Arc Cathode Spot Research Articles

Microstructural modifications in powder-metallurgically produced Al0.675Cr0.275Fe0.05 targets during cathodic arc evaporation

Recently, the authors showed that metallic droplets, originating from the Al0.675Cr0.275Fe0.05 cathode surface, play an essential role in the nucleation process of hexagonal crystallites in mixed cubic and hexagonal-structured cathodic arc-evaporated (Al0.70Cr0.25Fe0.05)2O3 films. Here, the authors investigated in detail the corresponding powder-metallurgically produced Al0.7Cr0.3 and Al0.675Cr0.275Fe0.05 targets (after the arc evaporation process) and the ejected macroparticles, which were intentionally separated and collected from the plasma stream. The 15–200 μm thick melting zone (under given process conditions not entirely covering the target surface) of both target materials predominately consists of intermetallic Al80Cr20, Al9Cr4 and dominating Al8Cr5 phases. The selective melting process, induced by the cathodic arc spot size and particle size distribution of the targets, led to the formation of Al-enriched areas. Oxide islands, which form on the target surfaces, especially contain Cr- and Fe-enriched particles. The latter are only present for Al0.675Cr0.275Fe0.05 targets, where the majority of Fe is basically dissolved in the intermetallic Al-Cr phases formed at the target surface due to the cathodic arc interaction. The chemical composition of the ejected macroparticles corresponds with these cathodic arc interaction zones. Based on here presented results, the authors can conclude that Fe-containing intermetallic phases, which are also present within the droplets, as well as Cr- and Fe-enriched particles, are the influential factors for the nucleation of hexagonal phases within arc-evaporated (Al,Cr,Fe)2O3 coatings.

Critical Point Parameters and Cathode Spot Cells in Vacuum Arcs

This paper presents a model for the estimation of specific parameters of vacuum arc cathode spot (CS) explosive cells. It involves the reactive force accelerating ions from the spot cell, what is known as the Tanberg effect, and the assumption that the average state of matter during the explosion is near the liquid–gas (plasma) critical point. Specifying the ions’ velocity and erosion rate known from measurements, and the critical pressure, one can estimate the average current density at the CS cell to be about 100 MA/cm2 for most of the materials considered.

Complex technology of vacuum-arc processing of structural material surface

The development of environmentally friendly and energy-resource-saving technologies based on vacuum arc discharge is a topical problem in science and engineering. In view of their unique properties, cathode spots of a vacuum arc induce cleaning of the surface of an article (cathode) from various contaminations and pulsed thermal action on the surface layers. These processes occur in complex with vacuum-arc deposition of coatings in the same technological cycle, which makes it possible to considerably increase the efficiency of methods for changing physical, mechanical, and chemical properties of the surface of steel articles, which considerably increase their service life. Analysis of the formation of the temperature regime of the surface during vacuum arc action and of the parameters of the deposited coating will make it possible to optimize the regimes of complex treatment of the surfaces of articles and is of considerable theoretical and practical importance.

Modeling Spots on Composite Copper–Chromium Contacts of Vacuum Arcs and their Stability

Cathode spots on copper–chromium contacts of vacuum interrupters are simulated by means of a self-consistent space-resolved numerical model of cathode spots in vacuum arcs developed on the basis of the COMSOL Multiphysics software. Attention is focused on spots attached to Cr grains in the Cu matrix in a wide range of values of the ratio of the grain radius to the radius of the spot. In the case where this ratio is close to unity, parameters of spot are strongly different from those operating on both pure-copper and pure-chromium cathodes; in particular, the spot is maintained by Joule heat generation in the cathode body and the net energy flux is directed from the cathode to the plasma and not the other way round. An investigation of stability has shown that stationary spots are stable if current controlled. However, under conditions of high-power circuit breakers, where the near-cathode voltage is not affected by ignition or extinction of separate spots, the spots are unstable and end up either in explosive-like behavior or in destruction by thermal conduction. On the other hand, spots live significantly longer-up to one order of magnitude-if the spot and grain sizes are close; else, typical spot lifetimes are of the order of 10 $\mu \text{s}$ . This result is very interesting theoretically and may explain the changes in grain size occurring in the beginning of the lifetime of contacts of high-power current breakers. A sensitivity study has shown that variations in different aspects of the simulation model produce quantitative changes but do not affect the results qualitatively.

