Negative Streamers Research Articles

A two-dimensional air streamer discharge modified model based on artificial stability term under non-uniform electric field at low temperature and sub-atmospheric pressure

In this paper, an improved air discharge fluid model under non-uniform electric field is constructed based on the plasma module COMSOL Multiphysics with artificial stability term, and the boundary conditions developed in the previous paper are applied to the calculation of photoionization rate. Based on the modified model, the characteristics of low temperature sub-atmospheric air discharge under 13 kV direct current voltage are discussed, including needle-plate and needle-needle electrode structures. Firstly, in order to verify the reliability of the model, a numerical example and an experimental verification were carried out for the modified model respectively. Both verification results show that the model can ensure the accuracy and repeatability of the calculation. Secondly, according to the calculation results of the modified model, under the same voltage and spacing, the reduced electric field under low temperature sub-atmosphere pressure is larger than that under normal temperature and atmospheric pressure. The high electric field leads to the air discharge at low temperature and sub atmospheric pressure entering the streamer initiation stage earlier, and has a faster propagation speed in the streamer development stage, which shortens the overall discharge time. Finally, the discharge characteristics of the two electrode structures are compared, and it is found that the biggest difference between them is that there is a pre-ionization region near the cathode in the needle-needle electrode structure. When the pre-ionization level reaches 1013 cm−3, the propagation speed of the positive streamer remains unchanged throughout the discharge process, and is no longer affected by the negative streamer. The peak value of electric field decreases with the increase of pre-ionization level, and tends to be constant during streamer propagation. Based on the previous paper, this paper constructs the air discharge model under non-uniform electric field, complements with the previous paper, and forms a relatively complete set of air discharge simulation system under low temperature and sub atmospheric pressure, which provides a certain reference for future research.

On a possible mechanism of space stem formation at the negative corona streamer burst periphery

It's a common knowledge for spark discharge researches that negative leaders in air always propagate in a step-wise manner. A new step forms when a space leader attaches to the primary negative one. Space leaders originate from small fusiform plasma formations which are called space stems. For now, there is no any established idea about how space stems form in conditions when the background electric field inside the negative corona volume is about three times less than the dielectric strength of air. In this study, we propose a new mechanism of space stem precursors formation which is based on the joint action of ionization and electron drifting processes occurring at the negative corona streamer burst boundary in the presence of strongly inhomogeneous stochastic electric field. It is supposed that electric field fluctuations are formed by chaotically positioned clusters of negative charge which can be considered as basic elements of spatio-temporal noise and are transported into the negative corona volume by negative streamer heads. The last are emanated from the newly-formed leader tip during the development of the negative corona streamer burst finishing the step-formation process. The model is applied to specify conditions under which space stem precursor genesis is possible and to analyze the peculiarities of proposed mechanism at different altitudes above sea level.

Generation of runaway electrons in plasma after a breakdown of a gap with a sharply non-uniform electric field strength distribution

The paper is devoted to the study of the initiation and formation of a negative streamer in a sharply inhomogeneous electric field and the generation of runaway electrons (REs) in air and helium at atmospheric pressure and below, as well as in sulfur hexafluoride at low pressure. Nanosecond voltage pulses of negative polarity with an amplitude of 18 kV were applied across a point-to-plane gap 8.5 mm long. The studies were carried out using broadband measuring sensors and equipment with picosecond time resolution, as well as using a four-channel ICCD camera. Using a special method for measuring the dynamic displacement current caused by the redistribution of the electric field during streamer formation, the waveforms of voltage, discharge current, RE current, and dynamic displacement current were synchronized to each other, as well as to ICCD images. Data on the generation of REs with respect to the dynamics of streamer formation were obtained. It was found that REs are generated not only during the breakdown of the gap, but also after that. It has been found that the formation time of explosive emission centers affects the generation of REs after breakdown. Based on the measurement data of the voltage, discharge current, and dynamic displacement current, the electron concentration in the plasma channel after breakdown and the electric field strength near the surface of the grounded electrode were calculated.

