Cathode Spot Of Vacuum Arc Research Articles

Research on the influence of grain size of CuCr contact on the characteristics of cathode spot and cathode surface erosion in vacuum arc

CuCr alloy is widely used in vacuum interrupters. The characteristics of cathode spot and erosion of cathode surface directly affects the lifetime and breaking capacity of vacuum interrupter. In order to control the cathode spot and crater, two types of contacts with different Cr phase size were used in this paper. The vacuum arc breaking experiments were conducted. The cathode spot images were recorded by ultra high speed camera and craters were measured by Scanning Electron Microscope and Profiler. It was found that the smaller the Cr phase size, the higher the expansion velocity of cathode spots, the larger the number of cathode spot, the smaller the cathode spot size and the lower the current carried by each cathode spot. The reduction of Cr phase size helps to improve the uniform distribution of cathode spots on the contact surface. Besides, the smaller Cr phase size leads to the decrease of radius and depth of crater. For CuCr contacts, thermal diffusion process was analyzed and there is a significant difference in the thermal diffusivity between Cu and Cr. The Cr phase uniformly distributed in Cu prevents the thermal diffusion process which reduces the size of crater and the erosion of cathode surface.

Threshold parameters of vacuum arcs with W-fuzz cathodes

An experimental technique was developed and performed to investigate the threshold currents and operation times of vacuum arcs with tungsten cathodes having a nanostructured (so-called “fuzz”) surface. The fuzz layer, pre-formed due to exposure of the tungsten samples with helium plasma, reduced the threshold current for arc initiation by an order of magnitude. For currents of tenths of an ampere, the average arc operation time was in the microsecond range. The experimental data were interpreted using the explosive emission (ecton) model of a vacuum arc cathode spot.

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.

Investigation of Vacuum Arc Droplets From Copper, Titanium, and Tungsten by Means of Light Scattering

Studies of molten droplets emitted from cathode spots of vacuum arcs on copper, tungsten, and titanium have been carried out using a time-of-flight measurement procedure. In the arrangement, two light barriers at different distances from the cathode surface are used. When particles traverse the barriers, the appearance of scattered light can be used to establish the flight time and, thereby, the velocity of particles. Simultaneously, the intensity of scattered light provides a measure of the size of the particle by applying the Mie-scattering theory together with calibrations. By turning the cathode surface with respect to the drift tube axis, an angular resolution of the parameters was achieved. Algorithms were developed for automated processing of the measured data to identify particles arriving from the arc spot unobstructed. Distributions of the particle sizes, a statistical relation between the sizes and the emission velocities of particles, and other properties are discovered for all three cathode materials.

Determination of the critical temperature of Nb–Al alloys from the plasma parameters of a vacuum arc cathode spot

The plasma of pulsed-periodic splashes of explosive electron emission forms a vacuum arc cathode spot. The average parameters of the plasma can be evaluated from the parameters of the critical state of a metal for different materials. According to the measurements (Zöhrer et al 2020 Plasma Sources Sci. Technol. 29 025022) of the average charge and the kinetic energy of plasma ions, the critical temperature and cohesive energy for Nb–Al alloys was evaluated within our model. It was close to the value corresponding to pure aluminum for all component ratios Nb/Al = ¼, ⅔, ¾. We have described the experimentally measured values of the cathode potential fall using the obtained magnitudes of critical temperature in the empirical ‘cohesive energy rule’ and have reproduced the observed deviation from linearity in form of U or V shaped dependencies. The obtained results make possible to evaluate the critical temperature and cohesive energy from the parameters of the vacuum arc plasma.

Numerical simulation of low-current vacuum arc jet considering anode evaporation in different axial magnetic fields

As the main source of the vacuum arc plasma, cathode spots (CSs) play an important role on the behaviors of the vacuum arc. Their characteristics are affected by many factors, especially by the magnetic field. In this paper, the characteristics of the plasma jet from a single CS in vacuum arc under external axial magnetic field (AMF) are studied. A multi-species magneto-hydro-dynamic (MHD) model is established to describe the vacuum arc. The anode temperature is calculated by the anode activity model based on the energy flux obtained from the MHD model. The simulation results indicate that the external AMF has a significant effect on the characteristic of the plasma jet. When the external AMF is high enough, a bright spot appears on the anode surface. This is because with a higher AMF, the contraction of the diffused arc becomes more obvious, leading to a higher energy flux to the anode and thus a higher anode temperature. Then more secondary plasma can be generated near the anode, and the brightness of the ‘anode spot’ increases. During this process, the arc appearance gradually changes from a cone to a dumbbell shape. In this condition, the arc is in the diffuse mode. The appearance of the plasma jet calculated in the model is consistent with the experimental results.

