Contact temperature and erosion in high-current diffuse vacuum arcson axial magnetic field contacts

We have investigated the surface heating effects of drawn vacuum arcs for several industrial designs of axial magnetic field (AMF) contacts, using near infrared (IR) photography of the Cu-Cr arcing surfaces with an image-intensified charge-coupled device (CCD) camera and an IR pyrometer. This enables detailed contact temperature mapping immediately after a half-cycle of arc current. The very homogeneous temperature distribution observed at current zero stands in contrast to the visually nonhomogeneous high-current diffuse arc, which was studied in separately reported experiments using high-speed digital photography and arc voltage measurements. The peak temperature at current zero increased relatively linearly with the peak current I_P, and reached well beyond the melting range. We combine the temperature maps with a heating model to determine the thermal sheath thickness after arcing and its dependence on I_P. The results suggest that near the interruption limit of AMF contacts, the interaction of the stable high-current arc with the anode and cathode is dominated by processes induced by flowing liquid metal, which redistributes the heat input from the axially concentrated arc over most of the contact surface. Furthermore, the flow of liquid metal off the cathode and anode faces contributes to the overall contact erosion     

Anode Melting in a Multicathode-Spot Vacuum Arc

Melting of the anode surface in a multicathode-spot vacuum arc is expected when the incident energy flux is not balanced. The anodic energy influx is proportional to the arc-current collected by the anode and melting of the anode should be observed when peak arc-current exceeds a critical value. In this work, the critical peak arc-current Ipt was measured, and its dependence on anode and cathode materials was determined. The arc was sustained between two parallel cylindrical electrodes, 14 mm in diameter and spaced 4 mm apart. The almost critically damped current pulse lasted for 30 ms with a 6-ms rise time to peak value. Peak currents were in the range of 500-2300 A. In most of the experiments the anode material differed from that of the cathode. In the runs where the cathode-anode materials were Cu-Al or Mo-Cu, respectively, the time dependence of a spectral line intensity radiated by the anode atoms located in the plasma near the anode surface was recorded. We found that Ipt depended on both the anode and cathode materials. Thus for an Al anode and Al and Cu cathodes, Ipt equaled to 1100 and 900 A, respectively. In arcs with a peak current larger or equal to Ipt, a sudden jump of the spectral line intensity was observed. In all experiments, even when strong melting of the anode was observed, the arc-voltage stayed quiescent and in the range 15-35 V, suggesting that no anode spot was formed.     

Enhancement and stabilization of cathodic arc using mesh anode

The performance and characteristics of a cathodic arc deposition apparatus consisting of a titanium cathode, an anode with and without a tungsten mesh, and a coil producing a focusing magnetic field between the anode and cathode arc investigated. The arc voltage V_a is measured with a fixed arc current for an anode diameter of 40 mm. The relationship between V_a and the magnetic field B with and without a mesh is obtained. In addition, the relationship between the arc current I_a and V_c, the voltage to which the artificial transmission line was charged, is measured with and without the mesh to determine the minimum ignition voltage for the arc when the anode hole diameter is 40 mm. The arc resistance increases with the focusing magnetic strength B and decreases when using the mesh. Our results indicate that the high transparency and large area of the mesh allows a high plasma flux to penetrate the anode from the cathodic arc. The mesh also stabilizes the cathodic arc and gives better performance when used in concert with a focusing magnetic field     

The Influence of the Self-Magnetic Field on the Steady-State Current Distribution in an Axially Flowing Conducting Medium

The steady-state electric current distribution in a multicathode-spot vacuum arc was determined by a solution of the magnetic transport equation subject to various boundary conditions. The inter-electrode region of the arc is modeled as a uniform plasma flowing from the cathode to the anode. Dimensional analysis shows that three parameters determine the magnetic field, and hence the current density which is derived from it: AR-the ratio of the electrode separation to the electrode radius, Rmm-magnetic Reynolds number of the axial material flow, and Rme-magnetic Reynolds number of the axial electron flow. While the anode side of the conducting medium is described as an equipotential surface, the following three cases of boundary conditions for the cathode side are examined: 1) a known current density distribution is assumed over the entire cathode side of the plasma surface; 2) the cathode side is an equipotential surface; and 3) the current is allowed to cross the cathode surface only through a finite number of ring shaped regions. Numerical solutions of the nonlinear magnetic transport equation show a constriction of the current at the anode side for all boundary conditions mentioned. On the other hand, the current moves to the perimeter of the cathode for boundary condition 2). When AR, Rmm, and Rme equal 0.72,-0.16, and 1.73, respectively, and a uniform current density flows at the cathode side, the on-axis current density at the anode is six times larger than its value at the cathode.     

