Cathode Spot Research Articles

Simulation of cathode spot crater formation and development on CuCr alloy in vacuum arc

The two-dimensional (2D) rotary axisymmetric model is used to describe the formation and development of a cathode spot on a copper-chromium alloy (CuCr) in a vacuum arc. The model includes hydrodynamic equations and the heat transfer equation. Parameters used in this model come from experiments and other researchers' work. The influence of parameters is analyzed, and the simulation results are compared with pure metal simulation results. In simulation, the depth of the cathode crater is from 0.5 μm to 1.1 μm, the radius of the cathode crater is from 1.6 μm to 2.6 μm, the maximum velocity of the droplet is from 200 m/s to 600 m/s, and the maximum temperature is from 3500 K to 5000 K which is located in the area with a radius of 0.5–1.5 μm. The simulation results show that a smooth cathode surface is advantageous for reducing ablation, the ablation on the CuCr alloy is smaller than that on the pure metal cathode electrode, and the cathode spot appears on the chromium grain only on CuCr. The simulation results are in good agreement with the experiment.

Cathode Spot Phenomena Observation by High Speed Camera in Variable Polarity Aluminum Arc Welding

Cathode Spot Phenomena Observation by High Speed Camera in Variable Polarity Aluminum Arc Welding

Highly porous micro-roughened structures developed on aluminum surface using the jet of rotating arc discharges at atmospheric pressure

Aluminum 6061 samples were exposed to the jet of an atmospheric pressure rotating arc discharge operated in either nitrogen or air. After multiple passes of treatment with an air-based plasma jet at very short source-to-substrate distances, scanning electron microscopy combined with x-ray photoelectron spectroscopy revealed a highly porous micro-roughened alumina-based structure on the surface of aluminum. Based on optical emission spectroscopy and high-speed optical imaging of the jet interacting with aluminum samples, it was found that the process is mainly driven by the energy transfer from the plasma source to the surface through transient plasma-transferred arcs. The occurrence of multiple arc discharges over very short time scales can induce rapid phase transformations of aluminum with characteristics similar to the ones usually observed during laser ablation of materials with femto- to nanosecond laser pulses or during the formation of cathode spots on the surface of metals.

Study of the ionization rate of the released deuterium in vacuum arc discharges with metal deuteride cathodes

The ionization rate of the released deuterium from a metal deuteride cathode in vacuum arc discharges is investigated by both experiments and modeling analysis. Experimental results show that the deuterium ionization rate increases from 2% to 30% with the increasing arc current in the range of 2–100 A. Thus the full ionization assumption, as is widely used in arc plasma simulations, is not satisfied for the released deuterium at low discharge current. According to the modeling results, the neutral-to-ion conversion efficiency for the deuterium traveling across the cathodic spot region can be significantly less than one, due to the fast plasma expansion and rarefaction in the vacuum. In addition, the model also reveals that, unlike the metal atoms which are mainly ionized in the sheath region and flow back to the cathode, the deuterium ionization primarily occurs in the quasi-neutral region and moves towards the anode. Consequently, the cathodic sheath layer acts like a filter that increases the deuterium fraction beyond the sheath region.

Influence of a Scanning Radial Magnetic Field on Macroparticle Reduction of Arc Ion-Plated Films

Cathode spot motion influences the physical characteristics of arc plasma and the related macroparticles (MPs) in resultant films; these MPs limit the application of arc ion plating (AIP). In this paper, a scanning radial magnetic field (SRMF) was applied to the cathode surface to control the cathode spot motion and reduce the MP contamination in the deposited films. It was shown that film surface morphologies prepared using SRMF were better than those using a static radial magnetic field (RMF). The improvement was greater with increased scanning range and frequency. Using SRMF, cathode spot motion was confined to a spiral trajectory on the cathode surface and the spots moved over a large area and at a fast-moving velocity. Both the large moving area and the fast velocity decreased the temperature on the cathode surface and thus reduced the emission of the MPs.

