Study of the direct current breakage process in an external nonuniform magnetic field

We study experimentally the influence of an external nonuniform magnetic field with transverse and longitudinal components on the electric characteristics, plasma configuration, and cathode spot arrangement of vaccum-arc discharge. It is revealed that for a cylindrical cathode, cathode spots are nonuniformly distributed on the cathode surface, the spot configuration has no axial symmetry, and the arrangement of spots changes in time depending on the induction value of the external pulsed magnetic field. With an increase in induction, spots (on average) are arranged closer to the cathode, i.e., displaced to the region of weaker transverse field. For two cathode geometries, the probabilities of direct current breakage depending on the induction of the external nonuniform magnetic field are experimentally determined. To determine the conditions of current breakage, a three-dimensional mathematical model of ion motion is suggested and the current breakage criterion is formulated. The trajectories of fast cathode ions in an electrode system with a ring cathode have been calculated using the model. Calculations were performed in a nonuniform magnetic field, the radial and axial components of which have been measured experimentally. It is shown that conditions of current breakage determined with this criterion agree with the results of experiment.     

The effect of axisymmetric two-dimensional magnetic field on the configuration of vacuum-arc plasma

An experimental investigation is made of the effect of axisymmetric two-dimensional magnetic field on the forming of plasma and on the configuration of cathode spots in a vacuum-arc discharge. It is demonstrated that a magnetic field with a transverse (relative to the discharge axis) component has a significant effect on the shape of plasma column and on the rate of expansion of the cathode spot region. In a magnetic field, arc plasma has the form of truncated cone expanding toward the anode. The cathode spots take up a part of the cathode area which decreases with increasing magnetic field. Arguments are given in support of the assumption that the arrangement of cathode spots and the form of arc plasma are defined by the minimum principle similar to the Steinbeck principle. In so doing, the displacement of spots is caused by their emergence in a new region corresponding to a lower arc voltage. Also discussed is the mechanism associated with retrograde motion of cathode spot in view of the effect of azimuthal magnetic field on the axial component of current and of the effect of axial magnetic field on the azimuthal component of current.     

Effect of tangential magnetic field on ecton processes in cathode spot of vacuum arc

Emission and erosion processes involved in the final stage of the cathode spot cell operation in vacuum arc in the presence of an external magnetic field have been numerically simulated. It is established that the application of a magnetic field leads to asymmetry in the distributions of current density and heat flux, so that their maxima shift in the “anti-Ampere” direction. For more detailed analysis of the phenomenon of retrograde motion of the cathode spot in a magnetic field, it is necessary to study the behavior of a liquid metal phase in the spot.     

Dynamics studies of cathode spots in a vacuum-arc discharge with ring electrodes

The effect of an external pulse magnetic field with axial and radial components on electric characteristics of the discharge and the configuration of cathode spots of a vacuum arc discharge with ring electrodes is studied experimentally. For arc currents within the range of 0.05–2 kA, shots of the cathode spots at different instants of time are obtained. The dependences of the number of the spots on the discharge current and the mean current per spot are determined. It was found that the expansion rate of the cathode spots area depends significantly on the instant value of the discharge current. It is shown that, when the pulse magnetic field is applied, the discharge voltage increases and the discharge current and number of the cathode spots decreases. It was found that the current interruption is a probability process. The probabilities of the current interruption depending on the maximal value of the external pulse magnetic field induction are determined.     

The effect of axial magnetic filed on the process of anode spot formation in a vacuum-arc discharge with CuCr50 electrodes

An experimental study is made of the effect of an external axial magnetic field on the process of anode spot formation in a pulsed vacuum-arc discharge in the range of currents from 5 to 12 kA in a discharge gap with CuCr50 electrodes. The times and currents, at which an anode spot is formed, are determined for each amplitude value of current depending on the magnitude of magnetic field. The minimal value of magnetic field preventing the anode spot formation is determined for each current. The measured values of diameters of the current channel are used to calculate the anode temperature. It is demonstrated that, under experimental conditions, the heating of anode is insufficient for marked evaporation, and the anode spot formation is associated with the critical flow of fast cathode ions.     

