Effects of rod radius and voltage on streamer discharge in a short air gap

Streamer discharge is the inaugural stage of gas discharge,and the average electron energy directly determines the electron collision reaction rate,which is a key parameter for studying streamer discharge.Therefore,taking into account the average electron energy,this work establishes a fluid chemical reaction model to simulate and study the course of evolution of a streamer discharge in a 5 mm rod–plate gap,considering 12 particles and 27 chemical reactions.It introduces the electron energy drift diffusion equation into the control equation,and analyzes the temporal and spatial changes of average electron energy,electric field intensity and electron density with change in rod radius and voltage.The effects of voltage and rod radius on the course of streamer discharge can be reflected more comprehensively by combining the average electron energies.Three different values of 0.3 mm,0.4 mm and 0.5 mm are set for the rod radius,and three different values of 5 k V,6 k V and 7 k V are set for the voltage.The influence of an excitation reaction on the streamer discharge is studied.The findings indicate that,as voltage raises,the streamer head’s electron density,electric field and average electron energy all rise,and the streamer develops more quickly.When the rod radius increases,the electron density,electric field and average electron energy of the streamer head all decrease,and the streamer’s evolution slows down.When an excitation reaction is added to the model,the average electron energy,the magnitude of the electric field and the density of electrons decrease,and the evolution of the streamer slows down.An increase in average electron energy will lead to an increase in electric field strength and electron density,and the development of the streamer will be faster.

Local electron mean energy profile of positive primary streamer discharge with pin-plate electrodes in oxygen nitrogen mixtures

Local electron mean energy （LEME） has a direct effect on the rates of collisional ionization of molecules and atoms by electrons. Electron-impact ionization plays an important role and is the main process for the production of charged particles in a primary streamer discharge. Detailed research on the LEME profile in a primary streamer discharge is extremely important for a comprehensive understanding of the local physical mechanism of a streamer. In this study, the LEME profile of the primary streamer discharge in oxygen-nitrogen mixtures with a pin-plate gap of 0.5 cm under an impulse voltage is investigated using a fluid model. The fluid model includes the electron mean energy density equation, as well as continuity equations for electrons and ions and Poisson＇s electric field equation. The study finds that, except in the initial stage of the primary streamer, the LEME in the primary streamer tip tends to increase as the oxygen-nitrogen mole ratio increases and the pressure decreases. When the primary streamer bridges the gap, the LEME in the primary streamer channel is smaller than the first ionization energies of oxygen and nitrogen. The LEME in the primary streamer channel then decreases as the oxygen-nitrogen mole ratio increases and the pressure increases. The LEME in the primary streamer tip is primarily dependent on the reduced electric field with mole ratios of oxygen-nitrogen given in the oxygen-nitrogen mixtures.

Synergistic Decolouration of Azo Dye by Pulsed Streamer Discharge Immobilized TiO_2 Photocatalysis

Photocatalyst was prepared by immobilizing TiO2 on glass beads using the traditional sol-gel method. Ultraviolet light （UV） produced by pulsed streamer discharge was then used to induce photocatalytic activity of TiO2 photocatalyst. Decolouration efficiency of the representative azo dye （acid orange 7, AOT） was investigated using the synergistic system of pulsed streamer discharge plasma and TiO2 photocatalysis. The obtained results showed that the decolouration rate of AO7 could be increased by 16.7% under the condition of adding supported TiO2 in the pulsed streamer discharge system, compared to that in the sole pulsed streamer discharge plasma system, due to the synergistic effect of pulsed streamer discharge and TiO2 photocatalysis induced by pulsed streamer discharge. The synergistic system of pulsed streamer discharge and TiO2 photocatalyst was found to have more reactive radicals for degradation of organic compounds in water.

Abnormal Polarity Effects of Streamer Discharge in Propylene Carbonate under Microsecond Pulses

Propylene carbonate （PC） has a great potential to be used as an energy storage medium in the compact pulsed power sources due to its high dielectric constant and large resistivity. We investigate both the positive and negative breakdown characteristics of PC. The streamer patterns are obtained by ultra-high-speed cameras. The experimental results show that the positive breakdown voltage of PC is about 135% higher than the negative one, which is abnormal compared with the common liquid. The shape of the positive streamer is filamentary and branchy, while the negative streamer is tree-like and less branched. According to these experimental results, a charge layer structure model at the interface between the metal electrode and liquid is presented. It is suggested that the abnormal polarity effect basically arises from the electric field strength difference in the interface between both electrodes and PC. What is more, the recombination radiation and photoionization also play an important role in the whole discharge process.

