Cathode Spot Crater Research Articles

Simulation of cathode spots crater formation on electrodes with micro-scale and nano-scale alloy materials in a vacuum arc

In the field of vacuum interrupting, an important research is to improve the erosion resistance properties of contacts. In this paper, a two-dimensional axisymmetric swirl model based on basic parameters has been proposed to describe the formation and development of cathode spots of micro-scale alloy and nano-scale alloy electrode in a vacuum arc. In this model, the peak ion density of the plasma cloud, the mean charge state in near-cathode region, the electron temperature, and the near-cathode voltage drop are adopted as external parameters. Ions and electrons originating from the plasma cloud, thermo-field (T-F) emission, vaporization of metal atoms, backing ions and back-diffused electrons are all taken into consideration. The morphology and temperature of cathode spots crater and the liquid metal velocity of pure copper electrode (Cu), copper-chromium (Cu-Cr) electrode and tungsten-copper (W-Cu) electrode under the conditions of micro-scale grains and nano-scale grains are compared, respectively. Simulation results show that the cathode spot crater of the Cu-Cr electrode is smaller than that of the pure copper electrode. Moreover, it can hold more current and inhibit the formation of new cathode spots nearby. Cathode spots appearing on tungsten grains are smaller in size and have a lower probability of forming liquid metal droplets than Cu-Cr electrodes. Contacts made of nano-scale grain alloys have stronger erosion resistance properties than that of micro-scale alloy contacts and can reduce the maximum temperature of the cathode. The simulation results are in good agreement with other researchers’ results.

Numerical simulation of cathode-spot crater formation and deuterium desorption process on hydrogen titanium cathode with flux boundary conditions and different impregnation degree

In this paper, the crater formation process of single cathode spot on hydrogen titanium electrode is modeled. In this model, combined with the deuterium diffusion equation, the flux boundary condition was considered to calculate the cathode spot desorption rate with different impregnation degree, and the deformation process of the cathode spot crater was simulated by a hydrodynamic model. The simulation results show that the current, size, and depth of a single cathode spot crater tend to decrease with increasing impregnation degree, which is consistent with many related experimental results. The simulation result shows that the desorption of deuterium in a single cathode spot crater mainly occurs in the ignition stage, and the positions of desorption are successively distributed in the liquid metal flow area on the side wall of the crater and the high temperature area in the center of the crater. The desorption rate of deuterium drops rapidly after cathode spot quenching. At the same time, the increase of arc current and impregnation degree can improve the deuterium desorption rate of a single cathode spot.

Mie scattering based on-line measurement of droplet from vacuum arc

Metal droplet is produced accompanied with vacuum arc discharge, which is important to the research of cathode spot and the application of vacuum arc. The droplet comes from the cathode spot crater and can reflect the physical process of the cathode spot. However, it will destroy the uniformity of surface deposition in engineering and should be avoided as much as possible. The measurement of metal droplet usually adopts off-line collector, which cannot obtain the signal of the whole space and singe arc. In order to on-line measure the droplet, a new method by the Mie scattering is developed in this work, and its feasibility is investigated. The characteristic of the scattering light of titanium droplet is computed by the simulation code. The results indicate that the scattering light beams of the small droplet are distributed at all angles. With the increase of the diameter, the scattered light beams are more and more concentrated in the forward direction, which allows the inversion of the signals of the droplets with different diameters. Then the detector is designed with different annuluses. When the detector is divided into 35 annuluses, the light energy coefficient matrix is easy to solve and the measurement system has a good resolution. The experimental setup is built and the preliminary experiment is carried out. The results indicate that the diameters of titanium droplets are mainly around 9.8 μm, which verifies the effectiveness of the Mie scattering method of measuring vacuum arc droplets. However, the small droplet information is not detected, so the droplet diameter distribution is quite different from the off-line measurement. The reason is that the signal-to-noise ratio of the measurement system is poor, thereby leading the scattered signals of the small droplet to fail to be obtained effectively. The experimental setup need to be further optimized.

