Application of vacuum arc cathode spot model to graphite cathode

A model of near-electrode processes is applied here to describe the behavior of cathode spots on graphite cathode in vacuum arc. The physical model is based on a kinetic treatment of cathode evaporation, electron emission from the cathode, and plasma production. The model consists of physical assumptions and a system of equations that are formulated in the paper. Spot parameters, such as cathode erosion rate, cathode potential drop, cathode surface temperature, current density, electric field, and plasma density, temperature, and velocity in the near-electrode region are calculated numerically. The calculation includes the dependence of spot parameters on spot current and spot lifetime. The variation of spot parameters as a function of spot lifetime are very strong at lifetimes shorter than 10 μs. The calculations indicate that Joule heating in the cathode body is significant, and may exceed cathode heating by the ion heat flux. Calculated spot parameters are compared with the corresponding experimental data for relatively low arc currents (<100 A) and their agreement is discussed     

Motion of the cathode spot of a vacuum arc in an external magnetic field

The problem of the motion of the cathode spot of a vacuum arc electrical discharge in a magnetic field applied tangential to the cathode surface is considered. The treatment is based on concepts of the nonstationary, cyclical nature of processes occurring in the cathode spot and the key role of return electrons falling out of the near-cathode plasma back onto the cathode. Zh. Tekh. Fiz. 68, 60–64 (June 1998)     

Vacuum arc investigation of dual-part cathode electrodes

The basic characteristics of a vacuum arc are investigated with a special electrode whose cathode consists of two half-disks. Pure copper is used for one part of the cathode, while copper, chromium, silver, and titanium are used for the other part. The tested arc current value is less than 4000 A, and the flux density of the axial magnetic field applied to a vacuum arc is less than 0.015 T. Experimental results show that the arc voltages of dual-part cathode electrodes are much lower than those of the individual pure metals, and that current sharing between the two parts is roughly determined by the arc current-voltage characteristics of the metals. The arc voltage of the dual-part cathode electrode is extremely low when the current is less than 1000 A     

Direct observation of cathode spot grouping using nanostructured electrode

Clear footprint of an arc spot, which is formed after arc spot runs on a fiberform nanostructured tungsten, reveals the detailed structure and motion of arc spot in a magnetic field. Fine inner structure of an arc trail is observed, exhibiting that many sub-arc spots existed inside the arc spot, with forming a group. The sub-arc spots move randomly but they globally move to some direction, which is affected by the axial and parallel magnetic field with respect to the specimen. The trails are analyzed by using box-counting method and fractality of the arc trails in magnetic field is discussed. It is shown that the nanostructured metal can clearly record the footprint of the arcs, similar as the man's footprints in the snow, and be a powerful tool for basic arc experiments.     

Ecton mechanism of the vacuum arc cathode spot

A new mechanism for the operation of a cathode spot in a vacuum arc, based on ecton processes, is proposed. An ecton is formed by the explosion of the tip of a jet of molten metal as it interacts with plasma. The time of ecton operation is assumed to be limited by the thermal conductivity of the liquid metal. For copper electrodes, the theoretical expressions are derived for the specific mass removal, ion erosion characteristics, current density, and the diameters of craters. The results agree well with the experimental data available     

Characteristics of the statistical distribution of the current transmitted by a cathode spot of a vacuum arc in variously oriented magnetic fields

The influence of variously oriented uniform magnetic fields on the cathodic attachment of a low-current vacuum arc with electrodes made of oxygen-free copper and CuCr30 composition is studied. It is found that, if the current is fixed, cathode spots in the arc attachment are distributed by the normal law in the entire range of variation of the amplitude of magnetic induction vector B and angle α between this vector and the normal to the cathode surface. The parameters of the distribution depend on the magnetic field and cathode material. The magnetic field dependence is appreciable only when angle α exceeds some critical value α* (α* ≈ 30° and ≈45° for cathodes made of copper and CuCr30, respectively). At α > α*, the parameters of the distribution become strongly dependent on α, while the B dependence remains weak. Only when α → π/2 does the field amplitude have a pronounced effect on the parameters of the distribution. From the obtained results, we determine the statistical characteristics of the distribution of the mean current transmitted by a cathode spot in variously oriented magnetic fields. The found relationships make it possible to explain the peculiarities of the structure of the cathodic attachment of the high-current vacuum arc stabilized by an external axial magnetic field.     

纵磁作用下真空电弧单阴极斑点等离子体射流三维混合模拟

真空电弧的特性直接受到从阴极斑点喷射出的等离子体射流的影响,对等离子体射流进行数值仿真有助于我们深入了解真空电弧的内部物理机制.然而,磁流体动力学和粒子云网格仿真方法受限于计算精度和计算效率的原因,无法有效地应用于真空电弧等离子体射流仿真模拟.本文开发了一套三维等离子体混合模拟算法,并在此基础上建立了真空电弧单阴极斑点...     

Physical model of the cathode spot retrograde motion

A physical model is proposed that describes the retrograde motion of a dc arc cathode spot as a process of plasma propagation in the direction opposite to the Poynting vector (against the Lorenz force).     

