Calculation of time dependence of the electrode surface temperature in the cathode spot of atmospheric pressure normal glow discharge

Non-stationary heat transfer in the volume of the electrode with the cathode spot of atmospheric pressure normal glow discharge on its surface is calculated in case the spot radius is much less than the electrode dimensions. A system of equations describing variation of the temperature in the spot and the discharge cathode sheath parameters after discharge ignition is formulated, and the time dependence of the electrode temperature in the spot for discharge in helium is found. It is shown that within a short time fast heating of the electrode surface in the spot proceeds and then its temperature is slowly approaching the equilibrium value.     

Influence of the electrode dimensions on its surface temperature in the cathode spot of normal glow discharge

The temperature distribution in a cylindrical electrode with the glow discharge cathode spot on its end is calculated, and an equation for the electrode surface average temperature in the spot is obtained taking into account the influence of gas heating near the spot upon its size. It is shown that, for the electrode the length of which is equal to the radius, the difference of temperature values found from this equation and under the assumption of infinite electrode dimensions is small, whereas for an electrode with the length substantially differing from the radius it can be quite large.     

Transition of a diffuse discharge to a spark at nanosecond breakdown of high-pressure nitrogen and air in a nonuniform electric field

The transition of a runaway-electron-induced diffuse discharge initiated in a nonuniform electric field under a high pressure of air and nitrogen to a spark is studied. High-voltage pulses with a rise time of 0.5 ns are applied to a discharge gap with a tubular cathode having a small radius of curvature. It is shown that the leader of the spark discharge propagates toward the tubular cathode along preproduced tracks and may pass from one track to another. For a pulse rise time of about 0.5 ns and a gap length of 12 mm or more, it is found that spark leaders originating at the cathode (which has a small radius of curvature) do not reach the anode and accordingly, do not cause the spark breakdown of the gap. It is confirmed that the spark breakdown of the gap is associated with a spark leader that moves away from the plane electrode after the appearance of a bright spot on it.     

Calculation of the electrode surface temperature in the cathode spot of normal glow discharge at atmospheric pressure

The system of equations relating the electrode surface temperature in the cathode spot of normal glow discharge at atmospheric pressure to the discharge cathode sheath parameters is formulated. An analytical expression for the cathode surface temperature in the spot has been obtained, and the parameters of the spot and discharge cathode sheath in helium have been calculated.     

Numerical study on the discharge characteristics and nonlinear behaviors of atmospheric pressure coaxial electrode dielectric barrier discharges

The discharge characteristics and temporal nonlinear behaviors of the atmospheric pressure coaxial electrode dielectric barrier discharges are studied by using a one-dimensional fluid model. It is shown that the discharge is always asymmetrical between the positive pulses and negative pulses. The gas gap severely affects this asymmetry. But it is hard to acquire a symmetrical discharge by changing the gas gap. This asymmetry is proportional to the asymmetric extent of electrode structure, namely the ratio of the outer electrode radius to the inner electrode radius. When this ratio is close to unity, a symmetrical discharge can be obtained. With the increase of frequency, the discharge can exhibit a series of nonlinear behaviors such as period-doubling bifurcation, secondary bifurcation and chaotic phenomena. In the period-doubling bifurcation sequence the period-n discharge becomes more and more unstable with the increase of n. The period-doubling bifurcation can also be obtained by altering the discharge gas gap. The mechanisms of two bifurcations are further studied.It is found that the residual quasineutral plasma from the previous discharges and corresponding electric field distribution can weaken the subsequent discharge, and leads to the occurrence of bifurcation.     

On Pulsed Modes of the Glowing Corona Region

Russian Physics Journal - A corona discharge is experimentally studied in atmospheric air on a needle electrode with the curvature radius 40 μm. For both electrode voltage polarities, but...     

