Cathode Surface State and Cathode Temperature Distribution After Current Zero of Different AMF-Contacts

Vacuum interrupters, particularly with high short-circuit interruption ability, are mostly equipped with contact systems based on two different principles: the widely used radial magnetic field (RMF) contact and the axial magnetic field (AMF) contact system. In this investigation, contact electrodes performance of an improved RMF system was compared with both an unipolar and a quadrupolar AMF contact system. By using a high-speed complementary metal oxide semiconductor digital video camera, the different systems were observed during arcing under short-circuit conditions at different current levels, concentrating on arc modes development with different arcing times. Contact erosion and thermal stress of the high-current vacuum arc on the contacts was basically evaluated on the basis of contact melting depth, with the result of comparable melting depths at insignificantly higher thermal stress of the RMF versus AMF systems. The microstructure of the copper and chromium compound contact material cross section was analyzed by means of a scanning electron microscope.

An experimental study of a high-current vacuum arc generated between two static CuCr25 contacts spaced 20 or 30 mm apart was conducted to characterize the arc appearance and the cathode spot (CS) distribution. The arc was ignited from the lateral surface of the cathode and controlled using an external axial magnetic field. Under the investigated experimental conditions, three distinct arc modes have been observed: multiple arc, diffuse, and diffuse columnar modes. The latter appeared at low $B_{\mathrm {AMF}}/I_{\mathrm {arc}}$ values, typically below 4 mT/kA. The temporal evolutions of the spot spatial distribution and the associated distribution of the current density were analyzed by processing high-speed video images of the cathode. Various types of distribution depending mainly on current were identified. At low currents (up to 13 kA), the CS distribution covered only a fraction of the cathode surface. At intermediate currents (in 17.2–31.1-kA range), CSs were present on the whole circumference of the cathode and the CS distribution included a closed region without spot. The latter was progressively filled by CSs, yet it totally disappeared before the end of arcing only when its location was off-centered with respect to the cathode axis. At high currents (up to 36.3 kA), the whole cathode surface was occupied by CSs at current peak. The azimuthally averaged radial distribution of the current density was found to be relatively uniform in the regions occupied by CSs. An average spot current of 36.5 ± 2.5 A has been estimated.

Plasma transport during continuous extinction processes from high-current arcs to full extinction is an important issue in the study of vacuum arcs. However, to describe this transition, the traditional methods, magneto hydrodynamics or particle-in-cell, are limited by either accuracy or efficiency. In this paper, we developed a fully 3D hybrid plasma simulation algorithm to study the extinction process of multiple cathode spots in a vacuum environment. Cathode spots are modelled as independent plasma sources rather than treating the whole cathode surface as an indivisible source. For a better balance between efficiency and accuracy, ions are modelled as particles and electrons are treated as a massless fluid. Using this model, the effects of cut-off time and the locations of cathode spots on the extinction process are studied. The results show that during an extinction process, the total number of ions between the electrodes gradually decreases and the mean velocity of the ions gradually increases. When the current approaches zero, two regions can be observed in the gap: the separated jet region, and the common plasma channel region. The last three surviving cathode spots have significant impacts on the ion distribution at current zero, whose values depend on the distances to them. The closer together the three cathode spots are, the higher the value that the local ion density will reach. In addition, at current zero, the maximum ion density appears near the last extinguished cathode spot.

The copper vapor density after the occurrence of a magnetically stabilized high-current vacuum arc was measured by the laser absorption method. Measurements were performed in a vacuum triggered gap with oxygen-free, axial magnetic field (AMF) Cu electrodes (40-mm-diameter cathode and 60-mm-diameter anode), separated by a gap of 30 mm. The arc current was a sinusoidal wave having a half-period of approximately 10 ms and a peak value ranging from 3 to 10 kA. The arc was stabilized by a magnetic field produced by the AMF electrodes. The copper vapor density at current zero increased with peak arc current and did not depend on the current zero slope. After current zero, the copper vapor density decayed with a time constant of approximately 400 /spl mu/s. These results can be explained by a model based on vapor expansion from cathode spot craters.

Axial magnetic field (AMF) contacts are applied within vacuum interrupters especially for high short-circuit currents. In this paper, two AMF contact designs based on a bipolar and a quadrupolar magnetic field arrangement and their characteristics are presented. In the first section, both principles are discussed. This is followed by three-dimensional field simulations performed by means of a finite-element-analysis program. The magnitude of the axial magnetic flux density, the phase shift between current and magnetic flux density, and the residual AMF after current zero are investigated during arcing with a constant contact gap. Furthermore, the influence of motion during the contact opening on the previously-mentioned parameters is investigated. An important parameter during the design phase of AMF electrodes is the electromagnetic attractive force, which is provided by the AMF contacts during current flow. This force can reduce the required contact force provided by the drive of the vacuum circuit breaker. Finally, it is reported about the short-circuit performance of both contact systems. Contact plates after high-current interruption are presented revealing the arcing behavior during the high-current phase.