Correlation of radius of cathode spot of vacuum arc of metals on the size of generated droplets

The simple relation to estimate the cathode spot radius of a vacuum arc of pure metals is obtained. On its basis, is established between the cathode spot radius and the size of droplets generated by the cathode spot a correlation. This enables to find ways to reduce droplets in the plasma flow, which forms coatings by the vacuum electric arc method. The paper presents the results of experimental study of the droplet sizes depending on the vacuum arc current iд. The size and amount of the droplets on an area of 1 mm2 of the coating surface are determined using the ImageSP program. As the initial data, the microstructures of the coatings are used with an increase of: ç100, ç200, ç500, ç1000, ç1500. The droplets have been generated by a cathode spot of a vacuum arc for the alloy of the composition, at.%: 68Al–8Cr–4Nb–20Si. It is established that the number of droplets with a diameter of < 2 μm is generated most of all, and the number of droplets with a diameter > 10 μm is generated least of all. The number of generated droplets with a diameter from 2 to 10 μm slightly depends on the arc current iд. It is noted that the diameter of the alloy droplet is smaller than the diameter of the droplets generated by the cathode spot on its components due to the fact that the radius of the cathode spot on the alloy is smaller than the radius of the cathode spot on its pure components.

Experimental Investigation on the Current Carried by a Cathode Spot of Vacuum Arc in Axial Magnetic Fields

The splitting and threshold currents of vacuum arc cathode spots in axial magnetic fields (AMFs) up to 220 mT are investigated experimentally. Measurements are made on electrodes of Cu and CuCr25 with electrode separations of 2 and 6 mm. The electrode diameter is 30 mm. All the experiments are conducted in a demountable vacuum chamber evacuated continuously by a turbo-molecular pump to a pressure of the order of 10−3 Pa. The spots are photographed using a high-speed digital camera with an exposure time of $2~\mu \text{s}$ . The AMF is applied by means of two permanent magnets placed symmetrically inside the chamber. The experimental results indicate that the splitting and threshold current decrease in the presence of AMF and tend to certain constant values when the field gets stronger. The splitting and threshold currents are considerably larger than Cu electrodes than those with CuCr25 electrodes. With increasing electrode separation, the splitting current decreases while the threshold current increases. Possible explanations are proposed to account for the experimental observations.

Simulation of vacuum arc cathode spot movement between transverse magnetic field contacts

Vacuum switches have been extensively used in the field of medium voltage. Carrying out relevant research on vacuum arc is particularly important for improving the performance and reliability of vacuum switches. The vacuum arc cathode spots provide metal vapor and electrons, which determine the breaking capacity of the vacuum switches to some extent. Therefore, based on the simulation model of the cathode spots between the transverse magnetic field (TMF) contacts, the motion phenomenon and distribution of the cathode spot were simulated and the characteristics of the initial diffusion stage of cathode spots between TMF contacts were studied. The simulation results showed that the probability of generating a cathode spot in the direction of retrograde motion increased with the increase in the TMF component and the direction of motion varied with the angle between the magnetic field and the contact. The diffusion process of cathode spots under wan-shaped contacts, cup-shaped contacts, and spiral-shaped contacts was simulated. The ring-shaped distribution of the cathode spots between cup-shaped contacts with a large current in the initial diffusion stage was formed by the self-generated magnetic field of cathode spots, while the axial magnetic field component and the clustered cathode spots destroyed the ring-shaped distribution. Taking the distribution of cathode spots and current from experiments as initial conditions, the simulation results of cathode spot distribution in the initial diffusion stage and the arc root radius were consistent with the experimental results.