Two-dimensional numerical study of an atmospheric pressure helium plasma jet with dual-power electrode

In this paper,the characteristics of an atmospheric pressure helium plasma jet generated by a dual-power electrode(DPE) configuration are investigated by using a two-dimensional fluid model.The effect of a needle electrode on the discharge is studied by comparing the results of the DPE configuration with those of the single ring electrode configuration.It is found that the existence of the needle leads to the generation of a helium plasma jet with a higher propagation velocity,higher species density,and larger discharge width.Furthermore,the influences of the needle radius and needle-to-ring discharge gap on the generation of a plasma jet are also studied.The simulation results indicate that the needle electrode has an evident influence on the plasma jet characteristics.     

Erosion Craters and Arc Cathode Spots in Vacuum

The development of erosion craters on clean, smooth cathodes in UHV has been investigated with a time resolution of nanoseconds (current range 10–200 A). Furthermore, crater size erosion rate and velocity of spot displacement have been measured in dependence on current and surface conditions. The relevance of the results for different cathode spot models is discussed in detail. From the measurements the following conclusions are drawn. The craters are caused by the action of the discharge pressure on the molten metal within a spot. The spots move in a random manner in elementary steps of a crater radius with time constants of the order of 10^?8 s. The main reason for the movement is the formation of micropoints at crater boundaries. Droplets and contaminations induce jumps of more than a crater radius. The impact of droplets causes considerable deformations of the cathode surface. Nanosecond pulse breakdowns and quasi-stationary arcs result in values of crater size, spot velocity and erosion rate that are comparable within an order of magnitude. These results support a non-stationary spot model that describes the cathode spot as a sequence of surface explosions.     

A self-consistent analytical model of arc spots on electrodes

A simple model of quasi-stationary electrode spots in arc discharges is presented. For a given dependence of parameters of the near-electrode plasma layer on the local surface temperature and on the voltage drop in the layer, the model allows one to evaluate integral parameters of the spots without using empirical parameters such as the value of the electric current per spot or arbitrary suppositions such as the principle of minimum voltage. Analytical formulas are obtained for the spot radius, integral heat flux removed by heat conduction, and the electric current per spot     

Role of Streamers in the Formation of a Corona Discharge in a Highly Nonuniform Electric Field

JETP Letters - The ignition of a corona discharge on a tip electrode with a small radius of curvature in air at atmospheric pressure is studied. It is established that the initiation of the corona...     

大脉冲电流下钨铜电极水和空气中的烧蚀特性

针对水中、空气中脉冲放电条件下金属电极烧蚀速率及烧蚀机理差异,对脉冲大电流作用下水中、空气中钨铜电极的烧蚀特性进行了对比研究。在保证放电电流波形一致性的前提下,通过采用高精度天平测量并获取了水中、空气中钨铜电极的阴、阳极烧蚀速率及总烧蚀速率,并对电极表面进行了二次电子观察和背散射电子观察分析。结果表明,大脉冲电流作用下,水中钨铜电极烧蚀较空气中更为严重,钨铜电极的烧蚀主要是金属蒸发引起的汽相侵蚀。由于水介质较空气具有不可压缩性,水中放电电弧集中,电极表面电弧斑点处电流密度和电流作用时间较空气中更为严重,同时由于水中脉冲放电时发生的高温物理化学反应,是造成水中电极烧蚀要高于空气中的根本原因。     

Structure of the glow of a nanosecond diffuse discharge in a strongly nonuniform electric field

The structure of the glow of a diffuse discharge in air under atmospheric pressure is studied in detail in the “rod (cathode)-plane” geometry for an electrode gap of 10 cm and a cathode tip radius from 3 cm to 2.4 μm. The amplitude of voltage across the gap was ～ 220 kV for voltage growth rate of ～10¹³ V/s and a pulse duration of 180 ns. It is found that the shape of the discharge glow strongly depends on the radius of the cathode tip. For large values of the radius, the multichannel form of the glow prevails, which is statistically transformed into a volume glow as the radius decreases from 5 mm to 60 μm. This transition is accompanied by a decrease in the amplitude of the discharge current from 440 to 140 A on the average. For ultrasmall radii of the cathode (2.4–7.7 μm), the multichannel form of the glow prevails again and the amplitude of the discharge current increases up to ～300 A.     

