaches the anode, the field strength on its surface increases to values close to those achieved in the positive head. After that, the field at the anode weakens rapidly to a level at which effective impact ionization becomes impossible. The article will be of interest for specialists and graduate students working in the field of electric power engineering and high-voltage electrical technologies.

Close View of the Lightning Attachment Process Unveils the Streamer Zone Fine Structure

AbstractA very close high‐speed video observation of lightning attachment to a building revealed novel details regarding the leader streamer zone dynamics. Upward leaders propagate in a steady and unbranched manner, displaying a uniformly luminous corona brush. The exception being the upward connecting leader (UCL) just before connection, when its streamer zone increases in size and develops a more filamentary pattern. Downward negative leaders have 3‐m long multiple streamers emanating from each negative leader tip. On some occasions, plasma formations known as space stems are seen to form in the location previously occupied by negative streamers. Space stems have luminosities comparable to the main leader channel, but are detached from it by 4 m. Some space stems display streamers of their own, including cases where streamers are emanating from both ends. The space stem formation hampered the propagation of the negative leader that was closest to the UCL.

Formation of wide negative streamers in air and helium: the role of fast electrons

Available experimental data show that the use of voltage pulses with subnanosecond rise times and amplitudes that essentially exceed the breakdown voltage leads to the formation of wide spherical or conical streamers. In this paper, the structure and dynamics of atmospheric pressure wide negative streamers in air and helium by applying high overvoltages with a short rise time to a sharp needle electrode are investigated experimentally and computationally. In the simulations, the two-dimensional fluid and kinetic electron Monte Carlo simulation models are used. All the streamers were simulated with the conventional photoionization term S ph that was never turned off. By including an additional source S MC, responsible for the generation of fast electrons, wide and diffuse streamers are obtained. We compare the shapes, width and velocities of conventional streamers in air and helium with those for streamers driven by fast electrons. We show that a conventional streamer in air has a cylindrical form. The conventional streamer in helium is wider than that in air and has a shape of an expanding cone. While accounting for fast electrons, different streamer shapes were obtained. In air, the gap was closed by a spherical streamer. In helium, the shape of a streamer resembles that of a pumpkin. We also demonstrate that near the flat anode, velocities of conventional streamers in air and helium were as high as 5 × 109 cm s−1 and reached values greater than 1010 cm s−1 when fast electrons were taken into account. By the application of high (by a factor of four or greater) overvoltages to a sharp needle electrode, the formation of a discharge with several parallel streamers was observed. In this regime, the trajectories of fast electrons originated not only from the cathode, but also from the region of a streamer front where the electric field is high. As a result, the so-called diffuse discharge was formed with high intensity plasma channels surrounded by an aureole of smaller electron density.

Transmission and multiple reflection mechanisms of guided streamers propagating through grounded annular electrode and interacting with grounded surface electrode

The repeatable dynamics and the reversal propagation of guided streamers remains a major question of fundamental physics. In this article, trains of positive guided streamers are generated within an atmospheric pressure plasma jet supplied in helium and polarized by a high-voltage nanosecond pulse generator. The device is completed by two distant targets: a grounded annular electrode (GAEL) coaxially centered around the capillary through which guided streamers can propagate, and a grounded surface electrode (GSEL) on which they can interact. The resulting transmitted and multiple reflected guided streamers are measured combining optical characterization (fast intensified charge-coupled device (ICCD) imaging) and electrical characterization (high voltage probe and current monitors). While the electrical approach provides information on the capacitive/conductive nature of the current peaks as well as on their positive/negative value, fast ICCD imaging distinguishes whether the guided streamers are incident, reflected or transmitted. Combining these two techniques allow us to demonstrate experimentally that the reflected streamers are negative contrarily to the others. Besides, four types of reflections have been highlighted: a reflection (r) at the outlet of the capillary, a reflection on the GSEL (R) and two reflections (r′ and r″) observed when an incident guided streamer passes through the GAEL. The two techniques agree that the characteristic propagation times are always shorter for reflected negative streamers than for the positive ones propagating forward. Hence, for a GAEL placed 3 cm away from the high voltage electrode, propagation time is 80 ns for reflection versus 250 ns for transmission. These characteristic propagation times are even shorter when the annular electrode is brought closer to the surface electrode with velocities typically higher than 300 km s−1. In addition, the intensity ratios of reflected/incident guided currents drop sharply, typically losing one decade over a counter-propagation length of only 3–5 cm. Finally, all these experimental data are utilized to build an equivalent electrical model that allow to better understand the dynamics of the guided streamers and explain their transmission and reflection modes upon their interaction with the two distant grounded electrodes.

Experimental investigation on streamer inception from artificial hydrometeors

In this study we use an experimental investigation to shed light on the lightning inception problem. From atmospheric observations, it is known that the electric fields in thunderclouds are significantly lower than required for electric breakdown in air. One theory to explain lightning inception is that hydrometeors, i.e. any liquid or solid water particles formed in the atmosphere, greatly enhance the local electric field and can thereby initiate an electron avalanche leading to a streamer discharge. In this study, we investigate streamer initiation in the presence of artificial particles with different shapes. A metal or dielectric (TiO2) particle is suspended between a high-voltage and a grounded planar electrode which are separated by 16 cm in 50 mbar air. The particles are shaped as ellipsoids with a length of 8, 4, 2, and 1 cm and with different aspect ratios. A negative high voltage pulse is applied with a rise time of 30 ns, a pulse width of 1–10 μs, a repetition rate of 1 Hz, and a maximum voltage between 1 and 50 kV. Results show that the required background electric field for breakdown in the presence of a dielectric particle is decreased to 0.4 times the air breakdown field. Moreover, we observed bipolar streamer development from the particles where negative streamers are thicker and slightly slower than positive streamers. Finally, we found that streamers from longer particles are thicker and faster.