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     

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.     

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.     

Investigation of unstable motion characteristics of vacuum arc cathode spots between transverse magnetic field contacts

With the improvement of the current level of power grids, the requirements of the opening level of the vacuum switches are also increasing. Vacuum arc cathode spots provide steam and electrons and, to a certain extent, determine the opening capacity of the vacuum switch. In this paper, a vacuum arc cathode spot research platform based on the de-mountable vacuum chamber is constructed. The characteristics of the vacuum arc cathode spots under the transverse magnetic field (TMF) contacts are assessed by a high-speed charge coupled device. The experimental results show that the cathode spot diffusion process can be divided into three processes through cathode spot distribution, arc voltage and current: initial diffusion stage of cathode spots, unstable motion stage of cathode spots, and extinguishing stage. The motion mode of cathode spots during unstable motion stage can be divided into cathode spots group stagnation (CSGS) to multi-cathode jet (MCJ) switch mode, cathode spots group motion (CSGM) to MCJ switch mode, CSGM mode, and MCJ mode. The effects of peak current and contact diameter on unstable motion mode were analysed.     

Anode Phenomena in Vacuum and Atmospheric Pressure Arcs

This paper summarizes recent experimental data related to anode phenomena in both vacuum and atmospheric pressure arcs. Currents in the range 10A to 3OkA are discussed, and particular emphasis is placed on the effect of plasma flow from the cathode. For vacuum arcs this plasma flow is the directed motion of metal ions from the cathode spots. These ions reduce the anode voltage drop, and maintain a diffuse anode termination. At atmospheric pressure the ion flow is impeded by gas-atom collisions. However, a plasma flow towards the anode can result from magnetic pinch forces at the constricted cathode termination. In the absence of plasma flow, the anode termination constricts to a vigorously evaporating anode spot. For a typical non-refractory electrode such as copper, the spot operates at a temperature close to the boiling point irrespective of the gas pressure. The spot temperature is dictated by the balance between electrical input power and evaporative losses. These anode phenomena are discussed in relation to vacuum switchgear, arc welding and arc furnaces.     

Structure and parameters of vacuum arc cathode spots

The effect of such parameters of cathode materials as the heat of atom evaporation, atomic weight, work function of electrons on the structure of cathode spots of a vacuum arc, conditions of charged particle generation, and, most important, the F-emission of electrons, is considered. Determining the interrelation of cathode parameters and processes in a vacuum arc cathode spot helps develop conditions for a vacuum arc to effectively modify the surfaces of materials.     

Vacuum breakdown on metal surfaces

The unipolar arc model is described. Experimental proof that unipolar arcing represents a discharge form which easily leads to explosive plasma formation is provided. Using a laser-produced plasma, it has been demonstrated that unipolar arcs ignite and burn on a nanosecond time scale without any external electric field being applied. Similar unipolar arc craters have been observed on the cathode surface of a pulsed vacuum diode with an externally applied field of 0.5 MV/cm. The experimental results show that cathode spots are formed by unipolar arching. The localized buildup of plasma above an electron-emitting spot naturally leads to a pressure gradient and electric field distribution which drives the unipolar arc. The high current density of a unipolar arc provides explosive plasma formation     

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.     

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.     

From Field Emission to Vacuum Arc Ignition: A New Tool for Simulating Copper Vacuum Arcs

Understanding plasma initiation in vacuum arc discharges can help to bridge the gap between nano‐scale triggering phenomena and the macroscopic surface damage caused by vacuum arcs. We present a new twodimensional particle‐in‐cell tool to simulate plasma initiation in direct‐current (DC) copper vacuum arc discharges starting from a single, strong field emitter at the cathode. Our simulations describe in detail how a sub‐micron field emission site can evolve to a macroscopic vacuum arc discharge, and provide a possible explanation for why and how cathode spots can spread on the cathode surface. Furthermore, the model provides us with a prediction for the current and voltage characteristics, as well as for properties of the plasma like densities, fluxes and electric potentials in a simple DC discharge case, which are in agreement with the known experimental values. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The behavior of vacuum arcs between spiral contacts with small gaps

The application of small gaps in high-current vacuum interrupters highlights the interdependence of the contact design, the contact gap, and the arc behavior. In this investigation, a framing camera was used to record the appearance and motion of drawn vacuum arcs between spiral-petal contacts with final gaps of 2 to 3 mm. After the rupture of the molten metal bridge, a high-pressure arc column formed and expanded across the width of the spiral arm. With a single arc column for the duration of the half-cycle, an intense anode spot formed if the peak current exceeded ~15 kA. Compared to results previously obtained at larger gaps, the arc motion was greatly reduced, and severe contact damage was observed at lower currents     

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.     