Axial magnetic field (AMF) contacts are applied in vacuum circuit breakers to interrupt high short-circuit currents. A magnetic field parallel to the current flow in the arc column improves the breaking capacity by affecting the arc mode. Depending on the design of the contacts, this magnetic field can take different directions over the contact surface. In this paper, measurements of bipolar and quadrupolar AMF contact systems are presented. For bipolar arrangements, the polarity of the field changes once and for quadrupolar arrangements twice. Due to different eddy current paths, the residual magnetic field at current zero is significantly higher for the quadrupolar arrangement. The breaking capability of two contact systems was investigated and completed by measuring the post arc current.

In order to explore the feasibility of axial magnetic field (AMF) contacts for applications requiring large contacts at large contact separation, i. e. at voltages above the typical medium voltage regime, experiments have been performed to investigate the arc behavior under these conditions. The AMF arcs were produced between AMF contacts in a synthetic test circuit; the contacts which were numerically optimized by 3- dimensional finite element modeling of the magnetic field distribution. High-speed video recording was used as a major diagnostics to investigate the arc behavior under different conditions. It was found that successful interruptions could be performed at RMS currrents of over 30 kA, at contact strokes of several tens of mm. Even at the highest currents investigated (42 kArms) the arcs are evenly distributed over most of the contact surface, indicating the suitability of AMF contacts for a current interruption capability of 40 kA of nominal short circuit current at these contact strokes.

The influence of lagged axial magnetic field (AMF) due to eddy current induced in contacts system on the distribution of cathode spots (CSs) in current-zero stage of vacuum arc with different current falling rates was investigated experimentally. Experiments were conducted with cup-shaped AMF contacts in a demountable vacuum chamber. The characteristic of CSs distribution of triggered vacuum arc in current-zero stage with arc current frequency of 50Hz was investigated at first with the aid of a high-speed digital camera with an exposure time of 2 microseconds. Then the characteristic of CSs distribution with faster current falling rate, which was realized by injecting a countercurrent with frequency of 500Hz or 1kHz, was investigated. Moreover, the lagged AMF flux density in current-zero stage with different current falling rates was calculated by ANSYS. Research results showed that CSs located in the center of contact plate tended to extinguish earlier than those at periphery of contact plate in fast current-zero stage due to the stronger lagged AMF in contact center.

Arc behaviour during filtered vacuum arc deposition of Sn-O thin films

Arc ion plating(AIP) has been widely utilized in the deposition of various kinds of thin solid films due to the excellent characteristics of the arc plasma produced from an active cathode spot that emits ions of cathode material,as well as electrons.In AIP process,the cathode spot is usually steered by an external magnetic field.Cathode spot motion is the key factor because it affects the physical characteristics of the vacuum arc plasma,the utilization of the cathode material, the emission of macroparticles(MPs) and the quality of subsequent films containing these MPs. Therefore,cathode spot dynamics should be understood practically under a compound external magnetic field,such as in axisymmetric magnetic field(AMF),for industrial applications.An AMF produced by using an adjustable electromagnetic coil associated with a concentric magnetic flux guider was applied to the cathode surface to investigate the influence of the AMF on the arc cathode spot motion.The distribution of the magnetic field was simulated by the finite element method (FEM) software.The magnetic field intensity was measured by an SHT-V magnetometer and the distributions of magnetic field with different intensities were analyzed.Based on the results of FEM simulation and the physical mechanism of the arc cathode spot discharge,the effects of magnetic-field components and AMF on the cathode spot movement were discussed.The results showed that increasing the AMF intensity can strongly influence cathode spot movement.In the case of a weak AMF, the cathode spot moves randomly on the cathode surface.With increasing AMF,there is an increasing tendency for the cathode spot to rotate and drift toward the cathode target edge.The increase in the transverse magnetic field(TMF) intensity,B_T,can accelerate the rotational velocity of the cathode spot,increase the arc voltage and decrease the arc current.With a relatively strong AMF(B_T≈30 Gs), the cathode spot rotates near the edge of the cathode surface and is confined to a circular trajectory. A new arc cathode spot is ignited,splits,and is extinguished repeatedly on the cathode surface,which can be observed at intervals of about 0.5 s,while there is an obvious erosion track left at the bottom of the cathode edge.