The Current Distribution and the Magnetic Pressure Profile in a Vacuum Arc Subject to an Axial Magnetic Field

The steady-state electric-current distribution and the magnetic pressure in a uniform conducting medium, flowing in a cylindrical configuration between two circular electrodes, was determined by solving the magnetic field transport equation with a superimposed axial magnetic field. This medium models the interelectrode plasma of the diffuse mode metal vapor vacuum arc. The results show the following. a) The electric current and the flux of the poloidal magnetic field are constricted at the anode side of the flowing plasma. Most of the constriction takes place within a boundary layer, with a characteristic length of 1/Rme, where Rme is the magnetic-Reynolds number for axial electron flow. b) The electric-current constriction inversely depends on K?, where K? is the azimuthal surface current density which produces the axial magnetic field. c) The magnetic-pressure profile shows a radial pinch force in most of the interelectrode region, but in the anode boundary layer it is axially directed, thus retarding the plasma flow. d) The peak of the magnetic pressure is at the anode, and its amplitude directly depends on K?. As K? increases, the peak location moves toward the anode center.     

THE EFFECT OF MESH ANODE ON CATHODIC ARC IN FOCUSING MAGNETIC FIELD

The performance and characteristics of a cathodic arc deposition apparatus consisting of a titanium cathode, an anode with and without a tungsten mesh, and a coil producing a focusing magnetic field between the anode and cathode are investigated. The arc voltage V_a is measured with a fixed arc current. The relationship between V_a and the magnetic field B with and without a mesh is obtained. In addition, the relationship between the arc current I_a and V_c, the voltage to which the artificial transmission line was charged, is measured with and without the mesh to determine the minimum ignition voltage for the arc. The arc resistance increases with the focusing magnetic strength B and decreases when using the mesh. Our results indicate that the high transparency and large area of the mesh allows a high plasma flux to penetrate the anode from the cathodic arc. The mesh also stabilizes the cathodic arc and gives better performance when used in concert with a focusing magnetic field.     

Angular distribution of ion current emerging from an aperture anodein a vacuum arc

The ion current distribution emerging from a vacuum arc between a Cu cathode and a conical ring anode was measured by a set of five probes. It was found that: (1) the total ion current emerging through the anode was 8.5% of the arc current; (2) the measured ion distribution without a magnetic field was a slightly flattened cosinusoidal function; (3) with an axial magnetic field, the ion current distribution became peaked along the z axis; (4) the total ion current extracted through the anode aperture slightly increased with the magnetic field; and (5) an anode with a larger aperture exhibited less magnetic collimation     

A physical model of the low-current-density vacuum arc

A one-dimensional (1-D) physical model of the low-current-density steady-state vacuum arc is proposed. The model is based on the continuity equations for ions and electrons and the energy balance for the discharge system; the electric potential distribution in the discharge gap is assumed to be nonmonotonic. It is supposed that the ion current at the cathode is generated within the cathode potential fall region due to the ionization of the evaporated atoms by the plasma thermal electrons having Boltzmann's energy distribution. The model offers a satisfactory explanation for the principal regularities of a hot-cathode vacuum arc with diffuse attachment of the current. The applicability of the model proposed to the explanation of some processes occurring in a vacuum arc, such as the flow of fast ions toward the anode, the current cutoffs and voltage bursts, and the backward motion of a cathode spot in a transverse magnetic field is discussed     

Voltage of the vacuum arc with a ring anode in an axial magneticfield

The plasma jet focusing and voltage distribution in the interelectrode gap of a vacuum arc with a ring anode and subjected to an axial magnetic field were studied theoretically. A two-dimensional model was developed based on the free plasma jet expansion into vacuum, and the steady-state solution of the fully ionized plasma in the hydrodynamic approximation was analyzed. It was found that the imposition of an axial magnetic field reduces the radial expansion of the plasma jet. The characteristic jet angle decreases from about 40° in the zero magnetic field case and approaches a value of about 20° with a 0.02 T magnetic field. The arc voltage consisting of the cathode drop, the plasma voltage drop, and anode sheath drop increased, with the imposition of a magnetic field, and decreased with the anode length. The model was compared to experimental measurements of the vacuum arc voltage behavior in an axial magnetic field, and good agreement was found     