Cathode Spot Phenomena in Low Current Vacuum Arcs on Arc-Cleaned Electrode Surfaces I. Spot Size

Cathode spots of vacuum arcs on pre-arced surfaces have been studied using image converter photographs with suitable temporal and spatial resolution. Part I describes experimental details and presents observations of two coexisting spot modes: the smaller spots tend to move behind the larger spots. A possible relationship to the general classification scheme for arc cathode phenomena (Anders and Anders, 1989) is discussed.     

Improved Model for Anode Spot Formation in Vacuum Arcs

The evaporation instability model for anode spot formation in high-current vacuum arcs shows one severe deficiency: it needs a critical vapor density at the anode, that is by two orders of magnitude higher than the measured value. The discrepancy can be bridged, if it is assumed that due to the relatively cool anode a low vapor pressure exists near the anode and thus the self magnetic field constricts the arc in the vicinity of the anode considerably. In consequence, the vapor density is higher near the anode than far away from that electrode. The mathematical analysis of that model shows that the predicted constriction near the anode exists indeed. The vapor density obtained at the anode surface is by more than two orders of magnitude higher than in the column and the absolute value is high enough to start the anode spot instability due to evaporation of the anode. The model shows that neither a pure magnetic constriction model nor a pure anode evaporation model can account for the effects observed, but that both effects contribute considerably to the phenomenon of anode spot formation in high-current vacuum arcs.     

Theoretische Modelle des katodischen Brennflecks von Bogenentladungen. Teil I: Grundlagen

Presuppositions, theoretical basis, results and general experiences of numerical model calculations of quasi-stationary cathodic spots in (vacuum) arcs are summarized. The results (mainly related to copper cathodes) comprise the essential physical quantities of the cathode spot surface and (partially) of the cathode spot plasma, i.e. possible states of existence within the space of parameters. Especially, the dependence of these results on changes of the parameters and equations is investigated. Several examples are presented and interpreted. In any case, the surface roughness of the cathode is taken into account. Finally, the limitations of the model are discussed, and the great importance of non-stationary (e.g. explosive) processes within the arc spot is emphasized, though such processes are beyond the scope of this model. The paper is published in two parts. This first part comprises an introduction to several aspects of arc spot modeling, the discussion of the physical processes in cathode arc spots and the development of relevant equations to be used in the model.     

Computer Simulation of the Dynamics of Plasma-Surface Interactions in Vacuum Arc Cathode Spots

After an introductory definition of the vacuum arc cathode spot, its main theoretical aspects are highlighted. Most of the review is being devoted to the E-diagram method as well as to advanced nonstationary cathode spot models. A survey of the aim of the present computer simulation and its development following the author's very first concept of "dynamic field emission" is also given. The cathode spot model subjected to the computer simulation is described in detail, considering the laws of conservation of charge, mass, and energy in the complete cathode spot system. A short survey of the computational procedure is also given here. The main computational results may be summarized as follows: Within the actual model assumptions, a steady-state self-sustaining cathode spot does not exist. On the one hand, plasma-wall interactions with steady-state thermal equilibrium exist below the balance conditions of mass and energy, while on the other hand, real cathode spots that meet the requirements of both mass and energy balance never show thermal equilibrium but are characterized by thermal runaway. Therefore, instability is an inherent feature of cathode spots. Finally, a comparison between computational values and representative experimental results of cathode spot characteristics shows a rather good agreement, further supporting the principal validity of a model devoted particularly to a theoretical specification of the vacuum arc cathode spot.     

Theoretische Modelle des katodischen Brennflecks von Bogenentladungen

Presuppositions, theoretical basis, results and general experiences of numerical model calculations of quasi-stationary cathodic spots in (vacuum) arcs are summarized. The results (mainly related to copper cathodes) comprise the essential physical quantities of the cathode spot surface and (partially) of the cathode spot plasma, i.e. possible states of existence within the space of parameters. Especially, the dependence of these results on changes of the parameters and equations is investigated. Several examples are presented and interpreted. In any case, the surface roughness of the cathode is taken into account. Finally, the limitations of the model are discussed, and the great importance of non-stationary (e.g. explosive) processes within the arc spot is emphasized, though such processes are beyond the scope of this model. The paper is published in two parts. This second part comprises the application of the model equations to the numerical calculation of arc spot parameters, the discussion of the results and some aspects of further developments in the theory of cathode arc spots.     

Direct observation of cathode spot grouping using nanostructured electrode

Clear footprint of an arc spot, which is formed after arc spot runs on a fiberform nanostructured tungsten, reveals the detailed structure and motion of arc spot in a magnetic field. Fine inner structure of an arc trail is observed, exhibiting that many sub-arc spots existed inside the arc spot, with forming a group. The sub-arc spots move randomly but they globally move to some direction, which is affected by the axial and parallel magnetic field with respect to the specimen. The trails are analyzed by using box-counting method and fractality of the arc trails in magnetic field is discussed. It is shown that the nanostructured metal can clearly record the footprint of the arcs, similar as the man's footprints in the snow, and be a powerful tool for basic arc experiments.