Cathode Spot Of Vacuum Arc Research Articles

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.

Strange collective behaviors of cathode spots in low vacuum arc

Removal of an oxide layer from all surfaces of a metal plate using an arc in vacuum is conducted for this study. Results confirmed that cathode spots that appeared on the side faced to anode were able to exist on another side opposite from the anode as well. Although some features of the cathode spots are not unusual, some characteristics disagree with conventional explanations for low-vacuum arc behavior.

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.

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.

Charge distribution of ions in low-current vacuum-arc plasma

The charge distribution of vacuum-arc ions has been investigated for the arc between copper electrodes at currents varied from 8 to 60 A (with the arc voltage increased from 21 to 24 V). Plasma contains copper ions with charge states from +1 to +4. The mean ion charge state increases from 2.15 to 2.24 (i.e., by only 4%) due to a small increase in the fractions of multiply charged ions (Cu2+ and Cu3+) in the total ion flux. This conclusion does not contradict that of the ecton model of vacuum-arc cathode spot, according to which the number of cells in the spot that are due to microexplosions on the cathode increases proportionally to the arc current.

Characteristics of Vacuum-Arc Cathode Spots on the Refractory Metal Electrodes

The paper describes investigation of characteristics of vacuum-arc cathode spots on refractory metal electrodes (Mo, W) that depend on magnetic field and the arc current and length: velocity of spot motion, average current per spot, and Robson angle. In the experiments, the arcs have lengths <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$h={2}$</tex></formula> and 6 mm and are subjected to magnetic field inclined to the cathode surface. The cathode surface is filmed during arcing by high-speed photography camera. The results obtained are compared with ones found in the literature.

On the problem of electron-emission mechanisms in the cathode spot of a vacuum arc discharge

A criterion for the F-type mechanism of electron emission in a vacuum-arc cathode spot is developed. It is shown that the implementation of F-electron emission in the cathode spot necessitates the fulfillment of three conditions: first, generation of the optimal electric field Eopt, second, implementation of atomic-ionic balance at which the Eopt field is generated and, third, retention of the cathode temperature lower than the inversion temperature of Nottingham’s effect. The calculations show that the heat of the evaporation of cathode atoms which meets these requirements does not exceed λa ∼ 1.1–1.2 eV. Taking the possibility of F-T-emission into account, the evaporation heat can be slightly higher than λa ∼ 1.5–1.6 eV. However, in this case, it turns out to be fairly small. Note that the number of such metals is not very large.

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.

Measuring the robson angle in vacuum arcs of various length

It is established that the Robson angle, which determines the direction of motion of a cathode spot of vacuum arc in a homogeneous electric field sloped relative to the cathode, depends mostly on the inter-electrode gap width. The dependence of the Robson angle on the field slope at various gap widths has been measured for molybdenum and tungsten cathodes. The knowledge of this angle is necessary for correct choice of the optimum magnetic field configuration controlling the cathode spot dynamics in setups for vacuum-arc deposition of various coatings and in vacuum commutation devices.

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.

Formation of plasma technological influence of vacuum arc on the internal surfaces of metal pipes and putting of the protecting coatings

In many cases of industrial applications it is economically profitable or technically necessary to combine bulk features of one material with surface features of another one in a single material. Vacuum-arc treatment of products from inexpensive materials to form new required features of surface layers is more perspective owing to its unlimited feasibility in process efficiency. Pipes are in great demand in oil-gas industries and power engineering. But there are no effective methods for pipe inner surface treatment and pipe coating deposition, which makes it impossible to use inexpensive steel pipes under conditions of aggressive liquids, high temperature, high pressure and other parameters leading to oxidation, erosion and pipe life. Therefore, development of plasma-arc technologies of pipe inner surface treatment and pipe coating deposition is very topical and actual. A specific effect of vacuum-arc discharge on material surface is due to high concentration of energy in a chaotically rapidly moving cathode spot, short-term local heating of surface and its rapid cooling. The technological effect of cathode plasma on surface is based on cathode spot movement. The effect of a plasma flow of cathode spots formed on the outer surface of a pipe cathode and controlled by a system of vacuum-arc cathode spot fixation with magnets placed inside an electrode has been considered. A great number of studies have been devoted to explaining and investigating “anomalous” movement of the cathode spots. It has been shown that the cathode spot moves in the direction of the greatest action of discharge’s own magnetic field and outer magnetic field. The magnets initiate an arc magnetic field that confines the cathode spots and makes it possible to scan (control) them on the electrode-cathode surface and act with a plasma flow and deposit coats on inner surface of a coaxially placed pipe-product. The high efficiency of the plasma vacuum-arc method for pipe inner surface treatment and coat deposition has been found.

Investigations on the Motion of High-Current Vacuum-Arc Cathode Spots Under a Magnetic Field

Much research has been done on the motion of low-current vacuum-arc cathode spots (CSs). The relationship between the motion velocity of CSs and the magnetic field, particularly the relationship between the direction of CS motion and the ratio of Bz/Bt, has been revealed. However, little investigation on the motion of CSs in high-current vacuum arcs has been conducted before. In this paper, the motion of CSs of high-current vacuum arcs under different axial magnetic fields was experimentally investigated. Simultaneously, calculation of the motion of CSs based on the empirical formula has been done, and comparison with experiments and analysis were given. Experiments were conducted in a detachable vacuum chamber with an arc current of 10 kA (root mean square). Images of CSs were photographed with a high-speed digital camera with an exposure time of 2 μs. With computer-aided numerical processing, the spatial distribution of the CSs was obtained, based on which the magnitude and direction of the CS motion velocity were calculated, and the CS motion trajectories around the current peak were plotted. Results indicated that the properties of the CS motion in high-current vacuum arcs are similar to those in low-current vacuum arcs.