Cathode spot motion in a vacuum arc with a long roof-shaped cathode under magnetic field

Vacuum arc cathode spot motion was investigated on a “roof-shaped” aluminum cathode under an external magnetic field. The cathode included a flat roof-top and four sloped sides, inclined by an angle α. A high speed camera was used to observe cathode spot motion. The spot velocity on the slopes and the distribution of cathode spots on the roof and slopes was determined. It was obtained that under a magnetic field, the spot motion on the roof was slow (<1 m/s) and mainly random, while on the slopes fast retrograde motion was observed. This velocity increased linearly with the magnetic field and decreased slightly with α.

The Rayleigh–Plateau instability and jet formation during the extrusion of liquid metal from craters in a vacuum arc cathode spot

We consider the displacement of molten metal from a crater being formed on the cathode during the operation of a vacuum arc under the pressure of the cathode plasma and formulate a criterion for the formation of a thin ridge of expelled liquid metal (a sheet-like jet) at the crater edge. When the ridge height is substantially greater than its thickness, conditions arise for the development of the Rayleigh–Plateau capillary instability, which breaks the axial symmetry of the problem. Estimates are presented, which suggest that this instability is responsible for the breakup of the liquid ridge into jets, which play an important role in the self-sustained operation of a discharge.

酸化膜の厚さが及ぼす真空アーク陰極点の移動形態への寄与

A remarkable characteristic of a vacuum arc cathode spot, which can remove an oxide layer, is that it moves across the metal surface. However, the contribution to the moving mode of the vacuum arc cathode spot remains unclear. This study elucidated the contribution to moving mode of the vacuum arc cathode spot as affected by the oxide layer thickness. Experiments were conducted using an SS400 cathode and a cylindrical copper anode. A high-speed video camera recorded the cathode spot movement with ND (Neutral Density) and a band pass filter. The obtained images were then analyzed using plasma image processing. The cathode spots have four moving modes: oxide layer removal, split, stagnation at the boundary between the oxide layer and the processed surface, and high-speed movement on the processed surface. Oxide layer thickness affects the cathode spot moving mode because of the maintenance of the current continuity of the cathode spot with oxide layer vaporization and ionization. Oxide layer thickness affects the cathode spot movement.

Numerical investigation of the stability of stationary solutions in the theory of cathode spots in arcs in vacuum and ambient gas

The stability of stationary spots on cathodes of arcs in vacuum and ambient gas is investigated by means of the simulation of the temporal evolution of perturbations imposed over steady-state solutions. Two cases of loading conditions are considered, namely, spots operating at a fixed current (the case typical of small-scale experiments) and spots operating at a fixed voltage (the case typical of high-power circuit breakers). Results are reported on spots on large copper cathodes of vacuum arcs and on spots on tungsten cathodes of high-pressure argon arcs. It is shown, in particular, that if the ballast resistance in small-scale laboratory experiments with a high-current arc is insufficient, the potential consequence may be a thermal explosion of a spot, if the arc burns in vacuum, and massive melting of the cathode surface, if the arc burns in ambient gas. This conclusion conforms to trends observed in the experiment.

Effect of laser on the welding process of short-circuiting transfer MIG welding of aluminum alloys

In this paper, the effect of laser on the welding process in metal inert gas (MIG) welding of aluminum alloys was studied exploiting high-speed imaging and welding electrical signals collecting. Bead-on-plate welding experiments were conducted using conventional short-circuiting transfer MIG welding and laser-short-circuiting transfer MIG hybrid welding. The obtained results point out that it took less time to strike the arc in laser-MIG hybrid welding, and the stability of arc striking was increased. When the laser was introduced into the arc atmosphere, the arc was compressed, and the welding energy was more concentrated. The arc cathode spot was fixed to the laser beam impinging spot, so the arc could burn stably. In laser-MIG hybrid welding, the droplet short-circuiting transfer stability and the weld appearance were improved significantly. Moreover, an analysis method of judging droplet transfer stability which bases on statistical techniques was proposed. By this method, it can be concluded that the short-circuiting transfer process became more stable in laser-MIG hybrid welding.