Parameters of plasma layer within the vacuum arc cathode spot

The paper gives a formulation of the closed loop model developed on the basis of the integral balances method to determine parameters of the plasma layer and boundary conditions at the cathode surface for the processes of energy, mass, and current transportation. Presented are the results of the self-consistent calculations for the copper cathode     

Vacuum discharge instability at laser initiation of a cathode spot

The dynamics of fast (a current rise time of ≤10¹¹ A/s) laser-induced vacuum discharges with moderate amplitudes of the current (≤10 kA) and voltage (≤20 kV), as well as medium storage energy (20 J), is studied. It is shown experimentally that the initial conditions specified by the energy and duration of laser radiation are a decisive factor governing the discharge dynamics. Two types of beam-plasma instabilities separated in space and time are discovered, and their occurrence conditions are analyzed. The first type of instability, observed early in the discharge, is associated with pinch structures at the front of the cathode jet expanding into a vacuum. The second type arises either at the peak or at the trailing edge of the current pulse and is accompanied by generation of hard (with an energy of ≥100 keV) bremsstrahlung from the anode region. The increase of the hard component energy over the current source potential is attributed to breaking due to plasma erosion.     

Cold-hollow-cathode arc discharge in crossed electric and magnetic fields

A crossed-field cold-hollow-cathode arc is stable at low working gas pressures of 10⁻²–10⁻¹ Pa, magnetic-field-and gas-dependent arcing voltages of 20–50 V, and discharge currents of 20–200 A. This is because electrons come from a cathode spot produced on the inner cathode surface by a discharge over the dielectric surface. The magnetic field influences the arcing voltage and discharge current most significantly. When the plasma conductivity in the cathode region decreases in the electric field direction, the magnetic field increases, causing the discharge current to decline and the discharge voltage to rise. The discharge is quenched when a critical magnetic field depending on the type of gas is reached. Because of the absence of heated elements, the hollow cathode remains efficient for long when an arc is initiated in both inert and chemically active gases.     

Analytical calculation of cathode spot parameters on the electrode surface in arc discharge

The model of a heat transfer in the cathode spot on an electrode in the arc discharge is developed and analytical expressions for the spot radius and temperature as the functions of basic characteristics of the discharge and the electrode material are obtained. The dependence of the spot radius on the voltage drop of the cathodic discharge, which is consistent with results of numerical simulation, is calculated.     

Model of liquid-metal splashing in the cathode spot of a vacuum arc discharge

The formation of microjets is studied during the extrusion of a melted metal by the plasma pressure from craters formed on a cathode in a burning vacuum arc. An analytic model of liquid-metal splashing that includes two stages is proposed. At the first stage, the liquid motion has the axial symmetry and a liquid-metal wall surrounding the crater is formed. At the second stage, the axial symmetry is broken due to the development of the Plateau–Rayleigh instability in the upper part of the wall. The wall breakup process is shown to have a threshold. The minimal plasma pressure and the minimal electric current flowing through the crater required for obtaining the liquid-metal splashing regime are found. The basic spatial and temporal characteristics of the jet formation process are found using the analytic model.     

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 E _opt, second, implementation of atomic-ionic balance at which the E _opt 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 λ_a ～ 1.1–1.2 eV. Taking the possibility of F-T-emission into account, the evaporation heat can be slightly higher than λ_a ～ 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.     

Gyrocenter shift of low-temperature plasmas and the retrograde motion of cathode spots in arc discharges

The gyrocenter shift phenomenon explained the mechanism of radial electric field formation at the high confinement mode transition in fusion devices. This Letter reports that the theory of gyrocenter shift is also applicable to low temperature high collisional plasmas such as arc discharges by the generalization of the theory resulting from a short mean free path compared with the gyroradius. The retrograde motion of cathode spots in the arc discharge is investigated through a model with the expanded formula of gyrocenter shift. It is found that a reversed electric field is formed in front of the cathode spots when they are under a magnetic field, and this reversed electric field generates a rotation of cathode spots opposite to the Amperian direction. The ion drift velocity profiles calculated from the model are in agreement with the experimental results as functions of magnetic flux density and gas pressure.     

Vacuum interrupter design based on arc magnetic field interactionfor horseshoe electrode

Computational methods for evaluating the complex magnetic field structure in axial magnetic field type vacuum interrupters based on horseshoe electrode geometry are demonstrated. Arcing behavior can only be understood by incorporating an arcing model. An essentially low-current arcing model is capable of predicting the high-current arcing behavior and can be used to optimize the contact structure     

Cathode jets and the retrograde motion of arcs in magnetic fields

The influence of a transverse magnetic field on the jets of an impulse discharge was investigated. It was found that the jet trajectories show a curvature. The amount of the curvature depends on the pressure, the magnetic field intensity, the discharge current, the material and the polarity of the electrodes. The curvature of the jets was explained as a result of the action of forces arising in a plasma flow through a magnetic and electric field; their analysis was carried out on the bases of a MHD model. On hand of a derived solution, a possible influence of the jets on the origin of a retrograde motion is discussed. It is shown that the existence of certain discharge parameters and of a certain magnitude of the magnetic field brings about, in a plasma flying through a magnetic field, an induction of fields and currents under the action of which, on the one hand, the plasma itself flows in a retrograde direction and, on the other hand, there are influenced the trajectories of the carriers (particularly those of ions) of the current in the channel of discharge in front of the cathode spot. The proposed model of the retrograde motion is discussed and compared to the known results.     

Influence of microstructure of CuCr25 cathode on the motion of vacuum arc spots

The motion of vacuum arc spots on nanocrystalline and coarse-grained CuCr25 alloys were observed by a digital high speed video camera and SEM. Experimental results show that without an external magnetic field a spot can move a long distance in a direction and leave a long straight arc trace (50–100 μm) on the surface of nanocrystalline CuCr25 cathode. This kind of spot motion can be defined as sub-directional motion. The spot motion is totally random and restricted on coarse Cr particles for coarse-grained CuCr25 cathode. Arc spots move sub-directionally and more easily on nanocrystalline cathode maybe results from an active surface formed by the special electronic structure due to Cu–Cr internal electric field.