The computer simulation of the vacuum arc emission center

The paper presents a two dimensional (2-D) hydrodynamic model of vacuum arc emission center-“ecton”-which is an outgrowth of the one-dimensional problem that we have solved earlier. The numerical simulation was carried out for the initial stage of the operation of the cathode spot of a low-current vacuum arc on a copper cathode. It has been shown that, in terms of this model, the current flows in the main through the edges of the cathode spot. Based on a these data, a mechanism for the motion of a cathode spot has been proposed     

On the Nature and the Motion of Arc Cathode Spots in UHV

Cathode spot types and spot motion of arcs in ultra high vacuum have been investigated with large area cathodes that consisted of two adjacent pieces of Mo and Cu. Arc currents were 20–60 A dc and 8–20 kA pulse (duration about 1 ms). Two spot types occured with different velocities and surface erosion: Type 1 spots are typical for surfaces covered by oxides or thick adsorption layers, whereas clean surfaces show only type 2 spots. During arc-conditioning both types exist simultaneously in a complex mutual dependence. Type 1 spots react weakly on the cathode material, while type 2 spots burn preferentially on Cu and at the boundary line between Mo and Cu. The motion of type 1 spots is determined by the expanding spot plasma, whereas type 2 spots show a step-by step motion, determined by explosions in the arc craters. Generally a spontaneous formation of type 2 spots beneath the arc plasma takes place only with contaminated surfaces (probably by a transition from type 1 to type 2 spots). Thus a breakdown between plasma and cathode surface requires the presence of contaminations. The observed effects occur in low current dc-arcs as well as in high current pulse arcs. They are discussed for different spot models.     

Influence of microstructure of CuCr25 cathode on the motion of vacuum arc spots

The motion of vacuum arc spots on nanocrystalline and coarse-grained CuCr25 alloys were observed by a digital high speed video camera and SEM. Experimental results show that without an external magnetic field a spot can move a long distance in a direction and leave a long straight arc trace (50–100 μm) on the surface of nanocrystalline CuCr25 cathode. This kind of spot motion can be defined as sub-directional motion. The spot motion is totally random and restricted on coarse Cr particles for coarse-grained CuCr25 cathode. Arc spots move sub-directionally and more easily on nanocrystalline cathode maybe results from an active surface formed by the special electronic structure due to Cu–Cr internal electric field.     

纵磁作用下真空电弧单阴极斑点等离子体射流三维混合模拟

真空电弧的特性直接受到从阴极斑点喷射出的等离子体射流的影响,对等离子体射流进行数值仿真有助于我们深入了解真空电弧的内部物理机制.然而,磁流体动力学和粒子云网格仿真方法受限于计算精度和计算效率的原因,无法有效地应用于真空电弧等离子体射流仿真模拟.本文开发了一套三维等离子体混合模拟算法,并在此基础上建立了真空电弧单阴极斑点射流仿真模型,模型中将离子作宏粒子考虑,而电子作无质量流体处理,仿真计算了自生电磁场与外施纵向磁场作用下等离子体的分布运动状态.仿真结果表明,单个阴极斑点情况下真空等离子体射流在离开阴极斑点后扩散至极板间,其整体几何形状为圆锥形,离子密度从阴极到阳极快速下降.外施纵向磁场会压缩等离子体,使得等离子体射流径向的扩散减少并且轴线上的离子密度升高.随着外施纵向磁场的增大,其对等离子体射流的压缩效应增强,表现为等离子体射流的扩散角度逐渐减小.此外,外施纵向磁场对等离子体射流的影响也受到电弧电流大小的影响,压缩效应随电弧电流的增加而逐渐减弱.     

Anode-Spot Formation and Motion of Vacuum Arcs

This paper reports about experimental investigations on high-current vacuum-arc phenomena, especially anode-spot formation, arc states, and motion. The presented work was stimulated by lack of information about the transition process from the diffuse low-current mode to the high-current mode characterized by anode spot(s). Optoelectronic measurements, streak photographs, high-speed movies, and correlated arc voltage/current records yielded remarkable results on power-frequency vacuum arcs. Three different high-current vacuum arc modes can be observed beyond a certain threshold current. Which mode appears depends mainly on the momentary electrode distance. The modes are characterized by different anode-spot behavior and interelectrode phenomena. The transition between different arc modes is continuous. The arc modes observed on ring electrodes producing a magnetic blast field are the same as those appearing on butt-type electrodes. Anode-spot formation is preceded by congregations of cathode spots and may be initiated by thermal overload of the anode surface opposite to these cathode-spot clusters.     