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.

Effect of the cathode spots of a vacuum arc on the properties of the surface layer of structural materials

The application of vacuum-arc discharge provides great opportunities for metallic-surface cleaning and the development of products with the required functional surface properties. The effect of vacuumarc treatment on the roughness of the surface, its mechanical properties, and the structure and composition of the surface layer is studied. The processing and cleaning of the surface of steel samples by cathode spots of a vacuum-arc discharge form a structurally changed layer with a thickness of up to 10 μm and a microhardness exceeding the initial one by 1.4–1.5 times. The deposition of 20% solution of KOH and NaOH alkalis (substances which reduce the electronic work function of the scale surface) on the processed steel surface reduces the specific power inputs of vacuum-arc cleaning by a factor of 1.5–2.4. The depletion of the surface layer of low-grade steels by impurity elements enhances its anticorrosion properties.

Relationships governing the movement of the cathode and anode spots of an electric arc

The main relationships governing the movement of the cathode (anode) spot of an electric arc, in particular, the effect of rotation of the arc spots, which can be visually investigated at low arc currents, are investigated. The reasons for this phenomenon are explained on the basis of the vortex electrodynamic model of the electric arc in which the current-conducting channel in the plasma arc column has the structure of the dynamic cylindrical coaxial condenser. This phenomenon must be taken in the analysis of the processes taking place in the weld pool because the rotation of the channel of the plasma arc column is the reason for the effective rotational displacement of the material of the weld pool.

Space-Resolved Modeling of Stationary Spots on Copper Vacuum Arc Cathodes and on Composite CuCr Cathodes With Large Grains

A self-consistent space-resolved numerical model of cathode spots in vacuum arcs is realized on the computational platform COMSOL Multiphysics. The model is applied to the investigation of stationary spots on planar cathodes made of copper or composite CuCr material with large $({\gtrsim}{\rm 20}~\mu{\rm m})$ chromium grains. The modeling results reveal a well defined spot with a structure, which is in agreement with the general theory of stationary cathode arc spots and similar to that of spots on cathodes of arcs in ambient gas. In the case of CuCr contacts with large chromium grains, spots with currents of the order of tens of amperes on copper coexist with spots on chromium with currents of the order of one or few amperes. The main effect of change of the cathode material from copper to chromium is a reduction of thermal conductivity of the cathode material, which causes a reduction of the radius of the spot and a corresponding reduction of the spot current.

Spectroscopy of Single Vacuum Arc Cathode Spots With Improved Sensitivity

Recently, we reported on the optical emission of vacuum arc discharges with a single cathode spot. The discharges were initiated by rectangular voltage pulses of 100-ns length produced by a cable generator. In combination with the application of a liquid Ga-In alloy cathode and ultrahigh vacuum conditions, a high reproducibility was obtained. An improvement of the sensitivity and the signal-to-noise ratio was achieved by the replacement of the streak camera by an intensified charge-coupled device camera. Thus, the accumulation of calibrated spectra with high temporal, spatial, and wavelength resolution was possible. Distinct differences in the temporal development of the atomic and ionic line intensities were found depending on the spatial position, that is, comparing the spot core and the spot surrounding.