Effect of tangential magnetic field on ecton processes in cathode spot of vacuum arc

Emission and erosion processes involved in the final stage of the cathode spot cell operation in vacuum arc in the presence of an external magnetic field have been numerically simulated. It is established that the application of a magnetic field leads to asymmetry in the distributions of current density and heat flux, so that their maxima shift in the “anti-Ampere” direction. For more detailed analysis of the phenomenon of retrograde motion of the cathode spot in a magnetic field, it is necessary to study the behavior of a liquid metal phase in the spot.

Generation of multicomponent ion beams by a vacuum arc ion source with compound cathode

This paper presents the results of time-of-flight mass spectrometry studies of the elemental and mass-to-charge state compositions of metal ion beams produced by a vacuum arc ion source with compound cathode (WC-Co(0.5), Cu-Cr(0.25), Ti-Cu(0.1)). We found that the ion beam composition agrees well with the stoichiometric composition of the cathode material from which the beam is derived, and the maximum ion charge state of the different plasma components is determined by the ionization capability of electrons within the cathode spot plasma, which is common to all components. The beam mass-to-charge state spectrum from a compound cathode features a greater fraction of multiply charged ions for those materials with lower electron temperature in the vacuum arc cathode spot, and a smaller fraction for those with higher electron temperature within the spot. We propose a potential diagram method for determination of attainable ion charge states for all components of the compound cathodes.

Hydrodynamic Model of Plasma Jet of Cathode Spot of Vacuum Arc in the Presence of External Axial Magnetic Field

This paper concerns the paraxial structure of a plasma jet, escaping a cathode spot of a vacuum arc in the presence of an external axial magnetic field. The jet is described in the stationary single-temperature single-fluid approximation with viscosity, thermal conductivity, and radiation disregarded. To take account of the ion composition of the plasma, the average-charge approach is used. Allowance is made for the transonic nature of the flow and the presence of a free boundary. The problem is solved by the method of paraxial expansions and is reduced to a closed system of ordinary differential equations, corresponding to the principal order of the expansion. System singularities relating to the critical section are isolated explicitly and processed. The equation set is integrated in the hydrodynamic region to either side of the critical section. The number of free parameters in the critical section is reduced to two, which are the average charge and temperature. The values of all other parameters of a jet in a wide range of their variation are constructed on the plane of free parameters. The same is performed for jet parameters in the plane, considerably away from the critical section. Axial distributions of the parameters are built. The obtained results generalize those of an earlier paper in the case of the presence of the external axial magnetic field. The results describe the contraction of a jet by the external axial field and the corresponding changes in the behavior of the jet parameters along the axis. It turns out here that, because of the jet contraction, the region of validity of the hydrodynamic description of a jet extends rather far toward the anode.

Optical Investigation of the Plasma Jet of Vacuum-Arc Cathode Spot

The results of the optical study of the plasma jet of the cathode spot of a freely burning vacuum arc with copper electrodes at a current of 60 A are given, as well as of the arc stabilized with a uniform axial magnetic field of induction up to 0.18 T. The axial and radial profiles of radiation intensity for different ionization states of copper - the atom and the single and double ions - were studied. The profiles of the distribution temperatures of the atom and the single ion were determined. The temperature of electrons is estimated on the periphery of the jet. The obtained results are discussed and compared with those of a mathematical simulation.

Spectroscopic Study of a Single Vacuum-Arc Cathode Spot

The cathode-spot plasma in spark and arc stages of a vacuum discharge was studied spectroscopically. A single spot was generated with high reproducibility in a gap under ultrahigh-vacuum conditions with a liquid-metal cathode of GaIn alloy. The self-breakdown of the vacuum gap resulting in a short discharge of less than 100 ns as well as a discharge over and up to 3 mus has been considered. The combination of a 0.5-m spectrograph with a streak camera enabled observation of spot evolution with a time resolution in the nanosecond range. Applying the streak camera as an image converter, time-integrated spectra resolved in the direction along the arc axis have been obtained. Limits concerning wavelength and time resolution as well as the emission intensity are discussed. Spectral lines of Ga and In atoms and single- and double-charged ions have been observed simultaneously. At the beginning of the discharge, ionic lines of higher charge state and wide line broadening dominate the spectrum. With a delay of several hundreds of nanoseconds, atomic lines appear and fall down in intensity to a much lesser degree than the ionic lines. Hence, atomic lines finally dominate in the arc stage of the discharge.

A study of vacuum arc ion velocities using a linear set of probes

The most likely velocity of ions moving away from vacuum arc cathode spots was measured using a set of probes along the path of plasma expansion. The goal was to determine how much, if any, change in the ion drift velocity occurs in the expanded plasma. The arc discharge current was perturbed to create plasma density markers whose travel is picked up by the set of probes. It was found that the perturbation with current oscillations did not result in consistent data because ion current maxima and minima are determined not only by the plasma production but also by the transients of the arc pulse and by the asymmetry of the ion velocity distribution function. Perturbation with a short current spike was more conclusive. The most likely ion velocity, which depends on the cathode material but is generally (1–3) × 104 m s−1, was measured to be reduced (about 25%) with increasing distance (0.5 m) from the cathode, which can be explained by collisions of ions with the background of neutrals. The ion velocity was slightly increased when the arc current was increased (about 15% when going from 50 to 400 A), which correlated with enhanced arc voltage and power dissipation. The ion velocity could be enhanced (by about 20%) when the plasma was produced in a non-uniform magnetic field (up to 300 mT).