Плазменные колебания в области катодного пятна вакуумной дуги

Плазменные колебания в области катодного пятна вакуумной дуги

Cathode and plasma phenomena in vacuum-arc sources of hydrogen isotope ions: I. Desorption of hydrogen isotopes during the operation of vacuum arc cathode spots

A model is proposed to describe the desorption of deuterium from a deuterated cathode during the operation of the vacuum arc cathode spot. The model treats a cathode spot as consisting of individual cells and involves a numerical simulation of the temperature fields that arise during the hydrodynamic processes responsible for the formation of microcraters on the cathode surface. Using a deuterated ZrD0.67 cathode as an example, it is shown that the amount of deuterium desorbed immediately from the crater formed during the operation of the cathode spot cells is several times smaller than the total amount of desorbed deuterium. The main portion of deuterium is desorbed from the hot cathode region adjacent to the crater and from the crater at the stage of its cooling. For a deuterated ZrD0.67 cathode, the percentage of desorbed deuterium can reach 80% of the total number of evaporated atoms, and it can be even greater if the cathode spot cells operate in the immediate vicinity of each other.

Two-dimensional particle simulation on the behavior of multi-charged copper ions in the cathode spot of a pulsed vacuum arc discharge

The behavior of multicharged ions in the cathode spot of pulsed copper vacuum arc is studied by the 2D3V electrostatic Particle-In-Cell Direct Simulation Monte Carlo method. This method tracks the position and velocity of electrons, neutrals, and copper ions charged from +1 to +4 simultaneously, which couples with external circuit physics as well as heat conduction at the cathode. The general thermofield electron emission developed in recent years is used in the vacuum arc simulation. The simulation starts from complete vacuum until the arc current reaches the steady state at about 3 A limited by the external circuit, and the arc voltage is between 20 and 30 V. During the discharge, the cathode temperature increases from room temperature to around 8000 K. The breakdown process is visualized by the distribution of ion density at different stages of arc discharge: from a small volume of cathode spot to a conductive current path between electrode gaps. The vacuum arc plasma is found to be highly ionized, with an average charge state slightly above two and electron density on the order of 1020 cm−3. The positively charged ions move in the direction from the cathode to anode, which is opposite to the direction of the applied external field. Ion energies at the steady state increase from 20 to 200 eV when charge states increase from +1 to +4. This indicates that the electrostatic acceleration of ions is caused by a dynamic space-charge field in the breakdown process of pulsed vacuum arc discharge.

Investigation of unstable motion characteristics of vacuum arc cathode spots between transverse magnetic field contacts

AbstractWith the improvement of the current level of power grids, the requirements of the opening level of the vacuum switches are also increasing. Vacuum arc cathode spots provide steam and electrons and, to a certain extent, determine the opening capacity of the vacuum switch. In this paper, a vacuum arc cathode spot research platform based on the de‐mountable vacuum chamber is constructed. The characteristics of the vacuum arc cathode spots under the transverse magnetic field (TMF) contacts are assessed by a high‐speed charge coupled device. The experimental results show that the cathode spot diffusion process can be divided into three processes through cathode spot distribution, arc voltage and current: initial diffusion stage of cathode spots, unstable motion stage of cathode spots, and extinguishing stage. The motion mode of cathode spots during unstable motion stage can be divided into cathode spots group stagnation (CSGS) to multi‐cathode jet (MCJ) switch mode, cathode spots group motion (CSGM) to MCJ switch mode, CSGM mode, and MCJ mode. The effects of peak current and contact diameter on unstable motion mode were analysed.

Numerical Investigation on the Robson Drift of a Single Cathode Spot of Vacuum Arc

The Robson drift phenomenon is a macroscopic reflection of the movement of a cathode spot (CS) in oblique magnetic field (OMF). Although it was discovered many years ago, its physical mechanism remains unclear. In this paper, the influence of OMF on the Robson drift of CS motion is simulated. A 2-D stepwise model of the movement of a single CS in OMF is established. It is assumed that the probability of generating a new spot in directions of Amperian force, anti-Amperian force, and directions perpendicular to Amperian force, is proportional to the magnetic pressure around the old spot. The drift angle and velocity of the CS in OMF are simulated. The simulation results show that the drift angle of the CS in OMF has liner relationship with the inclination angle of magnetic field, and it has no obvious relationship with the strength of the applied magnetic field and the amplitude of the arc current, which are in a good agreement with the experimental results. Besides, under the same angle of magnetic field, the velocity of spot motion increases linearly with the strength of OMF. Moreover, the simulation results show that the transverse magnetic field component has a much more predominant influence on the CS velocity than the axial magnetic field component, which are also in agreement with the experimental results.