First‐mode of negative streamers: Inception at liquid/solid interfaces

First‐mode of negative streamers: Inception at liquid/solid interfaces

Lightning Initiation From Fast Negative Breakdown is Led by Positive Polarity Dominated Streamers

AbstractEvidence of positive polarity dominated streamers preceding fast negative breakdown (FNB) and of simultaneous positive and negative polarity streamer development in lightning initiation is reported. Observations of lightning initiation as FNB have remained a puzzle because simulations of lightning initiation have shown that negative streamers are not produced in virgin air without simultaneous positive streamers. Here, the authors observe positive streamer development forms first or at least simultaneously with negative streamers. Further evidence comes from observations of mixed fast breakdown (FB). The overall trajectory of the positive breakdown during such mixed events indicates that the positive streamers continuously propagate during the burst of strong negative breakdown. These observations indicate that even when negative streamers dominate the overall very high frequency (VHF) emissions, both positive and negative streamers are propagating simultaneously from the initiation point. These findings on the structure and dynamics of FB provide key new insight to our understanding of lightning initiation.

Inside Front Cover: Plasma Process. Polym. 4/2021

Inside Front Cover: The interaction of plasma streamers with material surfaces made of different structures is of outmost importance for various applications. However, the discharge characteristics are found to be completely different for positive and negative streamers. Negative surface streamer develops along the dielectric surface, while the positive surface streamer floatingly develops above the dielectric; Comparing to positive streamer, negative streamer can propagate into much smaller dielectric pores. Further details can be found in the article by Quan-Zhi Zhang, Li Zhang, De-Zheng Yang, Julian Schulze, You-Nian Wang, and Annemie Bogaerts (e2000234).

High field conduction in mineral oil based ZnO nanofluids prior to negative streamer inception

The electric conduction under intense electric fields (up to ∼ 109 V/m) in nanofluids using surface-modified ZnO–C18 nanoparticles dispersed in mineral oil as host, is investigated with both experiments and numerical simulations. The measurements are used to estimate unknown parameters necessary to represent the generation and loss of electrons in an electrohydrodynamic model for mineral oil with and without ZnO–C18 nanoparticles in a needle-plane configuration. The model suggests that ZnO–C18 nanoparticles induce an enhanced field emission from negative needles, explaining the significantly larger conduction currents measured in the nanofluid compared with those in the host liquid. It is also found that the scavenging of electrons by ZnO–C18 nanoparticles is a process which is negligible compared with the loss of electrons due to attachment in mineral oil. It is shown that ZnO–C18 nanoparticles hinder the streamer initiation process by reducing the effective electric field at the tip of the needle. This electric field reduction is caused by the combined effect of enhanced electron injection through ZnO–C18 nanoparticles and strong electron attachment in mineral oil. Thus, the electric field on the needle tip reaches the same threshold value when the streamer is incepted in the nanofluid as in mineral oil, although at a larger voltage. Solid evidence indicating that the additional electron scavenging and the reduced electron mobility introduced by nanoparticles has no effect in the conduction currents and in the negative streamer inception in the tested ZnO–C18 nanofluids is shown.

Electronegative microchannel guided streamer propagation for in-liquid spark breakdown applications

One of the well-known challenging issues of in-liquid spark breakdown is electrode wear and wear-dependent deposit energy fluctuation, regardless of the electrode materials. This work suggests a method that can reduce the breakdown threshold by an order of magnitude and hence enhances the likelihood of breakdown, regardless of wear. Generally, the negative streamer propagates in a branching way; however, the present experiment indicates that the electronegative microchannel is converged with the streamer propagation and extends the breakdown gap distance between the electrodes. Subsequently, the breakdown-possible gap distance was extended by 14.3 times, leading to an enhancement of shockwave intensity by 33%. Such an extension of the breakdown conditions was achievable without any additional source energy input or changing the substance of dielectric electrodes. Thus, the results provide a favorable scheme for energy reduction in high-voltage systems, cost saving for electrode replacement, and enhancement of the propagating shock pressure.