三维轴对称折叠腔CO2激光器的位相锁定

在一新型三维轴对称腔CO2激光器的基础上提出了一种对此激光器位相锁定的方法。通过在输出镜后放置一具有反射和整形作用的凹凸镜，让作为输出注入光束的谐振腔优先起振。此腔输出的光束到达凹凸镜时被其反射。利用重新定义光斑半径的方法，证明此光束能够覆盖其他放电管输出的光束，根据几何光学，此光束能反射进入各个放电管。然后利用光子简并度的概念证明了反射光束能够控制腔内的本征模式，并且数值分析了注入锁定后腔内模式匹配。在反射注入锁定的情况下，给出了数值计算的光强输出及其模拟，结果证明经位相锁定后，激光器输出光束的质量有了很大的提高。     

Pseudospark discharge in long time current pulse: electricalcharacterization and recovery phenomena

At high currents, the performance of triggered vacuum gaps is limited by the constriction of the arc discharge on the anode. In a symmetrical system within flat electrodes, a motionless anode spot appears in front of the trigger spark. A pseudo-spark system within electrode cavities and specific gas pressure provides a good mechanism for avoiding anode spot phenomena. It appears possible to obtain a high intensity diffuse discharge. In this paper, we present the experimental measurements of the instantaneous voltage and discharge current, (peak current less than 50 kA, discharge time between 20 and 300 μs). These measurements allow us to determine the dynamical resistance, mean voltage and energy lost during the conduction phase. Consequently, we refine our knowledge of discharge development. Additionally, we present a complementary investigation on quenching behavior     

Modelling of corona discharge in cylinder-wire-plate electrode configuration

Numerical computation of the electric field strength and ionic space charge density in electrode systems consisting of ionizing wire and non-ionizing cylinder, connected to the same DC high-voltage supply and facing a grounded plate, is a difficult problem, which is of interest to several electrostatic processes applications. In a previous study a simple numerical method has been proposed to calculate the spatial distributions of electric field and ionic space charge in a case of a continuum and uniform corona discharge originating at the surface of the wire. The aim of the present paper is to improve the physical model of the corona discharge in this particular electrode configuration, by assuming a more realistic law of charge injection on the wire circumference. The computations were carried out for an ionizing wire of radius r=0.1 mm, located at different distances h from a metallic tubular support of radius R=13.4 mm.The initial conditions of the corona discharge took into account the non-uniformity of the charge injection around the ionizing wire electrode. The computational results were compared with those obtained under the assumption of uniform corona discharge. The comparison pointed out that neither the non-uniformity of the electric field nor that of the charge injection can be neglected. They depend on the geometry of the electrode system and affect the distribution of the electric field and of the space charge density in the inter-electrode gap.     

A model of the cathode spot glow discharge for a cathode withinclusions of low work function material

A model that describes the self-maintained glow discharge for a composite cathode in which cathode spots are formed on low work function material inclusions is presented. Based on this model, expressions are obtained for the radius of the spot, the cathode voltage, and the spot current as functions of Pd (where P is the gas pressure and d is the thickness of the cathode layer), the parameters of the inclusions, and the basic material matrix are obtained. Numerical results for the case of a glow discharge in 75 torr neon gas with a copper cathode matrix and inclusions of LaB₆ are presented. The average radii of inclusions were 0.05 and 0.015 mm. Conditions under which the glow discharge migrates from the inclusions and begins to spread onto the surface of the basic matrix are obtained. The transition of the glow discharge into a contracted state, depending on the structure of the composite material, is also considered     

The plasma in high-current pseudospark switches

The discharge behavior and the erosion rate of pseudospark switches for high currents (50-150 kA) and pulse lengths of several microseconds were investigated for different electrode materials. A capacitor discharge (3.3 μF) without any load was used at a maximum voltage of 30 kV. Side-on optical investigations were performed either with a streak camera or a fast shutter camera. Using metal electrodes, the discharge ignites on axis, then widens up radially and burns homogeneously at the edge of the central apertures. After about 500 ns, a stable anode spot is observed on the plane electrode surface (at currents exceeding 45 kA), the location of which is statistical. The discharge is transformed to a metal vapor are discharge and the erosion rate increases by more than one order of magnitude. With semiconductor electrodes (i.e., silicon carbide), a different discharge behavior is observed, After ignition on axis, the discharge burns homogeneously on the whole carbide surface. No contraction to a small area occurs in comparison to metal electrodes. The reignition of later current half cycles starts at the triple point metal-carbide-gas. Then the discharge again spreads homogeneously over the total carbide surface. The erosion rate is about two magnitudes lower in comparison to metals. We assume that the current is conducted in a thin surface sheath which is heated to more than 2000 K     

