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 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.     

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.     

Characteristics of Macroparticle Emission from a High-Current-Density Multi-Cathode Spot Vacuum Arc

Macroparticle mass transport, size distribution, and spatial distribution were studied in a 6.5-MA/M2 25-ms Cu multi-cathode spot (MCS) vacuum arc. The macroparticle erosion rate was determined to be 105 ?g/C, and together with ionic emission, accounted for most of the cathodic erosion. The number of macroparticles emitted decreased exponentially with macroparticle diameter, with 20-80-?m macroparticles carrying the bulk of the mass transport. Macroparticles are emitted preferentially at an angle of 20° with respect to the cathode surface. In comparison to previous investigations, higher macroparticle erosion rates, a larger proportion of large macroparticles, and a higher emission angle are observed, and the differences are attributed to the large current density used in the present experiment.     

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.     

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.     

In Situ Determination of Macroparticle Velocities in a Copper Vacuum Arc

The velocity components of individual macroparticles (molten droplets) moving through the interelectrode plasma of copper vacuum arc were measured by applying the forward-scattering laser Doppler anemometry method (LDA). The arc was sustained between two cylindrical copper electrodes, 14 mm in diameter and spaced 4 mm apart. Two current waveforms, with rise times to peak currents of 1 and 6 ms and duration of about 5 and 30 ms, respectively, were used in the experiment, while in both cases peak currents were either 1 or 2 kA. Macroparticles traversing through the ellipsoid shaped "probe-volume," which was produced by the intersection of the two He-Ne laser beams, scattered the laser light, through a monochromator, used as a 1.7-A bandpass filter, onto a photomultiplier. The Doppler-frequency component of the photomultiplier was recorded after appropriate filtering and amplification. The macroparticle velocity component obtained from the Doppler frequency was in the plane defined by the illuminating laser beams and directed perpendicularly to the optical axis. Macroparticles were detected during the whole period of the discharge, and their velocity was determined either at the instant of peak current or when the current decreased to 10 percent of its peak value, at several spatial locations inside the discharge volume. The measured macroparticle velocity components ranged from about 10-20 m/s up to about 700 m/s, showing a systematic dependence on the instantaneous value of the arc-current and on the spatial position of the "probe-volume," e. g.     

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.     

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.     

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.     

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.     

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.     

Cathode Spot Phenomena in Low Current Vacuum Arcs on Arc-Cleaned Electrode Surfaces. II. Spot Dynamics

Part II of the paper presents results of high temporal and spatial resolution experiments with vacuum arcs on pre-arced surfaces. Immobile spots, continuous spot motion and spot position oscillations have been observed. Tentative explanations of these phenomena are offered.     

Behavior of Sustained High-Current Arcs on Molten Alloy Electrodes during Vacuum Consumable Arc Remelting

Vacuum consumable arc remelting is a casting process carried out in a vacuum with the aim of remelting the consumable electrode in such a way that the new ingot has improved chemical and physical homogeneity. The power which causes the melting is supplied by a vacuum arc burning between the electrodes. In order to determine the furnace partitions of electrical power and current, experiments were conducted on molten-faced round electrodes. The quasi-steady melt rate was determined for both horizontally opposed 15-cm-diameter Ni electrodes and for vertically suspended 40-cm-diameter Inconel 718 electrodes. The cathode thermal power is directly proportional to the melt rate which, for the horizontally opposed electrode experiment, agrees to within 10 percent with the Ni breaker switch calorimetry measurements and with predictions from retarded potential analyzer plasma data. However, for the vertically suspended electrode experiments, the measured thermal power at the cathode is 50 percent higher than for nickel. When CO is introduced into the vertical alloy electrode system and electrode gap is increased, the cathode thermal power is reduced by approximately 50 percent. Furthermore, the electrode position measurements and observation of the ingot surface suggest that a concentrated arc is formed under these conditions.     

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.     

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.     

Generation and Measurements of Ion Species from Vacuum Arcs

We have measured the ion flux for different electrode materials in a vacuum arc. The vacuum arc has a point-plane geometry. The ion species in the generated plasma are identified using a time-of-flight (TOF) spectrometer. Ion species that have been generated to date include D+, Mg+, Mg++, Al+, Al++, Al+++, Ti+, Ti++, Ni+, Ni++, Cu+, Cu++, Zn+, Zn++, and In+. We found that in all cases, the ion flux measured is directly proportional to the interelectrode gap spacing and to the arc current. Typical current densities measured were ~300 mA · cm-2 at a distance of 10 cm from the gap for 150-?s pulse. The study will be used for the development of a multiple-arc array source for application to intense ion beam generation.     

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.