Vacuum Arc Cathode Research Articles

Development of Numerical Analysis Method for Cathode Sheath Considering Electron Emission From Cathode in Vacuum Arc

ABSTRACTIt is imperative to develop the simulation of vacuum arcs as a tool to aid electrode design in vacuum circuit breakers. Although the development of electromagnetic thermo‐fluid simulations for vacuum arcs has been reported, most of them do not take cathode sheath phenomena into account and have not been able to reproduce vacuum arc phenomena, especially cathode spots. As the first step, the cathode sheath voltage, cathode surface electric field, and temperature and field (T‐F) electron emission current were analyzed numerically on the basis of the space charge in the cathode sheath. In this study, the cathode sheath was assumed to exist when the charge density induced on the cathode surface is positive, and the temperature and density of vacuum arc plasma and cathode temperature, which are physical quantities obtained by electromagnetic thermo‐fluid simulation, were used as parameters of the analysis. As a result, the cathode sheath voltage, electric field, and electron emission current density can be calculated from the vacuum arc plasma temperature, density, and cathode temperature. Numerical results show that the electron emission current density has a dominant effect on the presence or absence of the cathode sheath.

Development of Numerical Analysis Method for Cathode Sheath Considering Electron Emission from Cathode in Vacuum Arc

Development of Numerical Analysis Method for Cathode Sheath Considering Electron Emission from Cathode in Vacuum Arc

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 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.

A study on surface-temperature measurement by means of a vacuum-arc cathode under different current conditions

Accurate detection and analysis of the cathode surface temperature delivers an important reference value as regards the prediction of electrode life and corrosion degree, a thorough study of the vacuum-breakdown phenomenon, design and application of the vacuum-arc ion source and vacuum switch. In this paper, multispectral true temperature measurement T of the surface temperature field of pulsed vacuum arc cathode materials under different discharge currents is carried out. Firstly, aiming at solving the problem of synchronous imaging measurement of the temperature field across the cathode of pulsed vacuum arc, a multi-wavelength monochromatic image-acquisition optical path is designed, based upon the image acquisition module of a high-speed monochrome camera. Also, multiple monochromatic spectral radiation images of the target are synchronously acquired on the focal plane of the same monochrome camera detector. As a result, the spectral-radiation image information of multiple wavelengths can be obtained in a single exposure time, which greatly shortens the temperature-field image acquisition time. Secondly, based upon the multispectral true-temperature calculation method, high-speed multispectral imaging measurement of the target temperature field is realized. Finally, the two-dimensional true temperature distribution of the cathode temperature field under different current conditions was studied by setting up an experimental platform for measuring the cathode temperature of the vacuum arc under different current conditions.

Generation of a Metal Ion Beam Using a Vacuum Magnetron Discharge

We have designed, fabricated and characterized an ion source based on a vacuum magnetron discharge. The magnetron discharge is initiated by a vacuum arc discharge, the plasma of which flows onto the magnetron sputtering target working surface. The vacuum arc material is usually the same as that of the magnetron target. The discharges operate at a residual pressure of 3 × 10−6 Torr without working gas feed. Pulses of vacuum arc (30 μs) and magnetron discharge (up to 300 μs) are applied simultaneously. After ignition by the vacuum arc, the magnetron discharge runs in a self-sustained mode. Cu–Cu, Ag–Ag, Zn–Zn, and Pb–Pb pairs of magnetron target material and vacuum arc cathode material were tested, as well as mixed pairs; for example, Cu vacuum arc cathode and Pb magnetron target. An ion beam was extracted from the discharge plasma by applying an accelerating voltage of up to 20 kV between the plasma expander and grounded electrodes. The ion beam collector current reached 80 mA. The ion beam composition, analyzed by a time-of-flight spectrometer, shows that the beam consists mainly of singly-charged (about 90%) and doubly-charged (about 10% current fraction) magnetron target material ions. The ion beam radial current density non-uniformity was as low as ±5% over a diameter of 6.6 cm, which is the diameter of the source output aperture.

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.