Control of vacuum arc source cathode spots contraction motion by changing electromagnetic field

This paper investigates the magnetic field component impact on cathode spots motion trajectory and the mechanism of periodic contraction. Electromagnetic coils and permanent magnets were installed at the different sides of cathode surface, the photographs of cathode spots motion trajectory were captured by a camera. Increasing the number of magnets and decreasing the distance between magnets and cathode both lead to enhancing cathode spots motion velocity. Radii of cathode spots trajectory decrease gradually with the increasing of electromagnetic coil’s current, from 40 mm at 0 A to 10 mm at 2.7 A. Parallel magnetic field component intensity influence the speed of cathode spots rotate motion, and perpendicular magnetic field component drives spots drift in the radial direction. Cathode spot’s radial drift is controlled by changing the location of the ‘zero line’ where perpendicular magnetic component shifts direction and the radius of cathode spots trajectory almost equal to ‘zero line’.

Structure of the Welding Arc Cathode Spot with a Nonconsumable Electrode

The cathode spot autographs on a glass surface are investigated through an electron microscope, with a magnification up to 100000. We discover that the welding arc cathode spot consists of separate cells with the substructure, current channels, 10–60 nm in diameter. We acquire color photographs of the welding arc cathode spot with the cathode torch. Photograph processing through different filters reveals the shape of the cathode spot and the cathode torch. We perform the estimate calculations of the current density in the welding arc cathode spot substructure.

Formation of liquid-metal jets in a vacuum arc cathode spot: Analogy with drop impact on a solid surface

Conditions of the liquid-metal jets formation in a cathode spot of a vacuum arc discharge are studied. Our consideration is based on the analogy between the processes, occurring in the liquid phase of the cathode spot, and the processes, accompanying a liquid drop impact on a flat solid surface. In the latter case there exists a wide variety of experimental data on the conditions under which the spreading regime of fluid motion (i.e., without formation of jets and secondary droplets) changes into the splashing one. In the present work, using the hydrodynamic similarity principle (processes in geometrically similar systems will proceed similarly when their Weber and Reynolds numbers coincide), criteria for molten metal splashing are formulated for different materials of the cathode. They are compared with the experimental data on the threshold conditions for vacuum arc burning.

Behaviors of Cathode Spot in Alternative Current Helium TIG Welding of Aluminum

Behaviors of Cathode Spot in Alternative Current Helium TIG Welding of Aluminum

Motion and production of microparticles in vacuum interrupter

Vacuum circuit breakers are widely used in medium-voltage class. In vacuum circuit breakers, restrikes and non-sustained disruptive discharges sometimes occur after current interruptions. It is said that these late breakdowns are triggered by the microparticles. This paper aims to investigate the relationship between the motion of microparticles and subsequent late breakdown phenomena. It does by artificially injecting microparticles into the vacuum gap and observing their motion using a highspeed video camera. We focused on the sputtering of the electrode material during the current interruption and the mechanical stress of the switching operation as being the origin of these microparticles. We found that the motion of microparticles can be grouped into two distinct patterns: bouncing and attaching. Bouncing microparticles seem to be made of metal, while most of the attaching microparticles attached themselves to the cathode, and their charge was about five times greater than that of the bouncing microparticles. The attaching microparticles seem to be positively charged by secondary electron emissions from the surface under electron bombardment due to the field emission current from the cathode. From these results, we believe that the surfaces of the attaching particles are covered with an insulating layer. From our observations of the production of microparticles by mechanical stress, we have found that the breakdown voltage of the gap drastically drops after a mechanical switching operation where microparticles are produced. The breakdown voltage is restored with repetitive mechanical switching operations and subsequent voltage application. However, the breakdown voltage drops again when the operating speed of the electrode is increased. We also found that microparticles are scattered from the cathode spots on electrodes that have not been current-conditioned, and they are hardly produced from electrodes that are current-conditioned. From these results, we believe that controlling the speed of the mechanical switching operation and the current conditioning are effective ways to prevent the microparticles from being produced.