磁控电弧离子镀阴极斑点特性的高速摄影研究

利用高速摄影技术，对磁控电弧离子镀中阴极斑点大小、寿命及运动轨迹进行了观测，获得了阴极斑点电流密度、运动速度随磁场、电弧电流及背景气体压力改变而变化的关系曲线。     

Two-dimensional kinetic model of short high-current vacuum-arc discharge

A two-dimensional mathematical model of a short high-current vacuum-arc discharge is developed, according to which magnetized electrons move in a hydrodynamic regime and fast cathode ions propagate in a free flight regime in a two-dimensional electric field. The proposed model takes into account the distribution of ions with respect to their escape angles from the cathode plasma boundary. A method for calculation of the plasma density distribution in the interelectrode gap is proposed. Two-dimensional distributions of the plasma density, electric field, and discharge current density in an external magnetic field are calculated. It is shown that ion trajectories exhibit mutual intersections, partly return to the cathode, and partly rotate in the oppositely oriented electric field at the side boundary of plasma. A decrease in the applied magnetic field intensity leads to a decrease in the number of ion trajectories reaching the anode (ion starvation), which can result in the violation of a stationary current transfer.     

Plasma flow crisis and the limiting electron temperature in a vacuum arc in axial magnetic field

The possibility of supersonic motion of cathode plasma in a low-current vacuum arc in an axial magnetic field has been studied. It is shown that an increase in the electron temperature unavoidably leads to a plasma flow crisis, whereby the plasma velocity decreases to the sound velocity. The dependence of the limiting length of a stationary flow on the magnetic field has been studied. The maximum possible electron temperature T _cr in the plasma is determined by the initial ion energy and can be estimated as T _cr ≈ 3T _m, where T _m is the maximum electron temperature in the cathode spot region.     

Measuring high current densities in the cathode spot of an ARC discharge

Conclusions Regarding a cathode spot a continuous sequence of breakdowns, as has been proposed in [7, 9, 10] for describing a fast-shifting cathode spot, leads to fundamental contradictions. It appears necessary to consider separately two stages in the existence of a spot: first comes the initiation stage when breakdown occurs and plasma is generated in the cathode spot, within about 5·10^–8 sec, and the emitted current enters the plasma, usually along the edges of the fading spot [4, 5], with a fast decreasing density; then, as the plasma scatters within about 10^–5 sec, the density of the emitted current decreases slowly and this is accompanied by an increase in the voltage drop so that, eventually, another breakdown occurs and a new spot is formed.Translated from Izmeritel'naya Tekhnika, No. 1, p. 69, January, 1977.     

Measuring the robson angle in vacuum arcs of various length

It is established that the Robson angle, which determines the direction of motion of a cathode spot of vacuum arc in a homogeneous electric field sloped relative to the cathode, depends mostly on the inter-electrode gap width. The dependence of the Robson angle on the field slope at various gap widths has been measured for molybdenum and tungsten cathodes. The knowledge of this angle is necessary for correct choice of the optimum magnetic field configuration controlling the cathode spot dynamics in setups for vacuum-arc deposition of various coatings and in vacuum commutation devices.     

Study on cathode spot motion and macroparticles reduction in axisymmetric magnetic field-enhanced vacuum arc deposition

Cathode spot motion and macroparticles (MPs) reduction on related films are the two main issues in the application of the vacuum arc deposition (VAD). In the present work, an axisymmetric magnetic field (AMF) was applied to the cathode surface to investigate the influence of the AMF on the cathode spot motion and the MPs reduction on TiN films. The results show that the AMF affected the cathode spot motion by redistributing the dense plasma connected with the initiation of the new spot. With increasing AMF, there is an increasing tendency for the cathode spot to rotate and drift toward the cathode target edge. Based on the results of FEM simulation and the physical mechanism of the cathode spot discharge, the mechanism of the cathode spot motion in the AMF was discussed. The morphology, detailed size distribution, and roughness of the resultant TiN films were systematically investigated. Fewer and smaller MPs ejection is observed with an increase in the transverse component of AMF. The effect of the AMF on the MPs reduction on TiN films was discussed, and the results were compared with the theoretical predictions.     

Magnetic‐Field Effect on Heat Transfer within a Cathode Arc Spot

The authors present results of an experimental study of the heat flux within the arc spot of the copper cathode of an electric‐arc heater with magnetic‐field‐induced displacement of the arc in an air medium — one of the most important parameters determining the erosion and service life of such a cathode. The investigations were carried out at a current strength of 110–?450 A, a magnetic induction of 0.009–0.38 T, and atmospheric pressure.     