Determination of Cooling Rate and Temperature Gradient during Formation of Cathode Spot Craters in a Vacuum Arc

Due to the extreme thermal conditions and short lifetimes, experimental exploration of cathode spots in vacuum arcs is very difficult. The intensive heat in the cathode spot is believed to be generated by ion bombardment and by Joule heating. However, thermal conditions occurring inside the re-melted material in craters created by cathode spots are not accurately known. During the exposure to cathodic arc plasmas, an Al-Cr cathode’s surface was locally melted by successive ignition and extinction of cathode spots. The melted layer, that quickly solidified, was characterized by the formation of several thin layers with a thickness of a few micrometers that were stacked on top of each other. The corresponding solidification patterns displayed cellular and dendritic microstructures. A phase field-based model was used to simulate and determine the thermal process conditions that led to the dendritic structures observed within the re-melted layer. Different combinations of cooling rates and temperature gradients were numerical explored to determine the most probable thermal conditions under which the cathode material re-solidifies. The results showed that the material in the vicinity of the cathode spot crater re-solidified under the condition of a cooling rate of about 3 × 105 K/s and a temperature gradient of about 6 × 107 K/m. These results constitute valuable data for the validation of numerical models dedicated to cathode spot formation.

Numerical simulation of the interaction between double cathode spot craters in vacuum arc

In this paper, a numerical simulation on the interaction between the double cathode spot craters in the vacuum arc is carried out. By establishing a double-cathode spot ablation model, the crater development process when two spots coexist is simulated, and the formation mechanism of cathode spot groups is analyzed. The simulation results show that the two cathode spots appearing on the flat electrode would squeeze each other to form a liquid metal ridge, which changed the ablation morphology of the cathode spots. When the double-spot arc craters appear simultaneously, the metal ridge will be squeezed into a straight line, otherwise, when the spots appear one after another, the liquid metal ridge will shift toward the side of the spot that appears first. The relative experimental results are adopted to compare with the simulation results. By comparison, it is found that the morphology of the multi-spot crater in the experiment is in agreement with the simulation results.

Model of Cathode Spots Crater Formation and Deuterium Desorption Process on Zirconium Deuterium Electrode

In this article, the model including cathode spot process and deuterium desorption process on zirconium deuterium cathode is developed. Five kinds of particle fluxes on cathode surface and three mechanisms affecting deuterium diffusion in cathode are considered. The model not only describes the deformation of cathode metal, but also describes the variation of deuterium concentration with time. On pure zirconium cathode, the mean charge state is larger but on zirconium deuterium cathode, the total ion number density is larger since there are additional deuterium ions, and simulation results show the discharge currents are very close on the two cathodes, the energy flux density on ZrD0.66 cathode is larger so cathode spot crater develops faster on ZrD0.66 cathode. Ablation rates caused by different mechanisms are calculated and the results show in cathode spot development process, mass loss caused by evaporation is ignored, most of the liquid metal will resolidify on cathode surface, and droplets are the main reason for cathode ablation. The ablation rates due to the ejection of droplets are close to experimental results. The ratios of deuterium ion from different mechanisms in this process are also calculated and it is 93.5% in simulation. Simulation results are in agreement with the other researchers’ works.

Improved model for cathode spot crater in vacuum arc

An improved two-dimensional axisymmetric swirl model based on former work has been developed to describe the detailed physical process in the formation and development of vacuum arc cathode spot. In the current model, the influence of plasma cloud, evaporation, backing ion, thermo-field (T-F) emission, back-diffused electron, self-generated magnetic field and Lorentz force are all considered. External parameters include electron temperature, near cathode voltage drop, average charge number, ion number density, space function and time function. The compositions of current density, energy flux density and external pressure are discussed and the influence of external parameters are studied. Simulation results show that electron current density from T-F emission is the most important component for current density; as for energy flux density and pressure, ion flux from plasma cloud is dominant. Ablation on a refractory metal is much less than that on a fusible metal under the same external conditions. The simulation results are in good agreement with other researchers’ results.

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.

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.

Cathode spot craters in pulse vacuum arc cleaning of a metal surface oxide layer

Cathode spot behavior during vacuum arc cleaning (VAC) of a metal surface covered with an oxide layer is investigated using a pulse discharge. During VAC, a bright spot is visible. A dot-like trace is marked on the cathode surface by the removed oxide layer. Detailed surface analyses show that the surface roughness is increased and that the oxide layer is stripped off almost uniformly in the trace. The cathode spot size adjustment is more precise than in our previous experiment. Spot characteristics change concomitantly with variation of the discharge electric power.

Laser measurement of copper vapor density after a high-current vacuum arc discharge in an axial magnetic field

The copper vapor density after the occurrence of a magnetically stabilized high-current vacuum arc was measured by the laser absorption method. Measurements were performed in a vacuum triggered gap with oxygen-free, axial magnetic field (AMF) Cu electrodes (40-mm-diameter cathode and 60-mm-diameter anode), separated by a gap of 30 mm. The arc current was a sinusoidal wave having a half-period of approximately 10 ms and a peak value ranging from 3 to 10 kA. The arc was stabilized by a magnetic field produced by the AMF electrodes. The copper vapor density at current zero increased with peak arc current and did not depend on the current zero slope. After current zero, the copper vapor density decayed with a time constant of approximately 400 /spl mu/s. These results can be explained by a model based on vapor expansion from cathode spot craters.