The distribution and dissipation of neutral atoms are crucial for understanding the dielectric recovery process after interrupting direct current (DC) vacuum arcs. This article aims to investigate the dissipation of copper atoms after a forced extinction of the vacuum arc experimentally, adopting the plane laser-induced fluorescence method. The change in the 2-D distribution of copper atoms with time is presented. The results show that the magnetic fields, the axial magnetic field (AMF), and the transverse magnetic field (TMF) have a limited effect on the initial density of copper atoms. For both the AMF and the TMF, the copper atom densities at current zero (CZ) vary in the range (6– $8) \times 10^{17}\,\,\text{m}^{-3}$ when 3-kA vacuum arcs are forced to 0 in 0.2 ms. Contrary to the TMF case, in the AMF case, the evaporation on the anode after the CZ results in long-existing atoms near it. Consequently, the atom density of the TMF decays faster than that of the AMF, which indicates that a vacuum interrupter with the TMF contacts has a better performance when interrupting a DC load. The difference between the two magnetic fields originates from the arc control patterns. Namely, the AMF tends to keep vacuum arcs in a stable mode, which is unfavorable for a DC interruption; on the contrary, the TMF drives the vacuum arcs to move at high velocities resulting in faster dissipation of copper atoms after the CZ. In addition, the composition proportion of CuCr contacts has a limited effect on the diffusion of copper atoms when a low-vacuum arc is interrupted.

Cathode spot is the source of plasma and metal vapor which affects the breaking capacity of vacuum circuit breakers. In this paper, the cup-shaped axial magnetic field contacts were used and the images of cathode spots were obtained by an ultra high-speed camera in the experiment. The dynamic characteristics of cathode spots during expansion process in vacuum arc were investigated. Under the conditions of different current amplitudes, different distributions of cathode spots were observed. During the expansion process of cathode spot, there are group spots and individual spots. The group spot only occurs in the cold electrode. The electrons emission characteristics of cathode spot as a function of temperature and electric field was calculated. The formation of group spot is related to the current and electron emission of cathode spot. The cathode spots were identified by Matlab. It was found that the variation of number of cathode spots is consistent with the current. However, at different stages, the reasons for the increase in the number of cathode spots are different, such as the increase in the area of cathode spots in the initial arcing stage and the splitting of cathode spot. It was observed that the cathode spots are not completely evenly distributed on the cathode surface. The farther away from the center of the contact, the longer the distance between the cathode spots. It is mainly effected by the transverse magnetic field (TMF) which is generated by the arc column. The closer to the contact edge, the higher the TMF at the cathode spot and the higher the expansion velocity of cathode spot.

This paper discusses the issues of a possible change of the type of cathode attachment of high-current vacuum arcs (HCVA) with an average cathode current density of more than 105 A/cm2. This type of HCVA is used as pumping plasma gun in experiments with plasma puff z-pinches. These experiments showed that the measured linear mass of the HCVA plasma jet is much higher (by a factor of 10 or more) than the expected mass, which can be obtained from the assumption that cathode attachment occurs only through a multitude of cathode spots emitting supersonic plasma jets. It is shown that in HCVA of the type under consideration, at some time instant there are two types of cathode attachments - cathode spots and thermionic erosion attachment (TEA). It can be said that HCVA of this type have a hybrid cathodic attachment. Unlike cathode spots, TEA produces a subsonic plasma flow, which contributes to an increase in the linear mass of the HCVA plasma jet.

The decay of neutral copper vapour density which follows the extinction of a high current (2-11 kA) vacuum arc between copper electrodes has been measured. The results show that during the period 1-8 ms from current zero the vapour density decays at least an order of magnitude slower than expected on the assumption that vapour atoms generated by the arc move freely to the electrode and vacuum chamber surfaces where they condense. The results can be explained, however, if it is assumed that the vapour is produced by evaporation from molten droplets which are emitted from the cathode spots.

Based on a 2-D magnetohydrodynamic model, high-current vacuum arcs (HCVAs) under axial magnetic fields (AMFs) with consideration of current density distribution at the cathode are simulated and analyzed. The current density distribution at the cathode used in the simulation is obtained by processing the image of cathode spots from experiments. In order to study the influence of current density distribution at the cathode on vacuum arc characteristics, HCVA is simulated under two kinds of current density distribution at the cathode. Then, for HCVA simulation in a half-sinusoidal period, the current density distribution at the cathode and the AMF delay are both taken into account. The simulation results show that the nonuniform current density at the cathode makes the values of the arc plasma parameters in the interelectrode region larger and their radial distribution more nonuniform than the uniform current density at the cathode. The more uniform current density at the cathode and the stronger AMF at the moments in the second 1/4 cycle make the values of the arc plasma parameters in the central region smaller and their radial distribution broader than those in the first 1/4 cycle. In addition, the simulation results will also be compared with the experimental results.