Vacuum Arc Anode Phenomena

This paper briefly reviews anode phenomena in vacuum arcs, specially experimental work. It discusses, in succession, arc modes at the anode, anode temperature measurements, anode ions, transitions of the arc into various modes (principally the anode spot mode), and theoretical explanations of anode phenomena. The two most common anode modes in a vacuum arc are a low current mode where the anode is basically passive, acting only as a collector of particles emitted from the cathode, and a high current mode with a fully developed anode spot. Characteristically this anode spot has a temperature near the atmospheric boiling point of the anode material and is a copious source of vapor and energetic ions. However, other anode modes can exist. A low current vacuum arc with electrodes of readily sputterable material may emit a flux of sputtered atoms from the anode. Usually this sputtered flux will have little effect upon the vacuum arc, but in certain circumstances it could be significant. A vacuum arc doesn't always transfer directly from a low current mode to the anode spot mode. In appropriate experimental conditions, formation of an anode spot may be preceded by the formation of an anode footpoint. This footpoint is luminous, but much cooler than a true anode spot. Finally, (again in appropriate circumstances) several small anode spots may form instead of one large anode spot. With sufficient increase in arc current or arcing time these will usually combine to form a single large active spot.     

Time dependence of vacuum arc parameters

Time-resolved investigations of the expanded plasma of vacuum arc cathode spots are described, including the study of the ion charge state distribution, the random cathode spot motion, and the crater formation. It was found that the ion charge state distribution changes over a timescale on the order of hundreds of microseconds. For the random spot motion two timescales were observed: a very short spot residence time of tens of nanoseconds which gives, combined with the step width, the diffusion parameter of the random motion, and a longer timescale on the order of 100 μs during which the diffusion parameter changes. Crater formation studies by scanning electron microscopy indicate the occurrence of larger craters at the end of crater chains. The existence of a timescale much longer than the elementary times for crater formation and spot residence can be explained by local heat accumulation     

Vacuum arc cathode spot grouping and motion in magnetic fields

Two of the important vacuum arc phenomena observed when the arc runs in a transverse magnetic field are cathode spot grouping and the cathode spot retrograde motion, i.e., in the anti-Amperian direction. This paper summarizes the main experimental observations and proposes a physical model for spot grouping and spot retrograde motion. The proposed spot motion model take in account the previous theoretical model of the cathode thermal regime and the plasma flow near the cathode surface that is based on two conditions: i) the heat loss in the cathode bulk is relatively small to the heat influx, and ii) the plasma flow in the Knudsen layer is impeded. In the present model, the current per group spot is calculated by assuming that the plasma kinetic pressure is comparable to the self-magnetic pressure in the acceleration region of cathode plasma jet. The model includes equations for the current per spot group, spot velocity dependence on the magnetic field and on the arc current in vacuum, as well as in gas filled arc gap. The calculated currents per spot group and spot velocity increase linearly with the magnetic field and arc current, and this dependencies well agree with previous observations. The cathode spot retrograde motion in short electrode gaps and at atmospheric pressure arcs, and the reversal motion in strong magnetic fields (>1 T) observed by Robson and Engel are discussed. The details of the retrograde motion observed in the last decades including the spot velocity dependence on the electrode gap, roughness, temperature, and material could be understood in the frame of the proposed model.     

Cathode spot dynamics on pure metals and composite materials inhigh-current vacuum arcs

An investigation has been carried out of cathode spot dynamics in a triggered vacuum arc in a demountable chamber. A rectangular current pulse of 1-5 kA, 1-5 ms has been used. Sufficient statistics were collected. The expansion of a cathode spot ring on a clean, pure metal surface was corroborated to be a retrograde movement in the self-magnetic field which obeys the same law as the movement of a single spot in an external magnetic field. The influence of a contact gap of 0.5-8 mm and current on the dynamics of cathode spots was investigated. The gap dependence of the proportional coefficient between the spot velocity and magnetic field in the case of a pure copper cathode was obtained. A phenomenon was discovered, where a group of cathode spots form in the short arcs on the CuCr cathodes after a transition diffuse arc stage. The follow-up investigation revealed that a close interrelation exists between the cathode and anode processes in short arcs. This interrelation is responsible for the appearance of the discovered phenomenon. Short-circuit performance tests conducted for a commercial vacuum interrupter proved cathode spot group formation to be responsible for the interruption failure at short contact gaps     