A Repetitive Nanosecond Pulse Source for Generation of Large Volume Streamer Discharge

Using a unipolar pulse with the rise time and the pulse duration in the order of microsecond as the primary pulse, a nanosecond pulse with the repetitive frequency of several kilohertz is generated by a spark gap switch. By varying both the inter-pulse duration and the pulse frequency, the voltage recovery rate of the spark gap switch is investigated at different working conditions such as the gas pressure, the gas composition as well as the bias voltage. The results reveal that either increase in gas pressure or addition of SF6 to the air can increase the voltage recovery rate. The effect of gas composition on the voltage recovery rate is discussed based on the transferring and distribution of the residual space charges. The repetitive nanosecond pulse source is also applied to the generation of large volume, and the discharge currents are measured to investigate the effect of pulse repetition rate on the large volume streamer discharge.

Characteristics of long-gap AC streamer discharges under low pressure conditions

The generation and propagation of a streamer is a significant physical process of air gap discharge. Research on the mechanism of streamers under low-pressure conditions is helpful for understanding the process of long-gap discharge in a high-altitude area. This paper describes laboratory investigations of streamer discharge under alternating current（AC） voltage in a low pressure test platform for a 60 cm rod–plane gap at 30 kPa, and analyzes the characteristics of streamer generation and propagation. The results show that the partial streamer and breakdown streamer all occur in the positive half-cycle of AC voltage near the peak voltage at 30 kPa. The partial streamer could cause the distortion of current and voltage waveform, and it appears as the branching characteristic at the initial stage. With the extension of the streamer, the branching and tortuosity phenomena become gradually obvious, but the branching is suppressed when the streamer crosses the gap. The low-pressure condition has little influence on the tortuosity length and the tortuosity number of the streamer, but affect the diameter of streamer obviously.

Electronic Excitation Temperature in DC Positive Streamer Discharge

The electronic excitation temperature in a direct current positive streamer discharge based on ultra-thin sheet electrodes was measured by optical emission spectrometry in order to deposit materials for potential future applications. It was remarkable that the electronic excitation temperature （Text） did not vary monotonically with the discharge current, but demonstrated a peak at a certain position. In a mixture of oxygen and argon （80% oxygen）, the maximum Texc reached about 6300 K at an average current of 600 pA. Both the positive ions accumulation in the discharge region and the increase of the local temperature around the streamer channel caused by Joule heating are considered to be the main reasons for the variations of Texc.

A Positivity-Preserving Scheme for the Simulation of Streamer Discharges in Non-Attaching and Attaching Gases

Assumed having axial symmetry,the streamer discharge is often described by a fluid model in cylindrical coordinate system,which consists of convection dominated(diffusion)equations with source terms,coupled with a Poisson’s equation.Without additional care for a stricter CFL condition or special treatment to the negative source term,popular methods used in streamer discharge simulations,e.g.,FEMFCT,FVM,cannot ensure the positivity of the particle densities for the cases in attaching gases.By introducing the positivity-preserving limiter proposed by Zhang and Shu[15]and Strang operator splitting,this paper proposes a finite difference scheme with a provable positivity-preserving property in cylindrical coordinate system,for the numerical simulation of streamer discharges in non-attaching and attaching gases.Numerical examples in non-attaching gas(N_(2))and attaching gas(SF_(6))are given to illustrate the effectiveness of the scheme.

NO conversion by positive streamer discharge——effects of gas compositions and reaction conditions

The effects of gas compositions and reaction conditions on NO conversion by positive streamer discharge were experimentally investigated by using a link tooth wheel-cylinder reactor.The results showed that NO conversion increased with increasing O_(2) concentration and NH3 concen-tration,but decreased with increasing inlet NO concentration and gas flow rate.The addition of CO_(2) or H_(2)O to the feed gas promoted NO conversion by increasing the maximum discharge voltage,and NH4NO3 was formed in the presence of NH_(3).There was a most suitable range interval between discharge tooth wheels if both NO conversion and energy consumption were considered.Increasing applied voltage resulted in the increase in the amount of O_(3) generated by streamer discharge.

Dynamic Characteristics of SF_6-N_2-CO_2 Gas Mixtures in DC Discharge Process

Dynamic characteristics of discharge particles are described within the framework of a two-dimensional photoionization-hydrodynamic numerical model for the discharge process of SF6-N2-CO2 gas mixtures at atmospheric pressure, under a uniform DC applied field. The finite difference flux corrected transport （FD-FCT） algorithm is used in the numerical implementation for improving the accuracy and efficiency. Then the tempo-spatial distributions of the gap space electric field and electron velocity are calculated from the microscopic mechanism, and the dynamic behaviors of charged particles are obtained in detail. Meanwhile, the tempo-spatial critical point of the avalanche-to-streamer in this model is discovered, and several microscopic parameters are also investigated. The results showed that the entire gap discharge process can be divided into two phases of avalanche and streamer according to Raether-Meek criterion; the electron density within the discharge channel is lower compared to that of positive and negative ions; space charge effect is a dominant factor for the distortion of spatial electric field, making the discharge channel expand toward both electrodes faster; photoionization provides seed electrons for a secondary electron avalanche, promoting the formation and development speed of the streamer.