Time Dependence of Anode Spot Formation Threshold Current in Vacuum Arcs

The variation of threshold current for the transition between the low current quiescent vacuum arc mode, and the high voltage noisy mode associated with anode spot formation, was measured as a function of peak current, current waveform frequency, and electrode separation on fixed diameter (25 mm) Cu and Ni electrodes. At current waveform frequencies of about 60 Hz on Cu electrodes, the threshold current depends mainly on electrode spacing, as has been observed by other investigators. However, at higher waveform frequencies, the threshold current becomes a strong function of peak current as well. At 347 Hz on 25 mm. diam. Cu electrodes separated by 10 mm, the threshold current rose from approximately 2 kA to 5.5 kA, as the peak current rose from 2 kA to 6 kA. At 543 Hz on 25 mm diam Ni electrodes separated by 9 mm, a saturation in threshold current at about 7.5 kA was observed for peak currents greater than 9 kA. Simultaneous anode temperature measurements indicated that the Ni anode surface temperature immediately prior to transition rose from about 1550° K to 2250° K with variations of peak current from 5 kA to 13 kA. Predictions of the variation of threshold current based on random transitions, and on cathode spot migration over the edge of the cathode, are compared with the experimental data.     

Ion flux from the cathode region of a vacuum arc

The properties of the ion flux generated in a vacuum arc are reviewed. The structure and distribution of mass erosion from individual cathode spots and the characteristics of current carriers from the cathode region at moderate arc currents are described. An appreciable ion flux (~10% of the total arc current) is emitted from the cathode of a vacuum arc. This ion flux is strongly peaked in the direction of the anode, although some ion flux may be seen even at angles below the plane of the cathode surface. The observed spatial distribution of the ion flux is expressed quite well as an exponential function of the solid angle. The ion flux is quite energetic, with average ion potentials much larger than the arc voltage, and generally contains a considerable fraction of multiply charged ions. The average ion potential and ion multiplicity increase significantly for cathode materials with higher arc voltages but decrease with increasing arc current for a particular material. The main theories concerning ion acceleration in cathode spots are the potential hump theory and the gas dynamic theory. Experimental data indicate that these theories serve reasonably well when used to predict the mean values of the charge state, ion potential, and ion energies for the ion flux, but are quite insufficient when compared with the results for the potentials and energies of individual ions     

外磁场作用下的磁等离子体动力学过程仿真

磁等离子体动力学推力器是空间高功率电推进装置的典型代表,磁等离子体动力学过程是其核心工作机制.为深入理解外磁场对其工作特性的影响,本文采用粒子云(particle in cell,PIC)方法结合基于自相似准则的缩比模型,进行外加磁场作用下磁等离子体动力学推力器工作过程的建模仿真,通过与实验结果对比验证模型和方法的可靠性,并重点分析推力器点火启动过程的等离子特性参数分布,以及外磁场和阴极电流对推力器工作性能的影响.研究结果表明:阴阳极放电电弧构建是推力器启动和高效工作的关键步骤;外磁场强度较低工况不利于构建稳定放电电弧,等离子体束流集中于轴线附近,推力主要产生机制是自身场加速;外磁场强度较高时,阴阳极放电电弧稳定,推力产生主要机制是涡旋加速,推力、比冲随外磁场强度线性增大;推力器效率随阴极电流和外磁场强度增大而增大;放电电压随阴极电流增大而增大,但随外磁场强度的增大表现出先减小后增大的趋势.     