Numerical modeling of the electrohydrodynamic and thermal instability of a conducting liquid surface in a strong electric field

The electrohydrodynamic and thermal instability development on a conducting liquid surface exposed to a strong electric field was studied by methods of two-dimensional numerical modeling. The Navier-Stokes equation was solved and the surface cone heating by the field emission current was described. It is demonstrated that the free surface evolution in this system leads to an avalanche-like growth in the cone tip temperature and the resulting explosion-like field electron emission. A considerable contribution to the liquid heating is due to a surface energy source (the Nottingham effect). The proposed computational method, based on the domain transformation to a canonical form, can be used to develop dynamic models of the liquid-metal ion source operation and to analyze the interaction of a gas plasma with a liquid metal surface in the cathode spot of a vacuum arc.     

The current column contraction and ion charge increase induced by the current build-up in a pulsed vacuum discharge

It is shown that three current build-up regimes may be realized in a pulsed vacuum discharge, depending on the relationship between the pulse duration τ and the characteristic times of the plasma flow (τ_L) and the cathode spot spreading (τ_S). For a rapid build-up (τ<τ_L), the cathode plasma jet exhibits the neck formation with high values of the plasma temperature and density. At a slow current increase (τ_L<τ<τ_S), the current column is subject to a uniform magnetic contraction, while at a very slow current growth rate (τ>τ_S), the contraction becomes insignificant. The first two cases give rise to additional nonequilibrium ionization in the current column, which leads to the ion charge increasing with the current. The third case is characterized by a “frozen” ion composition corresponding to the plasma parameters in the near-cathode region.     

Oscillations of the Hall current in a Hall thruster with an anode layer

Burning of an intense discharge in crossed electrical and magnetic fields in a Hall electrojet engine (Hall thruster) is considered. An engine of this type is an azimuthally symmetric device in which the discharge burns in an annular channel between the poles of the magnetic circuit. The anode is usually made in the form of a cavity through which the working gas is supplied (thruster with anode layer, TAL) or in the form of a planar ring mounted in a dielectric channel in a weak magnetic field (stationary plasma thruster, SPT), and the role of the cathode is played by the plasma surrounding the engine. The electrons trapped in the magnetic field between the poles of the magnetic circuit oscillate in the electric field between the anodic and cathodic regions of the discharge, forming the Hall current closed in the azimuthal direction. It is known from practice that discharge of this type is always nonstationary and the main self-sustained oscillations are excited on the atomic time-of-flight frequency (“accelerative” regime). In addition, there exists a region of parameters in which the self-sustained oscillations become stochastic and the discharge current increases sharply (“stochastic” regime). This work is devoted to the induction measurements of the oscillations of the Hall current arising in various regimes of operation.     

Minimum voltage principle in the theory of a high-current vacuum-arc discharge

The effect of an inhomogeneous magnetic field with axial and radial components on the structure of a high-current vacuum-arc discharge (VAD) has been theoretically studied. The characteristic features of current passage in a short VAD are considered using analytical expressions for the slope of the total current lines (TCLs) relative to the discharge axis, the axial components of the electric and magnetic fields, and the effective conductivity of discharge plasma. A two-dimensional mathematical model has been used to calculate the TCL shapes and the discharge voltage for various dimensions of the region of cathode spots. Calculations showed that the voltage drop on the discharge gap as a function of the arc discharge radius on the cathode has a minimum. The shape of TCLs and the arc radius on the cathode at this minimum agree with the available experimental data.     

The effect of cathode deuteration on the parameters of vacuum-arc plasma

We propose a model for determining the influence of the relative content of deuterium in a zirconium cathode on the properties of vacuum-arc plasma. It is shown that the occlusion of deuterium in the cathode leads to an additional energy consumption for its ionization and to the related decrease in the average charge of cathode material ions in the discharge plasma. Deuterium in the cathode spot is fully ionized, and the drift velocity of deuterium ions almost coincides with that of ions of the cathode material.     

Output microwave radiation power of low-voltage vircator with external inhomogeneous magnetic field

Dependence of the power of a broadband microwave radiation generated by a low-voltage oscillator with virtual cathode (vircator) on the parameters of an external inhomogeneous magnetic field has been studied by numerical simulations using a two-dimensional model. It is established that there are optimum parameters of the generator (configuration of the external magnetic field, electron beam current) for which the output radiation power is maximum. A relationship between the optimum conditions of virtual cathode formation in the electron beam and the microwave generation regime is established.     

Control of cathodic arc spot motion under external magnetic field

A closed circular transverse magnetic field is designed to control the spot motion of a cathodic arc on a large rectangular target. Stable and controllable spot motion is observed from both graphite and copper. The arc stability, spot velocity, and etched pattern on the cathode surface are studied systematically. Circular tracks are etched on the target surface by the repetitive spot movement and the surface morphology and resistivity of the cathode materials influence the spot velocity.