On-Line Measurement of Ion Density in Atmospheric Nitrogen Discharge Filaments via Radiation Signals from Plasma Oscillation

Diagnosis of the particle number density of plasma plays an important role in the understanding of plasma sources and processing.Regular radiation signals from plasma oscillation in filaments of atmospheric nitrogen discharge,which were excited by the injection of secondary electron beams during the propagation of the streamer,are employed to determine the ion density of plasma and its evolution in the filaments.Results show that the density of N4＋ in a filament of atmospheric nitrogen discharge is of the order of 1013 cm-3.It is also found that the recombination processes play a dominant role in plasma decay,and that the ion density decreases non-monotonically with time during streamer propagation.

Filament Discharge Phenomena in Fingerprint Acquisition by Dielectric Barrier Discharge

In this paper, the dielectric barrier discharge fingerprint acquisition technique is introduced. The filament discharge phenomena were observed in the process of fingerprint acquisition. The filament discharge reduced the quality of fingerprint images. Obviously, it was necessary to eliminate streamer discharges in order to get good fingerprint images. The streamer discharge was considered to be the cause of the filament discharge in the experiment. The relationship between the critical electric field and the discharge gap was calculated with the Raether＇s model of streamer discharge. The calculated results and our experiment proved that it would be difficult for the streamer discharge to occur when the discharge gap was narrow. With a narrow discharge gap, the discharge was homogeneous, and the fingerprint images were clear and large in area. The images obtained in the experiment are very suitable for fingerprint identification as they contain more information.

Plasma propagation in single-particle packed dielectric barrier discharges:joint effects of particle shape and discharge gap

In this work,a single Al_(2)O_(3) particle packed dielectric barrier discharge(DBD)reactor with adjustable discharge gap is built,and the influences of the particle shape(ball and column)and the residual gap between the top electrode and particle on the electrical and optical characteristics of plasma are studied.Our research confirms that streamer discharge and surface discharge are the two main discharge patterns in the single-particle packed DBD reactor.The strong electric field distortion at the top of the ball or column caused by the dielectric polarization effect is an important reason for the formation of streamer discharge.The length of streamer discharge is proportional to the size of the residual gap,but the number of discharge times of a single voltage cycle shows an opposite trend.Compared to the column,a smooth spherical surface is more conducive to the formation of large and uniform surface discharges.The surface discharge area and the discharge intensity reach a maximum when the gap is equal to the diameter of the ball.All in all,the results of this study will provide important theoretical support for the establishment of the synergistic characteristics of discharge and catalysis in plasma catalysis.

全尺寸交流500 kV输电杆塔复合横担的雷电闪络特性

架空输电线路(OTL)一直以来都是电力传输的主要方式。随着输电电压和输电容量的迅速提高,输电杆塔的尺寸也随之增加,造成土地资源的大量占用。考虑到线路复合绝缘子的优异性能,复合横担有望成为这一问题的解决方案。在本文中,作者制备了一个安装有复合横担的全尺寸交流(AC)500 kV输电杆塔,并施加了不同极性的雷电过电压。借助高速相机记录了流注-先导放电的过程,确认了闪络的主要路径。通过测量闪络电压并根据空气密度和湿度对结果进行修正,证实了放电的极性效应。继而,根据杆塔结构和闪络特性计算了杆塔的耐雷水平。基于全尺寸试验的结果与分析,确认了复合横担的可行性,并提出了结构优化方案。

Experimental Simulation of Red Sprites in a Laboratory

Over the past three decades,research of high-altitude atmospheric discharges has received a lot of attention.This paper presents the results of experimental modeling of red sprites during a discharge in low-pressure air.To initiate ionization waves in a quartz tube,an electrodeless pulse-periodic discharge fed by microsecond voltage pulses with an amplitude of a few kilovolts and a repetition rate of tens of kHz were formed.In this case ionization waves(streamers)have a length of tens of centimeters.The main plasma parameters were measured at various distances along the tube.The measurements confirm the fact that ionization waves propagate in opposite directions from the zone of the main electrodeless discharge,just as it happens during the formation of red sprites.