外磁场作用下的磁等离子体动力学过程仿真

磁等离子体动力学推力器是空间高功率电推进装置的典型代表,磁等离子体动力学过程是其核心工作机制.为深入理解外磁场对其工作特性的影响,本文采用粒子云(particle in cell,PIC)方法结合基于自相似准则的缩比模型,进行外加磁场作用下磁等离子体动力学推力器工作过程的建模仿真,通过与实验结果对比验证模型和方法的可靠性,并重点分析推力器点火启动过程的等离子特性参数分布,以及外磁场和阴极电流对推力器工作性能的影响.研究结果表明:阴阳极放电电弧构建是推力器启动和高效工作的关键步骤;外磁场强度较低工况不利于构建稳定放电电弧,等离子体束流集中于轴线附近,推力主要产生机制是自身场加速;外磁场强度较高时,阴阳极放电电弧稳定,推力产生主要机制是涡旋加速,推力、比冲随外磁场强度线性增大;推力器效率随阴极电流和外磁场强度增大而增大;放电电压随阴极电流增大而增大,但随外磁场强度的增大表现出先减小后增大的趋势.     

Ion Flux from the Cathode Region of a Vacuum Arc

This paper reviews the properties of the cathode ion flux generated in the vacuum arc. The structure and distribution of mass erosion from individual cathode spots and the characteristics of current carriers from the cathode region at moderate arc currents are described. An appreciable ion flux (～10% of total arc current) is emitted from the cathode of a vacuum arc. This ion flux is strongly peaked in the direction of the anode, though some ion flux may be seen even at angles below the plane of the cathode surface. The observed spatial distribution of the ion flux is expressed quite well as an exponential function of solid angle. The ion flux is quite energetic, with average ion potentials much larger than the arc voltage, and generally contains a considerable fraction of multiply-charged ions. The average ion potential and ion multiplicity increase significantly for cathode materials with higher arc voltages, but decrease with increasing arc current for a particular material. The main theories concerning ion acceleration in cathode spots are the potential hump theory (PH), which assumes that all ions are created at the same potential, and the gas dynamic theory (GD), which assumes that all ions are created with the same flow velocity. Experimental data on the potentials and energies of individual ions indicates that these theories in their original forms are not quite correct, however extensions or modifications of the PH and GD theories seem very likely to be able to predict correct values for the charge states, potentials, and energies of individual ions.     

Experimental Investigation of Anode Activities in High-Current Vacuum Arcs

It is well known that the melting of electrodes (mainly anode melting) in vacuum arc can increase the metal vapor density around current zero and even lead to interruption failure. In order to clarify the anode activities and their influence on arc appearance and interruption capacity, series experiments of cup-shaped axial magnetic field copper electrodes were conducted. Obvious anode melting was detected; the liquid copper flowed on the contact plate of anode and formed a clockwise swirl flow. The appearance of anode melting is likely to correlate to the transition of arc mode from high-current diffuse mode to high-current diffuse column mode. The melting of anode was severer than cathode and was influenced by the distribution of cathode spots. Various kinds of copper particles at macroscopic level can be seen in arc column. Even at the interruption limit, the majority of melted copper of anode sputtered out of gap in form of liquid droplets or was pressed into the cup of anode, the copper vapor evaporated into arc column only accounted for a few portion and no obvious anode jets was found due to large plasma pressure in arc column.      

Characteristic features of the emission properties of pulsed broad-beam ion and plasma sources utilizing the evaporation of metal by a vacuum arc

An experimental confirmation was obtained of the anode potential fall effect in pulsed broad-beam ion and plasma sources utilizing the evaporation of metal by a vacuum arc. An increase in the overall voltage across the arc discharge was discovered. The investigations demonstrated that the magnitude of the positive anode fall depends on the structural features of the ion source and are determined by the ratio of the plasma flux directed onto the lateral surface of the anode to the total plasma flux from the cathode spot. It was established that the anode fall effect is controlled and makes it possible to influence the homogeneity of the ion current distribution over the beam cross section, the efficiency of extracting ions from the plasma, and the charge composition of the ion flux.Scientific-Research Institute of Nuclear Physics, Polytechnic University, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 82–92, February, 1994.     