Positive and negative streamer propagation in volume dielectric barrier discharges with planar and porous electrodes

AbstractThe spatiotemporal dynamics of volume and surface positive and negative streamers in a pin‐to‐plate volume dielectric barrier discharge is investigated in this study. The discharge characteristics are found to be completely different for positive and negative streamers. First, the spatial propagation of a positive streamer is found to rely on electron avalanches caused by photoelectrons in front of the streamer head, whereas this is not the case for negative streamers. Second, our simulations reveal an interesting phenomenon of floating positive surface discharges, which develop when a positive streamer reaches a dielectric wall and which explain the experimentally observed branching characteristics. Third, we report for the first time, the interactions between a positive streamer and dielectric pores, in which both the pore diameter and depth affect the evolution of a positive streamer.

Simulation of decelerating streamers in inhomogeneous atmosphere with implications for runaway electron generation

The dynamics of positive and negative streamers is numerically simulated in atmospheric pressure air in the range of parameters corresponding to the streamer deceleration and termination in the middle of the discharge gap. A detailed comparison with experiments in air at constant and variable density demonstrates good agreement between the 2D simulation results and the observations. It is shown that positive and negative streamers behave in radically different ways when decelerating and stopping. When the head potential drops, the negative streamer transits to the mode in which the propagation is due to the forward electron drift. In this case, the radius of the ionization wave front increases, whereas the electric field at the streamer head decreases further and the streamer stops. Its head diameter continues to increase due to the slow drift of free electrons in the residual under-breakdown field. On the contrary, the only advancement mechanism for a positive streamer with a decreasing head potential is a decrease in the effective radius of the ionization wave, leading to a local increase in the electric field. This mechanism makes it possible to compensate for the reduction in the efficiency of gas photoionization at small head diameters. A qualitative 1D model is suggested to describe streamer deceleration and stopping for different discharge polarities. Estimates show that, during positive streamer stopping, the local electric field at the streamer head can exceed the threshold corresponding to the transition of electrons to the runaway mode when the head potential (relative to the surrounding space) decreases to ∼1.2 kV in atmospheric pressure air. In this case, pulsed generation of a beam of runaway electrons directed into the channel of a stopping positive streamer can occur. The energy of the formed pulsed electron beam depends on the intensity of photoionization in front of the streamer head. This energy can vary from 700 V (when increasing the photoionization rate by a factor of 10 with respect to the value in atmospheric pressure air) to 2.6 kV (when decreasing the photoionization rate by a factor of 1000). It is possible that this behavior of decelerating positive streamers can explain the observed bursts of x-ray radiation during the streamer propagation in long air gaps.

A Two-Dimensional Air Discharge Modified Model Under Unipolar Square Pulse Voltage at Low Temperature and Sub-Atmospheric Pressure

In this paper, a modified COMSOL Multiphysics model with artificial stability is adopted, and the boundary condition developed in the previous paper are applied to the calculation of the photoionization rate. The main processes involved in air discharge were simulated by 39 plasma chemical reactions composed of 12 kinds of particles. The characteristics of low-temperature sub-atmospheric pressure air discharge under unipolar square wave pulse voltage with 13kV applied amplitude, 25 kHz-50 kHz frequency and a duty cycle of 50%-75% are discussed. The results show that: When the duty cycle increases, the average density of electron, N2+, N4+, O2+, O4+, and N2O2+ show an increasing trend, and the average electron temperature changes little; When the frequency increases, the average electron density, the average density of N4+ and O4+ show a downward trend, the average density of N2+, O2+, N2O2+ change little; The average electron temperature shows an increasing trend, but the duration of high electron temperature becomes shorter. When discussing a single period, it is found that the potential difference between the electrode and the plasma determines the direction of the electric field in the space. For the photoionization reaction, even if its influence on the negative streamer is not important, it still needs to be considered in the simulation. The effect of temperature on discharge is also taken as the research emphasis, it includes the following aspects: Firstly, at low temperature, the collision reaction and attachment reaction rates are increased, while the recombination reaction rate is almost unaffected; Secondly, low temperature results in the decrease of secondary electron emission coefficient, which indirectly increases the electric field gradient and the peak value of electric field; Finally, at low temperature, the mobility of carriers, including electrons, positive and negative ions, is increased, but the streamer velocity is decreased.

