Cathode Erosion in Vacuum Arcs and Unipolar Arcs

The cathode spots from vacuum arcs on 316 stainless steel are compared with the tracks found on the same material after exposure to the plasma of the tokamak TFR 600. Further the erosion yields of vacuum arc cathodes of 316 stainless steel and titanium are determined from experiments and the measured values are compared with theoretical estimates. The velocity of the arc is investigated as a function of the applied magnetic cross-field. The scatter of both, the velocity data and the erosion yields is substantial. Improved experiments are planned.     

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.     

Small-Scale Anode Activity in Vacuum Arcs

Coordinated high-speed movies, streak photographs, and voltage/current oscillograms have been taken for vacuum arcs on copper-based electrodes at peak currents up to 70 kA in half-cycle pulses. These results show that small-scale transient luminous anode-spot activity is associated with the strong voltage noise that precedes the establishment of the conventional large anode spots. The characteristic dimensions of the small-scale spots go below a millimeter, and may be less than 100 ?m. Unlike cathode spots of that size, these small anode spots always move in the I × B direction. This small-scale activity is especially pronounced in experimental systems initially containing surface films of volatile matter. Good correlations have been established between bursts of anode light and corresponding bursts of arc voltage noise, both of which appear to be associated with variations in the small luminous structures. The practical importance of the small transient luminous anode activity reported here is in its clear tendency to advance the formation of electrode jets, particularly under experimental conditions favoring the evolution of gas or vapor from anode surfaces. It has theoretical significance as a precursor to the formation of the usual large anode spots and jets, and as a possible source of structure within large anode spots.     

Macroparticle Dynamics during Multi-Cathode-Spot Vacuum Arcs

Macroparticle dynamics in multi-cathode-spot (MCS) vacuum arcs were studied by utilizing laser Doppler anemometry (LDA) methods for in situ measurement of the cathodic macroparticle velocities and relative emission rates. Arc current pulses having peak values of 1-2 kA at either 6 or 1 ms after arc initiation were investigated. Systematic dependence of the macroparticle dynamics (i.e., speed and direction of flight) on cathodic thermophysical properties, location of the measurement probe in the interelectrode region, instantaneous value of the arc current, arc current waveform, and macroparticle size was determined. It was found that the macroparticle velocity increased with the melting temperature of the cathode metal, distance from the cathode surface, and the instantaneous value of the arc current, and decreased with macroparticle size and the rise time of the current waveform. All the above dependencies may be understood as direct indications of the plasma-macroparticle interaction during the discharge. The measured instantaneous relative emission rates were found to peak later than the arc current but before the peak average cathode surface temperature, which was estimated using a semi-empirical model. This result may be an indication of the dependence of cathodic erosion in the form of molten metal droplets on the average cathode surface temperature.     

Anode Spot Formation in Vacuum Arcs

Experiments on the transition of vacuum arcs from the diffuse mode into the constricted mode were carried out using both optoelectronic equipment and streak photography with time resolution in the range of microseconds. The transition occurs in a time range from 50 to 800 ?s and appears to be a continuous process.     

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.     

Ion Beam Generation in a Vacuum Arc Ion Source with a Multicomponent Cathode

Russian Physics Journal - Vacuum arc ion sources are widely used for modifying the properties of various surfaces using ion implantation. The use of cathodes consisting of multicomponent compounds...     

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.     

Temperature Measurements in Alternating Current Cross-Flow Arcs

Using optical techniques appropriate to columns having non-circular cross-section, temperature distributions within an alternating current cross-flow arc column have been obtained. Experiments were conducted upon a 61.1 A rms, 840 Torr argon AC plasma (having 11.2 mm electrode spacing) in a crossflow of 1.1 m/s. Isotherm distributions, obtained in horizontal planes, were generally non-circular with major axis in the direction of flow. Column deflections increased as arc current was reduced. The (characteristic) temperature-time history showed that temperature reached a minimum following current zero.     