针-板空气间隙流注放电起始过程的三维PIC/MCC仿真研究

流注放电作为自然界中闪电传播的预电离机制、高压输变线路间长空间间隙放电的重要初始阶段,在工业领域存在诸多潜在应用,近年来引起人们越来越多的关注.流注放电具有典型的多尺度、非线性的放电特征,实验观测中多呈现出分叉等不规则结构.为了研究其微观结构特性和行为特征,本文采用三维粒子仿真模型(PIC/MCC),着重研究了流注从针型正电极的起始和发展过程.模型采用了可变自适应网格、可变粒子权重以及并行计算等技术,有效地降低了三维粒子仿真的计算时间.通过调节针型电极上的施加电压幅值、改变气体组分及调整电极形状尺寸等,研究了放电参数变化对流注放电的分叉结构、半径等行为的影响.模拟结果表明:随着电压的升高,流注的半径及分叉数目增加;对比不同气体组分(纯氧以及不同比例氮氧混合气体),发现其对流注的分叉数目影响较为显著;针型电极结构直接影响了流注的起始时间和形貌.     

Experimental observation of negative differential characteristic of corona discharge in ultraviolet spectrum

Corona discharge is a self-sustained discharge which appears at electrodes with a small radius curvature in gas insulation. An almost invisible glow occurs just above the inception voltage. Corona phenomenon is mainly used in electro-technological processes to obtain space charge for electrostatic precipitation, separation of different particles, electrostatic liquid or solid coating, neutralization of space charge, etc. All of these processes rely on a strong nonhomogeneous electric field generated by a point – plate electrode system. When the critical value of the applied voltage is reached, the ionization processes near the point electrode start and give rise to the current between two electrodes. If the pointed electrode is positive, it is possible to observe an anomaly of the current – voltage (I-U) characteristic for the point-plate space. It means that while the voltage is raising the current density decreases in a narrow voltage area (2–3 kV). The anomaly was technically named as negative differential conductivity (dI/dU < 0). Unstable current can have a negative influence on electro-technological processes. The anomaly was detected for different shapes and materials of the electrode as well as for various temperatures and distances between electrodes. An oxidation layer, which appears on the metal electrode, also influences the ionization processes near the pointed electrode and causes a decrease of a current. In this paper measuring of the discharge activity in a point – plate electrode system is presented. Ionization of gas atoms and molecules in a high electric field and the following recombination of electrons and positive ions in the corona region can give rise to high-energy photons which produce new electrons in the field of discharge. Corona discharges are detected by DayCor Corona camera which can register UV emission generated by corona in a day light. The experiment was conducted with various shapes of the pointed electrode and distances between the high voltage and the grounded electrode under applied direct voltage with positive and negative polarity.     

Studying a kind of portable ultra-bright microfocus x-raysource

This paper studies the properties of a kind of portable ultra-bright microfocus x-ray source with the Monte-Carlo method in detail. The new x-ray source consists of an electron-emission system, an electrostatic focusing system and a metal target. A crystal Lanthanum Hexaboride cathode, a Wehnelt grid and an extracted electrode compose the triode electrode electron-gun system. Two equal radius cylinder electrodes form the focusing system. The key factors determining the focus properties of the electron beam such as the ratio D_w/H, grid bias V_g, and the properties of the extracted electrode are numerically studied. The calculated results reveal that when D_w/H, V_g, the length of the extracted electrode, and the distance between the grid and the extracted electrode equals 5, --0.6~kV, 10~mm, and 8~mm respectively, the electron beam focal spot can be concentrated down to 9~μm in radius and a reasonable focal length about 72.5~mm can be achieved, at the same time, the cathode emission currents can be as high as 30~mA.