Observations of the Origin of Downward Terrestrial Gamma‐Ray Flashes

AbstractIn this paper we report the first close, high‐resolution observations of downward‐directed terrestrial gamma‐ray flashes (TGFs) detected by the large‐area Telescope Array cosmic ray observatory, obtained in conjunction with broadband VHF interferometer and fast electric field change measurements of the parent discharge. The results show that the TGFs occur during strong initial breakdown pulses (IBPs) in the first few milliseconds of negative cloud‐to‐ground and low‐altitude intracloud flashes and that the IBPs are produced by a newly identified streamer‐based discharge process called fast negative breakdown. The observations indicate the relativistic runaway electron avalanches (RREAs) responsible for producing the TGFs are initiated by embedded spark‐like transient conducting events (TCEs) within the fast streamer system and potentially also by individual fast streamers themselves. The TCEs are inferred to be the cause of impulsive sub‐pulses that are characteristic features of classic IBP sferics. Additional development of the avalanches would be facilitated by the enhanced electric field ahead of the advancing front of the fast negative breakdown. In addition to showing the nature of IBPs and their enigmatic sub‐pulses, the observations also provide a possible explanation for the unsolved question of how the streamer to leader transition occurs during the initial negative breakdown, namely, as a result of strong currents flowing in the final stage of successive IBPs, extending backward through both the IBP itself and the negative streamer breakdown preceding the IBP.

Corona with Streamers in Atmospheric Pressure Air in a Highly Inhomogeneous Electric Field

The paper presents research data on positive and negative coronas in atmospheric pressure air in a highly inhomogeneous electric field. The data show that irrespective of the polarity of pointed electrodes placed in a high electric field (200 kV/cm), this type of discharge develops via ball streamers even if the gap voltage rises slowly (0.2 kV/ms). The start voltage of first positive streamers, compared to negative ones, is higher and the amplitude and the frequency of their current pulses are much lower: about two times and more than two orders of magnitude, respectively. The higher frequency of current pulses from negative streamers provides higher average currents and larger luminous areas of negative coronas compared to positive ones. Positive and negative cylindrical streamers from a pointed to a plane electrode are detected and successive discharge transitions at both polarities are identified.

Schlieren Images of Negative Streamer and Leader Formations in CO2 and a CF3I-CO2 Electronegative Gas Mixture

In electrical networks, SF6 gas is currently used to insulate high-voltage equipment, however, due to its high environmental impact, new alternatives such as CO2 and electronegative gas mixtures such as CF3I-CO2 are being trialled to replace SF6 and create a sustainable energy system. A high-voltage lightning impulse (1.2 μs/50 μs) was used to approximate the disturbance in a high-voltage electrical network caused by a lightning strike and helped to identify the likely streamer and leader formations in different gases insulating a piece of gas insulated switchgear. In this paper, the theoretical and practical aspects of electrical streamer and leader formations in pure CO2 and an electronegative gas mixture of CF3I-CO2 are examined using schlieren videography in small length rod-plane gas gaps between 20 and 50 mm in length. Schlieren allows the examination of gas density in streamer formations and for the differences in weakly attaching gases, such as CO2, and electronegative gas mixtures, such as CF3I-CO2, to be studied. The gas pressure is varied in order to examine the differences in streamer and leader formation as the gas density is varied and hence the probability of electron collision is varied.

Pressure-dependent propagation of streamers under step voltage in a long point-plane gap in transformer oil

The effect of pressure on positive and negative streamer propagation in a mineral transformer oil is studied in an 80 mm point-plane gap under step voltage. Increased pressure causes reduced stopping length for non-breakdown streamers, higher breakdown voltage, reduced background current, reduced branching in positive streamers and virtually no change in negative streamer shape. The velocities and acceleration voltages are close to being independent of pressure, indicating that the velocity controlling mechanisms take place in the liquid phase. Amplitude and frequency of small current pulses (probably small reilluminations) during negative 1st mode is reduced with increasing pressure, but the effect on reilluminations during 2nd and 3rd mode is small in both polarities.

Numerical simulations of nanosecond discharge in gas-dynamic flows

Numerical simulations were performed of the nanosecond high-voltage breakdown in nitrogen at a voltage of 25–35 kV in the sub-centimeter discharge gaps. The streamer configurations were studied at two constant pressures, as well as at the radially varying pressure, simulating the pressure distribution in the axial region of the vortex “columnar” gas-dynamic flow. In each case, the characteristic parameters and propagation velocities of the primary and secondary streamers were calculated. It was ascertained that the law of pressure change in the radial direction strongly affects the spatial structure of the negative streamer.