Diameters and Velocities of Droplets Emitted From the Cu Cathode of a Vacuum Arc

The cathode spots of vacuum arcs emit material in the form of plasma as well as droplets. Generated by arcs burning at the first wall of fusion devices, the droplets may effectively contaminate the fusion plasma. Essential characteristics of the droplets (diameter, velocity, and emission direction) and their interrelations are poorly known so far. In this paper, a new approach of optical droplet investigation is presented. Emitted from a pulsed vacuum arc, the droplets fly vertically inside a drift tube against gravity, finally passing two consecutive light beams. The time-of-flight and the detected intensity of scattered light allow a simultaneous determination of droplet velocity and size. Different solid angle directions have been realized by turning the cathode with respect to the flight tube axis. Using this technique, the parameters of droplets emitted from a Cu cathode into different directions were obtained. Two distinct groups could be identified at smaller ( 20°) angles between the surface plane and emission direction, respectively. They exhibit different velocity distributions and different relations between the particle diameter and the emission velocity.

Revisiting Theoretical Description of the Retrograde Motion of Cathode Spots of Vacuum Arcs

A fresh attempt to develop a self-consistent description of the retrograde motion of cathode spots on volatile cathodes is undertaken. Three potential mechanisms of effect of transversal magnetic field on the distribution of parameters in the spot are studied: the effect of magnetic field on hydrodynamics processes in the spot, in particular, on the formation of liquid-metal jet and the droplet detachment, and the effect of transversal magnetic field over the motion of ions and emitted electrons in the near-cathode space-charge sheath. It is found that for typical conditions of cathode spots in vacuum arcs the effect of magnetic field over the formation of liquid-metal jet and the droplet detachment is negligible; the motion of the ions in the near-cathode space-charge sheath is not disturbed; and the motion of the emitted electrons is disturbed only marginally. Thus, the above-mentioned potential mechanisms are hardly relevant and the first-principle understanding is still missing. A phenomenological description of the retrograde motion is developed as an alternative. The description employs general considerations without relying on specific assumptions and the (only) unknown parameter can be determined from comparison with the experiment.

Vacuum Arc Cathode Spot Theory: History and Evolution of the Mechanisms

Vacuum arc cathode spot theories are reviewed, beginning from the primary studies in second half of the 19th century up to the present. The evolution of the main ideas is presented, starting from those, which developed, for separate phenomena, up to systematic inclusive models. New spot type classifications are proposed based on observed characteristics, instead of the previous simple numbering. Different approaches were considered for generating plasma comprised of cathode material, based on cathode vaporization and local explosions. Models with closed systems of equations were developed. A kinetic model advantageously allows calculating the cathode potential drop. These closed models explain various phenomena and two base rules were derived: 1) impeded plasma flow enables cathode spot operation and 2) the heat loss in the cathode must be smaller than the energy input to the cathode spot. Based on these rules, mechanisms for the spot types were derived and observed spot behavior in transverse and oblique magnetic fields was described.

Energy Balance on Current Density Distribution of Cathode Spot in Vacuum Arc

Energy Balance on Current Density Distribution of Cathode Spot in Vacuum Arc

Splashing conditions for a liquid metal in vacuum arcs: Cyclic processes in a cathode spot

Molten metal extrusion from craters that form on the cathode during vacuum arc burning is considered. Using the hydrodynamic similarity principle, this process is compared with the well-studied splashing process that can develop within the impact of drops impinging one by one on a solid surface (the arc cycle duration is identified with the inverse frequency of the drop train). Based on this analogy, the conditions under which the regime of spreading of a liquid over the cathode surface will change into the splashing regime (accompanied by the formation of microjets and droplets) are analyzed. As it turns out, the conditions realized in vacuum arc cathode spot at near-threshold currents are close to the splashing threshold of liquid metal. This gives grounds to relate the existence of a threshold arc current to the existence of a threshold for the process of liquid metal jet formation.

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.