The Time-Resolved Characterization of Erosion Products from High-Current, Copper Vacuum Arcs

A vacuum arc at high enough current can produce gross melting on electrode surfaces as a consequence of anode spot formation and other high-current electrode phenomena. Erosion from the electrodes under this condition is much more rapid than at low-current (where material loss occurs principally from the cathode) and is a process that is presently poorly understood. The present work is aimed at characterizing the erosion products from cathode and anode surfaces during high-current arcs on copper electrodes for single half cycles (60 Hz) arcs having peak currents of 30 kA. Fully open gap lengths were approximately 18 mm. Among the findings were the following. a) Erosion rate determined by electrode weight loss was approximately 8 mg/C of arcing. b) Droplets ejected from the electrodes had masses varying from a few tenths to a few tens of micrograms and velocities typically up to 40 m/s, although higher velocities are seen. c) The greatest number of droplets are produced at, or just after the current peak, and higher droplet velocities are seen in this same time interval. d) Erosion in vapor form detected in the plane of the cathode surface and moving radially is a maximum just after the peak of current and is relatively abundant. Such vapor is essentially absent in the anode plane.     

A Review of Anode Phenomena in Vacuum Arcs

This paper discusses are modes at the anode, experimental results pertinent to anode phenomena, and theoretical explanations of anode phenomena. A vacuum are can exhibit five anode discharge modes: (1) a low current mode in which the anode is basically passive, acting only as a collector of particles emitted from the cathode; (2) a second low current mode that can occur if the electrode material is readily sputtered (a flux of sputtered atoms will be emitted by the anode); (3) a footpoint mode, characterized by the appearance of one or more small luminous spots on the anode (footpoints are generally much cooler than the true anode spots present in the last two modes); (4) an anode spot mode in which one large or several small anode spots are present (such spots are very luminous, have a temperature near the atmospheric boiling point of the anode material, and are a copious source of vapor and ions); and (5) an intense are mode where an anode spot is present, but accompanied by severe cathode erosion. The are voltage is relatively low and quiet in the two low current modes and the intense are mode. It is usually high and noisy in the footpoint mode, and it can be either in the anode spot mode. Anode erosion is low, indeed negative, in the two low current modes, and it is low to moderate in the footpoint mode. Severe anode erosion occurs in both the anode spot and intense are modes. The dominant mechanism controlling the formation of an anode spot appears to depend upon the electrode geometry, the electrode material, and the current waveform of the particular vacuum are being considered. In specific experimental conditions, either magnetic constriction in the gap plasma, or gross anode melting, or local anode evaporation can trigger the transition. However, the most probable explanation of anode spot formation is a combination theory, which considers magnetic constriction in the plasma together with the fluxes of material from the anode and cathode as well as the thermal, electrical, and geometric effects of the anode in analyzing the behavior of the anode and the nearby plasma.     

A Review of Anode Phenomena in Vacuum Arcs

This paper discusses arc modes at the anode, anode temperature measurments, anode ions, transitions of the arc into various modes (principally the anode-spot mode), and theoretical explanations of anode phenomena. A vacuum arc can exhibit five anode discharge modes: 1) a low-current mode in which the anode is basically passive, acting only as a collector of particles emitted from the cathode; 2) a second low-current mode that can occur if the electrode material is readily sputtered (a flux of sputtered atoms will be emitted by the anode); 3) a footpoint mode, characterized by the appearance of one or more luminous spots on the anode (footpoints are much cooler than the true anode spots present in the last two modes); 4) an anode-spot mode in which one large or several small anode spots are present (such spots are very luminous, have a temperature near the atmospheric boiling point of the anode material, and are a copious source of vapor and ions); and 5) an intense-arc mode where an anode spot is present, but accompanied by severe cathode erosion. The arc voltage is relatively low and quiet in the two low-current modes and the intense-arc mode. It is usually high and noisy in the footpoint mode, and it can be either in the anode-spot mode. Anode erosion is low, indeed negative, in the two low-current modes, and it is low to moderate in the footpoint mode. Severe anode erosion occurs in both the anode-spot and intense-arc modes.     