Spatiotemporal analysis of streamer discharge in a wire-to-wire reactor with positive nanosecond pulse supply

The spatial and temporal distribution of the discharge streamers in positive pulsed wire-to-wire electrode configuration in atmospheric air is investigated by an electrical-optical diagnostic system. Time-resolved ICCD images show that the discharge streamers in wire-to-wire electrode develop in three phases: the primary streamer, the secondary positive streamer, and the secondary negative streamer. It is observed that the evolution of discharge streamers is strongly influenced by the amplitude of the applied voltage. The optical emission spectroscopy measurement of hydroxyl radical OH indicates that the OH is mainly generated in the secondary positive streamer near the anode region. But in the region near the cathode the emission of OH radicals can also be detected due to the secondary negative streamer. The influences of rise time, fall time and pulse duration on streamer dynamics and the subsequent radical production are observed. It is shown that the average propagation velocity of the primary streamer decreases with the increase of the rise time, while the variation of pulse width and pulse duration parameters have little effect on that of the primary streamer. The response surface methodology based on Box–Behnken design model is implemented to evaluate the contribution of the three critical pulse parameters on ozone production. The results of the response surface quadratic model show that the pulse rise time plays the most prominent role in the generation of ozone among the three pulse parameters of rise time, fall time and pulse duration.

Double-propagation mode in short-gap spark discharges driven by HV pulses with sub-ns rise time

The object of this study is the investigation of spark discharges ignited by unipolar positive rectangular high voltage (HV) pulses with 200 ps rise time and (15 ± 2) kV amplitude with 3 ns duration full width at half maximum in synthetic air in a 1.2 mm pin-to-pin gap (tungsten electrodes) at atmospheric pressure. The discharge development was recorded by synchronised iCCD and streak camera measurements in single-shot operation, revealing a two-stage propagation mode. The discharge started with a fast initial breakdown across the entire gap (∼10 mm ns−1) during the HV slope, followed by a much slower (∼0.1 mm ns−1) propagation originating from both electrodes towards the gap centre. The combination of high-resolution diagnostics with numerical modelling indicated that the initial breakdown phase is caused by the rapid increase of electric field strength during the steep HV slope, which leads to the simultaneous fast propagation of a positive and a negative streamer.

Negative Streamer Initiation From an Isolated Hydrometeor in a Subbreakdown Electric Field

AbstractPlasma discharge fluid simulations indicate a negative streamer can be initiated from the narrow tip of a cone‐shaped, realistically sized hydrometeor in a uniform electric field as low as 0.65Ek. A negative streamer, however, cannot be formed from a cone‐shaped hydrometeor with a slightly smaller base radius or a column hydrometeor under otherwise the same conditions. The simulation also shows the negative streamer is accompanied by a positive streamer. The results present implications for understanding lightning initiation and the physical mechanism of the recently discovered thunderstorm‐related phenomenon called fast negative breakdown. Finally, the initiated negative streamer develops with a relatively constant head radius and velocity in contrast with the expanding and accelerating positive streamer. It appears that the differences between the properties of each polarity of streamer are more pronounced in an electric field below Ek.

How pulse polarity and photoionization control streamer discharge development in long air gaps

A 2D approach is used to simulate the properties of positive and negative streamers emerging from a high-voltage electrode in a long (14 cm) air gap for standard pressure and temperature. The applied voltage varies from 100 to 500 kV. To reveal the influence of photoionization, the calculations are made for various rates of seed electron generation in front of the streamer head. The difference between the properties of positive and negative streamers is associated with the different directions of the electron drift ahead of the streamer head. As a result, the peak electric field at the streamer head and the streamer velocity are higher for positive voltage polarity. The average electric field in the negative streamer channel is approximately twice that in the positive streamer channel, in agreement with available measurements in long air gaps. It is shown that photoionization in front of the streamer head is important not only for the development of strong positive discharges, but for the development of strong negative discharges as well. An increase in the photoionization rate increases the propagation velocity of the positive streamer and retards the propagation of the negative streamer.