Temperature Dependence of Retrograde Velocity of Vacuum Arcs in Magnetic Fields

Retrograde velocity of vacuum arcs in transverse magnetic fields is known to depend on, among other things, the magnetic induction, the arc current, the electrode spacing, the cathode material, and the cathode surface condition, and was also found to depend on the cathode temperature. Using the optical method, the retrograde velocity was measured as a function of the cathode temperature with copper, aluminum, and stainless steel as cathode materials. The optical measurement shows that by increasing the cathode temperature, the arc velocity decreases. It appears that with the increase in the cathode temperature, the decrease of the arc velocity is related to the increase of the cathode crater radius. The experimentally measured temperature dependence of the retrograde velocity of vacuum arcs can be explained by the ion jet model for retrograde motion of vacuum arcs [10]. The relative decrease of retrograde velocity as a function of the cathode temperature calculated according to this model agrees quantitatively with the reported measurements.     

Mechanisms of anode power deposition in a low pressure free burningarc

Anode power deposition is a dominant power loss mechanism for arcjets and magnetoplasmadynamic (MPD) thrusters. In this study, a free burning arc experiment was operated at pressures and current densities similar to those in arcjets and MPD thrusters in an attempt to identify the physics controlling this loss mechanism. Use of a free burning arc allowed for the isolation of independent variables controlling anode power deposition and provided a convenient and flexible way to cover a broad range of currents, anode surface pressures, and applied magnetic field strengths and orientations using an argon gas. Test results showed that anode power deposition decreased with increasing anode surface pressure up to 6.7 Pa and then became insensitive to pressure. Anode power increased with increasing arc current, while the electron number density near the anode surface increased linearly. Anode power also increased with increasing applied magnetic field strength due to an increasing anode fall voltage. Applied magnetic field orientation had an effect only at high currents and low anode surface pressures, where anode power decreased when applied-field lines intercepted the anode surface. The results demonstrated that anode power deposition was dominated by the kinetic energy of the current-carrying electrons acquired over the anode fall region. Furthermore, the results showed that anode power deposition can be reduced by operating at increased anode pressures, reduced arc currents, anode current densities, and applied magnetic field strengths, and with magnetic field lines intercepting the anode     

Photographic appearance of high-current vacuum arcs prior to and during anode spot formation

This article presents the results of research on the photographic appearance of a highcurrent vacuum arc between butt type copper electrodes a of 30–80 mm diameter and a fixed gap of 10 mm. Current pulses of up to 30 kA peak amplitude at an initial value of (di/dt)₀ from 1–10kA/ms and a duration of approximately 14 ms were applied. Arcs were photographed with a high-speed framing camera, mostly at 10⁴ frames/s. A detailed study of discharge modes in phase transition from a high-current diffuse arc to a constricted arc with an anode spot was conducted. Most of the measurements were obtained at a peak current slightly in excess of 10 kA for electrodes of 55 mm diameter. It was found that at peak current exceeding moderately the threshold value of the onset of anode spot formation, the arc is characterized by the following main features: the formation of an anode spot and an anode plasma jet occurs concurrently with a local concentration of cathode spots; the anode spot is, most often, formed on the electrode edge; the coexistence of very varied structures of spots on the cathode; the lack of considerable constriction of the cathode discharge; the pseudo-periodic shrinking and expansion of the area occupied by cathode spots; the existence of a relatively dark space separates the anode plasma jet from the plasma sheath near the cathode surface; the plasma space distribution in the interelectrode gap is non-uniform and non-stationary.This work was supported by State Committee for Scientific Research within the research project No. 3 P40101507.     

Cathode spot dynamics on pure metals and composite materials inhigh-current vacuum arcs

An investigation has been carried out of cathode spot dynamics in a triggered vacuum arc in a demountable chamber. A rectangular current pulse of 1-5 kA, 1-5 ms has been used. Sufficient statistics were collected. The expansion of a cathode spot ring on a clean, pure metal surface was corroborated to be a retrograde movement in the self-magnetic field which obeys the same law as the movement of a single spot in an external magnetic field. The influence of a contact gap of 0.5-8 mm and current on the dynamics of cathode spots was investigated. The gap dependence of the proportional coefficient between the spot velocity and magnetic field in the case of a pure copper cathode was obtained. A phenomenon was discovered, where a group of cathode spots form in the short arcs on the CuCr cathodes after a transition diffuse arc stage. The follow-up investigation revealed that a close interrelation exists between the cathode and anode processes in short arcs. This interrelation is responsible for the appearance of the discovered phenomenon. Short-circuit performance tests conducted for a commercial vacuum interrupter proved cathode spot group formation to be responsible for the interruption failure at short contact gaps