Transverse Forces and Motions at Cathode Spots in Vacuum Arcs

Consistent analyses are provided for the motion of cathode spot cells in the presence of magnetic fields parallel to the cathode surface (retrograde motion), for the spontaneous splitting of cells, and for the grouping of cathode spot cells in organized structures. The formulas for retrograde motion and cell splitting frequencies are evaluated for cells carrying 1-80 A on copper vacuum arc cathodes. The results for retrograde motion are shown to agree satisfactorily with published experimental data. It is concluded that retrograde motion can be explained simply, and that measurements of retrograde velocities can provide useful information concerning cell sizes in cathode spots. The close connection demonstrated between retrograde motion and cell splitting yields the conclusion that cell splitting should slow in the presence of an ambient gas, as does retrograde motion. Cathode spot sizes and energies of formation are evaluated for spots that are circular clusters containing up to twelve individual cells. It is concluded, in apparent agreement with experiment, that such clusters should not be stable under conditions of clean vacuum where the cells exhibit retrograde motion.     

氘化物阴极真空弧放电光斑分布

氘化物真空弧放电在许多领域均有应用,如无损检测、石油探井、中子活化分析等。和金属阴极不同,氘化物阴极放电时会释放大量的气体分子,表现出许多不同性质。采用放大镜头和ICCD相机观察了氘化物阴极真空弧放电光斑分布。测量系统的空间分辨率约为5 μm,时间分辨率最小2 ns。放电脉冲半高全宽（FWHM）0.9 μs,弧流波形为半周期正弦波。实验结果表明,氘化物真空弧放电时,所有阴极斑聚集为一个群落,表现为一个大光斑;在液滴作用下,阴极斑群落偶尔也会分裂为两个或多个群落;光斑形状不受弧流影响,但面积和亮度会随弧流增加而增大。氘化物阴极放电斑点聚集有利于产生高密度等离子体,提高放电效率。     

Investigations of the Cathode Spot Dynamics in a Vacuum Arc Coating Process

Influence of cathode materials (Ti, Al, Cu, TiN), ambient gases (Ar, N₂, p = 0.1-1 Pa) and the arc current itself on the motion and the velocity of cathode spots in an arc coating process have been investigated with the help of a new high speed framing camera. It was found, that the cathode material causes different spot currents but in general the spot arrangement and the motion on the surface are similar. Surface contaminations due to ambient gases affect this dynamics in several ways. Insulating layers like AIN can drastically increase the instantaneous spot velocity, for example from <5 m/s on Al up to 170 m/s on AIN contaminated areas. TiN layers with a high conductivity increase the spot mobility at first. But at nearly completely contaminated surfaces (simulated by a TiN cathode), the mobility is strongly decreased. The values change from an average velocity of 6.3 m/s with a diffusion constant of 54 cm²/s (Ti, 0.01 Pa) to 2 m/s and 6.4 cm²/s at TiN. The course of the instantaneous spot velocity during the spot splitting phase was investigated too. The instantaneous spot velocity of each of the two new spots originated from the starting spot is relatively high (30–40 m/s) within the first 50 μs. The cathode material and the ambient gases are of slight influence in this phase. The movement is directed. In the further development the instantaneous spot velocity is decreasing to values under 5–10 m/s. The motion is now more and more random. Additionally it could be proved, that the lower stability limit for a stable discharge is strongly connected with the spot current, which depends on discharge conditions.     

真空短间隙微弧级联效应观测

利用0.5 mm间隙微通道板成像器观测了微弧放电的斑点级联效应.实验发现,微弧阴极斑点的运动轨迹呈多类型的折线轨迹,斑点间隙在200—300 μm,首发射阴极斑点的放电强度比次级斑点高一个量级以上,而且次级斑点之间的放电强度相对稳定,次级斑点在放电阴极表面无融蚀现象.实验表明,次级斑点产生机制与首发射存在较强的依赖关系. 关键词： 阴极斑点 微弧级联 微通道板成像器     

Properties of Anchored Cathode Spots of a Dc Mercury Vacuum Arc

Ensembles of anchored cathode spots of a dc mercury vacuum arc have been studied by fast framing and streak photography. From these photographs, several statistical properties of the cathode spots have been determined: distribution functions for their diameters, velocities, and displacements, as well as spot shape and average values for the spot current and its density. The measurements showed that the anchored cathode spots were quasi-stationary. No indications of a microstructure within the individual cathode spots were found at an optical resolution of 0.37 ?m. Strong evidence is presented that the dc cathode spot parameter values reported here are typical for a clean mercury surface, and that those reported in the earlier literature are typical for impurity-covered surfaces.     