Average current per vacuum-arc cathode spot and spot velocity in a magnetic field on a CuCr50/50 nanocomposite

It has been shown that such cathode spot characteristics as the average current per spot and its dependence on tangential magnetic-field induction B t and the spot velocity and its dependence on B t for two CuCr50/50 specimens with very different structures (nanocomposite and “solid-state sintered” composite) almost coincide if the surface of contacts has been totally remelted before measurements with the use of moderate arc currents in the process of conditioning.

Numerical Simulation of Plasma Near the Cathode Spot of Vacuum Arc

Kinetic numerical simulation of plasma near the cathode spot (CS) was performed. Two types of possible solutions for the self-sustained CS on copper cathode were found. The firs type of solutions is valid for quasi-stationary spot with radius of about $100~\mu \text{m}$ . Stationary solution in this case is possible with cathode temperature 4.2–4.5 kK, current density $\sim 10^{6}$ A/cm2, and applied voltage 20–25 V. The second type of solution is obtained for the spot with radius of $1~\mu \text{m}$ and spherical geometry of plasma expansion. Possible solutions were obtained for the cathode temperature of above 6.2 kK, current density of about $10^{8}$ A/cm2 and applied voltage of 15–25 V. Transient simulations of the near-cathode plasma expansion have shown that if current density remains less than $10^{9}$ A/cm2 then the plasma expansion has a “quiet” self-similar character. At higher current density the plasma expansion has an essentially nonstationary character. The high-density current leads first to the development of Buneman instability and second to the plasma rupture with the creation of conditions for collective acceleration of ions toward the cathode and toward the anode. The accelerated ions create an additional powerful heat flux to the cathode, which should facilitate the reproduction of the CSs in the spark stage of vacuum discharge.

On transition from diffuse mode to the constricted one with high-current cathode spot in overvoltage open discharge in D2

So called “open discharges” in a narrow gap between the solid cathode and grid anode are widely used for generation of the pulsed high-current electron beams with energy up to 100 keV. The need to get high-energy e-beams leads to the necessity in using of strong overvoltage of the short gas gap with the reduced electric field of the order of 105 Td or higher. The discharge under strong overvoltage is unstable and tends to transit into high-current regime with low voltage. In the case of the open discharge in D2 at low pressure (about 0.5-2 Torr) and powered by stepwise voltage with amplitude up to 25 kV we revealed that this discharge exhibits two diffuse regimes which follow one by one and finally transits into the constricted mode with formation of high-current spots on the cathode. The physical properties of these gas discharge regimes have been explored in detail with the usage of the fast multi-frame camera synchronized with the current and voltage of discharge. Our findings promote more insight into physics of the overvoltage open discharge generating the e-beams with energy up to 25 keV.

Electric field formation in three different plasmas: A fusion reactor, arc discharge, and the ionosphere

Three examples of electric field formation in the plasma are analyzed based on a new mechanism driven by ion-neutral collisions. The Gyro-Center Shift analysis uses the iteration of three equations including perpendicular current induced by the momentum exchange between ions and neutrals when there is asymmetry over the gyro-motion. This method includes non-zero divergence of current that leads the solution of time dependent state. The first example is radial electric field formation at the boundary of the nuclear fusion device, which is a key factor in the high-confinement mode operation of future fusion reactors. The second example is the reversed rotation of the arc discharge cathode spot, which has been a mysterious subject for more than one hundred years. The third example is electric field formations in the earth's ionosphere, which are important components of the equatorial electrojet and black aurora. The use of one method that explains various examples from different plasmas is reported, along with a discussion of the applications.