Vacuum Arcs with Diffuse Cathode Attachment (Review)

The review covers the results of predominantly experimental studies of stationary vacuum arcs with diffuse cathode attachment that burns in vapor of the cathode material with a current density of less than 100 A/cm2. Such discharges are of great interest for a number of technologies that require the formation of high-intensity plasma flows without a droplet fraction. The discharges on cathodes made of graphite, pure metals, and oxides, as well as mixed cathodes, are considered. The specificity of the processes on vacuum arc cathodes is characterized by the ratio of the fluxes of thermally vaporized atoms and thermionic electrons. The review presents the results of studies of cathode materials with an atom-electron ratio of ~10–2 to ~108. Data on the working cathode temperature, the current–voltage characteristic of the discharge, the heat flux from the plasma to the cathode, the plasma parameters, its radiation spectrum, and the ion energy in the cathode jet are presented. Depending on the cathode material, the working temperature ranged from 1.2 to 2.5 kK. An analysis of the features of the charge-transfer processes on various cathodes is also presented.

Characterization of low-pressure high-current cascaded arc plasma in large volumes

A comprehensive study of behavior and parameters of the low-pressure (1–20 mTorr) large-area plane–symmetric direct current arc discharge plasma in pure Ar and Ar-N2 mixtures was carried out. A low-pressure arc discharge was ignited in a large chamber between a planar vacuum arc cathode with magnetic steering of arc spots and a surrounding grounded primary anode while a long-length remote arc discharge was extended toward a remote linear anode parallel to the cathode plate. The arc column moved up and down along the cylindrical cathode target positioned perpendicular to the cathode-to-remote anode direction, following the motion of the cathodic arc spots. Current-voltage characteristics of the low-pressure direct current arc discharge, electron density, electron temperature, electron energy distribution function (EEDF), ion energy distribution function (IEDF) and dissociation of nitrogen molecules in the discharge were studied using electro-technical methods, electrostatic probes, a microwave resonant probe, an ion energy analyzer and optical emission spectroscopy (OES) methods. A number of OES methods were used to determine temperature and density of electrons in the discharge: the Te was measured using 1) the IArI,415/IArI,306 intensity ratio, 2) an intensity ratio of the (0–0) vibrational bands of 1st negative and 2nd positive systems of N2; the ne was obtained by 1) the IArII,487/IArI,451 intensity ratio and 2) ratio of N2(C3Πu,v = 1) and N2(C3Πu,v = 0) populations. In addition, the nitrogen dissociation degree was monitored using INI,868.03/IArI,852.14 and INI,862.92/IArI,852.14 intensity ratios. Spectral and probe measurements of Te and ne in the large-area arc discharge show reasonable agreement. The electron population analysis based on the second derivative of the Langmuir probe current-voltage characteristics allowed accurate measurements of electron population parameters in the discharge including EEDF, electron drift velocity along with plasma potential, electron density and energy. Modeling of the low-pressure large-volume arc discharge using the axially symmetric drift-diffusion approximation exhibited good qualitative and acceptable quantitative agreement between calculated plasma parameters and experimental data.

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.

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.

A Simple Model of Distribution of Current Over Cathodes of Vacuum Circuit Breakers

There are several hundreds of spots operating simultaneously on cathodes of vacuum arcs in high-power vacuum circuit breakers. In this work, the spot distribution along the contact surface is simulated by means of an approach that is based on the concept of surface density of spots and represents a natural alternative to tracing individual spots. An equation governing the evolution of the surface density of the spots or, equivalently, the distribution of macroscopic (averaged over individual spots) current density over the cathode is obtained by generalizing the concept of random walk of a single cathode spot in low-current vacuum arcs. The model relies on empirical parameters characterizing individual spots (the diffusion coefficient of the random motion of cathode spots and the velocity of drift superimposed over the random motion), which may be taken from experiments with low-current arcs, and does not involve adjustable parameters. The model is simple and physically transparent and correctly reproduces the trends observed in the experiments under conditions where the cathode arc attachment is diffuse. The distribution of the macroscopic current density on the cathode, given by the model, represents the boundary condition that is required for existing numerical models of vacuum arcs in high-power vacuum circuit breakers.

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.

Droplet Erosion of a Cathode in a High-Current Vacuum Arc

It is shown that high-current vacuum arcs accompanied by partial melting of the cathode surface and its droplet erosion can generate, in addition to the well-known droplets formed immediately in the cathode spot (first type droplets), a second component of erosion. The liquid part of the cathode surface can emit droplets with significantly different sizes and velocities, which may be called second type droplets. The mechanisms of the formation of first and second type droplets are also substantially different.

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.

The sequence of processes in the ecton cycle of a vacuum arc

The paper deals with the temporal structure of the explosive electron emission cell of the vacuum arc cathode spot. Essential features of the cathode spot cells operation - the ignition, burning and extinction have been considered in frames of the ecton model.

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.

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.