Application of vacuum arc cathode spot model to graphite cathode

A model of near-electrode processes is applied here to describe the behavior of cathode spots on graphite cathode in vacuum arc. The physical model is based on a kinetic treatment of cathode evaporation, electron emission from the cathode, and plasma production. The model consists of physical assumptions and a system of equations that are formulated in the paper. Spot parameters, such as cathode erosion rate, cathode potential drop, cathode surface temperature, current density, electric field, and plasma density, temperature, and velocity in the near-electrode region are calculated numerically. The calculation includes the dependence of spot parameters on spot current and spot lifetime. The variation of spot parameters as a function of spot lifetime are very strong at lifetimes shorter than 10 μs. The calculations indicate that Joule heating in the cathode body is significant, and may exceed cathode heating by the ion heat flux. Calculated spot parameters are compared with the corresponding experimental data for relatively low arc currents (<100 A) and their agreement is discussed     

Effect of the discharge parameters on the generation of deuterium ions in the plasma of a high-current pulsed vacuum arc with a composite zirconium deuteride cathode

We have studied the mass and charge composition of an ion beam extracted from the plasma of a vacuum arc with a zirconium deuteride cathode for various durations of the arc current pulse (half width at half amplitude) of 2, 4, 7, and 17 μs. It has been established that the fraction of deuterium ions in the vacuum arc plasma increases with the current and the dependence achieve saturation for current of about 1 kA. For the fraction of deuterium atoms in the cathode at a level of 40%, the fraction of deuterium ions in the vacuum arc plasma can exceed 80%. The experimental results have been interpreted theoretically. It has been shown that the main sources of deuterium ions in a microsecond arc discharge are cathode spots. We have developed a model of deuterium desorption during the operation of cathode spots for quantitatively estimating the concentration of deuterium ions in the arc plasma.     

Cathode processes in free burning and stabilized by axial magneticfield vacuum arcs

Collective behavior of the cathode spots (CS) has been investigated in free burning and stabilized by axial magnetic field (AMF) vacuum arcs. Experiments carried out proved previously discovered phenomenon of CS group formation in free burning arc to be a general phenomenon for a short high-current vacuum arc. The dependency of CS group size in the developed are on arc current for different contact materials has been analyzed. Application of AMF with even relatively low intensity strongly affects on cathode processes. In short arcs, it hinders formation of the CS group and consequently reduces thermal stress applied to the electrodes. It has been revealed that high current vacuum arc under the action of AMF can exist only at current densities exceeding certain minimal value that depends on AMF intensity, contact gap, and does not depend on current itself. The dependency of this minimal (or normal) current density on AMF intensity has been studied for short and long vacuum arcs. A qualitative model of the cathode spot dynamics has also been proposed     

Arc Cathode Processes in the Transition Region between Vacuum Arcs and Gaseous Arcs

The cathode processes of electric ares on cleaned Cu cathodes were investigated in the transition region between vacuum and atmospheric pressure (argon). The plasma density in the cathode plane was estimated by probe measurements to be n = r are current, r – distance from the spot). It was observed that several cathode spot parameters have an extremum at p ～ 10⁴ Pa. The crater diameter has a minimum independently of the cathode temperature. The diffusion constant of the chaotic motion determined by framing photographs was found to have a maximum. Some additional, large displacements occurred at that pressure. The diameter of the bright plasma cloud obtained by open-shutter photographs showed a maximum, the current per spot was found to decrease from 20 A in vacuum to 10 A at atmospheric pressure. It is thus concluded that the spot with the smallest crater radius and a low current per spot, occurring at ～ 10⁴ Pa, represents the single spot, whereas the spot at higher pressures, and probably also in vacuum, has a complicated nature where the large craters are formed by a cooperation of single spots.