Detailed numerical simulation of cathode spots in vacuum arcs: Interplay of different mechanisms and ejection of droplets

A model of cathode spots in high-current vacuum arcs is developed with account of all the potentially relevant mechanisms: the bombardment of the cathode surface by ions coming from a pre-existing plasma cloud; vaporization of the cathode material in the spot, its ionization, and the interaction of the produced plasma with the cathode; the Joule heat generation in the cathode body; melting of the cathode material and motion of the melt under the effect of the plasma pressure and the Lorentz force and related phenomena. After the spot has been ignited by the action of the cloud (which takes a few nanoseconds), the metal in the spot is melted and accelerated toward the periphery of the spot, with the main driving force being the pressure due to incident ions. Electron emission cooling and convective heat transfer are dominant mechanisms of cooling in the spot, limiting the maximum temperature of the cathode to approximately 4700–4800 K. A crater is formed on the cathode surface in this way. After the plasma cloud has been extinguished, a liquid-metal jet is formed and a droplet is ejected. No explosions have been observed. The modeling results conform to estimates of different mechanisms of cathode erosion derived from the experimental data on the net and ion erosion of copper cathodes.

Modeling of cathode spot crater formation and development in vacuum arc

A two-dimensional (2D) rotary axisymmetric model has been developed to describe the formation and development of cathode spot in vacuum arc. The model includes hydrodynamic equations and heat transfer equation which considers surface evaporation and Joule heating. Parameters used in this model, such as the distributions of pressure, energy flux density, and current density, come from experiments and other researchers’ work. In this model, cathode spot maintains 30 ns and during this time, all parameters are fixed. The simulation results show that when the energy flux density is 1.5–3 × 1012 W, discharge current is 1–6 A and the pressure is 1–3 × 108 Pa, the crater radius is 1.4–4.1 µm, the crater depth is 1.4–2.1 µm, the velocity of liquid metal is 154–428 m s−1 and the maximum temperature is 2145–5342 K which is located in the area with radius 0.5–1.5 µm. Besides, on the chromium cathode, the maximum temperature is higher mainly because of the lower thermal conductivity.

A thorough numerical model of cathode spots challenges conventional wisdom

A new model of cathode spots in vacuum arcs shows that interactions of vacuum arc plasma with the cathode surface do not necessarily lead to explosions.

Research on unusual cathode erosion patterns in the process of filtered cathodic vacuum arc deposition

The cathode spot is crucial to vacuum arc technology and the cathode spot theories have been widely studied. However, the study on a type of unusual erosion patterns is still very rare so far. For blocking this erosion patterns’ formation, a new cathode water cooling system with higher cooling efficiency was designed. The paper studies the formation of this erosion patterns by macro-particle filtered vacuum arc plasma, arc spots splitting, and arc spots movement velocity in pure argon gas and mixture gas (oxygen and argon). The ratio of oxygen to argon is from 0 to 2.5. Meanwhile, energy dispersive spectrometer (EDS) experiment was completed on the cross-section of this erosion pattern to study on the oxygen and other trace elements’ redistribution. In addition, an erosion model was proposed to explain the format process of these etching patterns. The results of this paper make a contribution to the theory of arc etching.

Dynamics of cathode spots in an axial magnetic field on the fall of a current pulse in a high-current vacuum arc

The cathode attachment of a high-current vacuum arc on the fall of a current pulse was studied. Contacts generating an axial magnetic field were used. The revealed transformation of the attachment shape near zero current was explained quantitatively in a way that was in satisfactory agreement with experimental results.

Effect of vacuum arc cathode spot distribution on breaking capacity of the arc-extinguishing chamber

A DC circuit breaker performs a key function in breaking an intermediate-frequency (IF) current since breaking a pure IF current is equivalent to breaking a very small DC with a reverse IF current. In this study, it is found that cathode spots show a ring-shaped distribution at 2000 Hz. An arc with an uneven distribution of cathode spots has been simulated. The simulation results show that the distribution of cathode spots significantly affect the microparameter distribution of arc plasma. The current distribution on the anode side differs from that on the cathode side under the total radial electric field. Specifically, the anode current distribution is both uneven and concentrated. The applied axial magnetic field, which cannot reduce the concentrated anode current distribution effectively, might increase the concentration of the anode current. Finally, the uneven distribution of cathode spots reduces the breaking capacity of the arc-extinguishing chamber.