Current Density Per Cathode Spot in Vacuum Arcs

Two methods of computing the current density in a cathode spot of a metal arc are compared. The first method computes the spot area in terms of a crater left on the metal. Detailed arguments are presented as to why this method is not correct. Evidence is presented supporting a second method, that of estimating the cathode spot from the luminous glow observed during the discharge. The current density is estimated to be less than 105 A/cm2 during the lifetime of the spot.     

Cathode Heating by Vacuum Arcs: A Survey

Theoretical predictions about heat sources, power dissipation and heating time scales are compared with available experimental data. It is concluded that the heating process must have a much faster time scale than quasi-stationary heat conduction. This can be achieved only by ion impact heating. A special model is discussed (thin layer heating) which explains the high power dissipation within the cathode as well as the short heating times.     

Experimental Investigation of Anode Activities in High-Current Vacuum Arcs

It is well known that the melting of electrodes (mainly anode melting) in vacuum arc can increase the metal vapor density around current zero and even lead to interruption failure. In order to clarify the anode activities and their influence on arc appearance and interruption capacity, series experiments of cup-shaped axial magnetic field copper electrodes were conducted. Obvious anode melting was detected; the liquid copper flowed on the contact plate of anode and formed a clockwise swirl flow. The appearance of anode melting is likely to correlate to the transition of arc mode from high-current diffuse mode to high-current diffuse column mode. The melting of anode was severer than cathode and was influenced by the distribution of cathode spots. Various kinds of copper particles at macroscopic level can be seen in arc column. Even at the interruption limit, the majority of melted copper of anode sputtered out of gap in form of liquid droplets or was pressed into the cup of anode, the copper vapor evaporated into arc column only accounted for a few portion and no obvious anode jets was found due to large plasma pressure in arc column.      

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.     

New Possibilities of OD ESR: Vacuum Ultraviolet Generation of Radical Ion Pairs

A new version of the optically detected electron spin resonance (OD ESR) technique using vacuum ultraviolet radiation to generate radical ion pairs is discussed. The technique can be used to study the spin-correlated processes of charge recombination in thin films that can be useful in spintronics. Distinctive features of photoionization and high-energy (including X-ray) ionization are discussed. The first results obtained by the technique in the study of charge recombination in polymeric and liquid crystal films are presented. The results of the concurrent observation of the OD ESR and electron cyclotron resonance (ECR) in the low-pressure gas over the sample are outlined. A possible application of ECR in the context of the OD ESR spectroscopy is suggested.     

A Model for DC Interruption in Diffuse Vacuum Arcs

A theoretical model for current interruption in a diffuse vacuum arc with dc commutation is described. Before current zero the interelectrode plasma is modeled as an ion-neutral fluid through which electrons are flowing. After current zero a positive ion sheath grows into the plasma from the former anode, driven by the transient recovery voltage. Using the basic laws of conservation, the decay of the plasma during commutation is evaluated numerically, enabling the post-arc current, the electric field at the former anode, and the power input to this electrode after current zero to be calculated. For copper electrodes, with a commutation time of 30 ?s, the ion density and velocity at current zero are 23 percent and 35 percent of their respective steady state values. The calculated post-arc currents of tens of amps are in good agreement with experimental data. The post-arc data generated with this model can be used to study reignition mechanisms and the interrupting capability of different contact materials.     

The Ecton Mechanism for the Generation of Ion Flows in a Vacuum Arc

The principal characteristics of the process of generation of the cathode plasma in a vacuum arc (ion erosion, mean charge of ions) are considered in terms of the ecton model of the cathode spot of a vacuum arc. The estimates of the parameters of ions obtained for a unit cell of a cathode spot – an ecton – are in good qualitative and quantitative agreement with experimental data. The following mechanism for the generation of the cathode plasma of a vacuum arc is proposed. When a region of the cathode is destroyed in an explosive manner due to Joule heating, the material of the cathode sequentially goes over a series of states: the condensed state and the states of imperfect and ideal plasma. During this transition the charge state of the plasma is formed and the ions are accelerated under the action of the pressure gradient in the plasma whose density decreases by several orders of magnitude over distances of 10 m from the cathode surface. The increase in current results in an increase in number of cells, and the principal parameters of the ions are formed as a result of the operation of a unit cell of the spot.