Hydrodynamics of the molten metal in a vacuum arc cathode spot at near-threshold currents

The extrusion of the molten metal from a microcrater formed on a metal cathode during the operation of a vacuum arc is considered. The problem is thought to be similar to the classical hydrodynamic problem of a liquid drop impact on a solid surface. Based on this analogy, the conditions are analyzed under which the liquid will change its regular behavior (spreading over the cathode surface) into a singular behavior (formation of microjets and droplets). It is shown that the conditions realized in vacuum arc cathode spots at near-threshold currents are close to the threshold conditions for splashing of the molten metal. This points to a considerable contribution of hydrodynamic processes to the self-sustained operation of a vacuum arc and, in particular, gives grounds to relate the existence of a threshold arc current to the existence of a splashing threshold for liquid metal.

Ecton Mechanism of the Cathode Spot Phenomena in a Vacuum Arc

In this paper, we review the state of the art in studying the physical processes that occur in the cathode spots of vacuum arcs. The now available experimental data are interpreted in the context of the ecton mechanism of the operation of vacuum arc cathode spots. Central in this mechanism is the explosive electron emission, a phenomenon discovered by the author and his co-workers in the mid-1960s while studying high-voltage pulsed vacuum breakdown. In the light of the ecton mechanism, the cathode spot of a vacuum arc consists of individual cells which are explosive emission sites each emitting a portion of electrons termed an ecton. The cathode spot processes are cyclic in nature due to the finiteness of the ecton lifetime. It is shown that an arc is self-sustained due to the explosive emission processes initiated on the interaction of the cathode plasma either with nonmetal inclusions present in the cathode surface (first-type spots) or with liquid metal jets ejected from the zone of an active cathode spot (second-type spots). Attention is focused on the physical processes occurring during the operation of a cathode spot cell. A statistical model of a vacuum arc is used to interpret the effect of the spontaneous extinction of an arc. It is shown that an increase in the arc current is accompanied by a slight increase in the number of simultaneously operating ectons; therefore, as observed in the experiments, the parameters of a vacuum do not greatly depend on the current up to the kiloampere level.

Control of cathodic arc spot motion under external magnetic field

A closed circular transverse magnetic field is designed to control the spot motion of a cathodic arc on a large rectangular target. Stable and controllable spot motion is observed from both graphite and copper. The arc stability, spot velocity, and etched pattern on the cathode surface are studied systematically. Circular tracks are etched on the target surface by the repetitive spot movement and the surface morphology and resistivity of the cathode materials influence the spot velocity.

On the mechanism of reverse motion of a cathode spot

The mechanism of reverse motion of a vacuum arc cathode spot in an external magnetic field is explained based on a description the spot and its surroundings as a localized set of slow magnetosonic waves. Reverse motion of this plasma perturbation results from the anomalous dispersion property of slow magnetosonic waves. The transverse displacement component of the cathode spot is oriented in the direction of transverse motion of the total energy flux. The proposed mechanism can be one of the competing mechanisms of reverse motion and can be used to construct new models for the reverse motion of cathode spots.

Fizeau interferometer system for fast high resolution studies of spectral line shapes

A monochromator∕Fizeau interferometer∕intensified CCD camera system is described that was developed for the measurement of the shape of spectral lines that are rapidly time varying. The most important operating parameter that determines the performance of the instrument is the size of the entrance aperture as this determines both the light throughput and the effective interferometer wavelength resolution. This paper discusses, both theoretically and experimentally, the effect of the finite source area on the instrumental resolution to assist in optimizing the choice of this parameter. A second effect that often produces a practical limit to the quality of the spectra is drift of the interferometer plates. Measurements of the shapes of spectral lines of ions and atoms ejected from the cathode spot of continuous and pulsed cathodic arcs are presented to demonstrate the utility of this instrument.

Investigation of Vacuum Arc Voltage Characteristics Under Different Axial Magnetic Field Profiles

Characteristics of the arc voltage under different profiles of axial magnetic field were investigated experimentally in a detachable vacuum chamber with five pairs of specially designed electrodes generating both bell-shaped and saddle-shaped magnetic field profile. The arc column and cathode spot images were photographed by a high speed digital camera. The dependence of the arc voltage on arcing evolution is analyzed. It is indicated that the axial magnetic field profile could affect the arc behaviors significantly, and the arc voltage is closely related to the arc light intensity.