Measurement of the Copper Vapor Density in a Magnetically Stabilized High-current Vacuum Arc by the Laser Absorption Method

The high-current interruption ability of the vacuum interrupter depends on the state of the residual plasma remaining between electrodes after an current zero. In this study, the copper vapor density around the current zero with a half-period of 1 ms and a peak current of 3 kA arc was measured using the laser absorption method. The copper vapor density found to be 8×1017 m-3 at current zero and was found to be independent of current zero gradients and arc current peak. After current zero, the copper vapor density decayed with a time constant of approximately 400 μs. The decay time observation by LIF signal agreed well with that of the absorption method. This decay time of the copper vapor density can be explained based on the calculation of the cooling characteristics of the cathode spot crater which supplies copper vapor.

Evidence of columnar diamond growth structures within cathode spot craters of vacuum arcs on carbon

Cross-sections of erosion craters formed by vacuum arc discharges on graphite cathodes are examined using electron microscopy. These reveal erosion craters that are covered with a growth layer forming a continuous film on the arced surface. The film surface is composed of sphere-like structures similar to cauliflower diamond and to the emitted macro-particles (MPs) observed in coating experiments using graphite vacuum arc sources. The film thickness within the cathode erosion craters evolves with the mean values of the MP diameter and mean crater depth for four graphite material morphologies. These two parameters increase with a decrease in arc spot velocity. The growth layer shows a relatively porous columnar structure, with micro-Raman spectroscopy indicating an evolution from amorphous carbon (cathode base), to graphitic (middle), to diamond dominating the top layer.

Formation and Deformation of Multiwall Carbon Nanotubes in Arc Discharge

Running arcs under both vacuum and low pressure and a jumping arc under vacuum were generated with a pure graphite cathode for 1–3 s upon applying a magnetic field. For the running arcs under both vacuum and low pressure, multiwall carbon nanotubes (MWNTs) were observed only at the cathode spot craters where the arcs were extinguished by switching the power off. At the crater where the cathode spot had run over, nanotubes were hardly observed. For the jumping arc in vacuum that was self-extinguished, only a few nanotubes were observed at the cathode spot craters. No nanotubes were observed on the surface outside the cathode spot crater for all arcs. These results revealed that the nanotubes are formed, as well as destroyed, by the cathode spot. The process of nanotube formation and deformation is discussed, taking into account the cathode spot activity and electron emission.

Vacuum arc cathode spots as a self-similarity phenomenon

The determination of the basic physical parameters of vacuum arc cathode spots is very difficult due to their small dimensions and short existence times. Values given in the literature for the spot diameter range from less than 10 /spl mu/m up to some hundred micrometers, and spot existence times vary from nanoseconds to some microseconds, caused by the use of different measuring techniques. Using specially designed optical equipment for emission photography, we found structures of cathode spot clusters (short exposure times) and cathode spot traces (long exposure times) which are similar on different spatial scales covering the range from hundreds of micrometers down to 10 /spl mu/m. The light spot traces were found to be similar to scanning electron microscope (SEM) pictures of cathode spot craters, the latter having a higher spatial resolution. The self-similarity over three ranges of magnitude in spatial resolution shows the fractal nature of vacuum arc cathode spot traces.

Plasma expansion as a cause of metal displacement in vacuum-arc cathode spots

The pressure on the molten surface of a vacuum-arc cathode due to the recoil from outward-directed ion jets is calculated for copper arcs. The calculation uses current density data and electrode force per unit current data from experiment. In addition, the force per unit current measured by Tanberg is shown to be consistent with a calculation of the same quantity based on the energy of outgoing ions measured by Davis and Miller and the so-called saturated positive-ion current measured by Kimblin. The ion recoil pressure is shown to be sufficient to remove molten metal from a cathode-spot crater in time of the order of 25–250 nsec with a velocity of 2×103−2×104 cm sec−1. It is shown that motion of molten metal in the cathode-spot crater must be considered as a first-order effect in rigorous calculations of surface temperature and heat flow in the metal in contact with the cathode-spot plasma. It is suggested that the rapid removal of metal by the plasma pressure causes molten droplets to be ejected, as has been observed experimentally, and causes a preference for the cathode to operate, after the liquid is ejected, on the rim of the crater or nearby on the surface where hotter metal may exist due to liquid-metal overflow and spatter.