Analysis of Cross-Flow Arcs in Transverse Magnetic Fields

Through the specification of two independent variables (e. g., arc current and mainstream velocity), and using appropriate phenomenological relations, the flow fields, temperature fields, and arc boundary shapes of magnetically balanced cross-flow arcs have been determined. Flow through the arc occurs at lower Reynolds numbers, Reo ~ 1; at higher Reynolds numbers a dividing streamline is formed, inside of which double vortex flow is found. Isotherms are nearly circular for the case of flow through the arc. At the higher Reynolds numbers the isotherms are non-circular throughout the cross-section with major axis transverse to the direction of flow.     

Stable Configuration of Electric Arcs in Transverse Magnetic Fields

Deviations from rotational symmetry of arc columns exposed to transverse magnetic fields cause convection and a momentum transport between the column and its surrounding gas. As a consequence, the arc moves and changes its geometry until it reaches a stable configuration. Based on the momentum transport equation, the arc behavior is discussed under various operating conditions, and stable arc configurations are determined. In particular, the stability-criteria of a balanced arc column in a transverse gas flow and magnetic field is derived. It turns out that the arc becomes unstable with respect to kinks parallel to the applied magnetic field when ? = p?/(B2/8?)?1. (p? = ambient pressure, Bo = applied magnetic field).     

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.     

Transverse Forces and Motions at Cathode Spots in Vacuum Arcs

Consistent analyses are provided for the motion of cathode spot cells in the presence of magnetic fields parallel to the cathode surface (retrograde motion), for the spontaneous splitting of cells, and for the grouping of cathode spot cells in organized structures. The formulas for retrograde motion and cell splitting frequencies are evaluated for cells carrying 1-80 A on copper vacuum arc cathodes. The results for retrograde motion are shown to agree satisfactorily with published experimental data. It is concluded that retrograde motion can be explained simply, and that measurements of retrograde velocities can provide useful information concerning cell sizes in cathode spots. The close connection demonstrated between retrograde motion and cell splitting yields the conclusion that cell splitting should slow in the presence of an ambient gas, as does retrograde motion. Cathode spot sizes and energies of formation are evaluated for spots that are circular clusters containing up to twelve individual cells. It is concluded, in apparent agreement with experiment, that such clusters should not be stable under conditions of clean vacuum where the cells exhibit retrograde motion.     

The Interaction of Vacuum Arc Ion Currents with Axia I Magnetic Fields

We summarize a series of experiments in which we measured the distribution of ion currents leaving the interelectrode region of a vacuum arc with Cu electrodes. Ion currents were collected by an arrangement of cylindrical collectors surrounding the arcing space. A Helmholtz coil arrangement surrounding the arcing chamber generated the axial magnetic field. Arc currents ranged from 70 to 2400 A dc.     

Current Density Per Cathode Spot in Vacuum Arcs

Two methods of computing the current density in a cathode spot of a metal arc are compared. The first method computes the spot area in terms of a crater left on the metal. Detailed arguments are presented as to why this method is not correct. Evidence is presented supporting a second method, that of estimating the cathode spot from the luminous glow observed during the discharge. The current density is estimated to be less than 105 A/cm2 during the lifetime of the spot.     

On the Retrograde Motion of Arcs in Magnetic Fields

Experiments are reported which allow the determination of retrograde velocity of individual cathode spots and of the plasma flow in a pulsed discharge using various metals and carbon as cathode materials. For discharge currents from 10 to 40 amps, pressures of 3 mmHg and magnetic field strengths of 6.103G retrograde spot velocities from 30 to approximately 300 m/sec are observed and the corresponding plasma flow velocities are in the range from 4,400 to 8,600 m/sec. On cathode materials with low melting points, the splitting rate of spots and the motion of individual spots is small, whereas under identical conditions the spots on refractory materials are highly mobile, the splitting rate is large, and the lifetime of individual spots is short.     

Arc Cathode Processes in the Transition Region between Vacuum Arcs and Gaseous Arcs

The cathode processes of electric ares on cleaned Cu cathodes were investigated in the transition region between vacuum and atmospheric pressure (argon). The plasma density in the cathode plane was estimated by probe measurements to be n = r are current, r – distance from the spot). It was observed that several cathode spot parameters have an extremum at p ～ 10⁴ Pa. The crater diameter has a minimum independently of the cathode temperature. The diffusion constant of the chaotic motion determined by framing photographs was found to have a maximum. Some additional, large displacements occurred at that pressure. The diameter of the bright plasma cloud obtained by open-shutter photographs showed a maximum, the current per spot was found to decrease from 20 A in vacuum to 10 A at atmospheric pressure. It is thus concluded that the spot with the smallest crater radius and a low current per spot, occurring at ～ 10⁴ Pa, represents the single spot, whereas the spot at higher pressures, and probably also in vacuum, has a complicated nature where the large craters are formed by a cooperation of single spots.     

Cathode Erosion in Vacuum Arcs and Unipolar Arcs

The cathode spots from vacuum arcs on 316 stainless steel are compared with the tracks found on the same material after exposure to the plasma of the tokamak TFR 600. Further the erosion yields of vacuum arc cathodes of 316 stainless steel and titanium are determined from experiments and the measured values are compared with theoretical estimates. The velocity of the arc is investigated as a function of the applied magnetic cross-field. The scatter of both, the velocity data and the erosion yields is substantial. Improved experiments are planned.     

Basic Characteristics of Vacuum Arcs Subjected to a Magnetic Field Parallel to Their Positive Columns

As vacuum arcs subjected to a magnetic field parallel to their positive column (an axial magnetic field) spread uniformly over all the electrodes and burn in the interelectrode region, arc voltages of these arcs are low and quiescent. When the magnetic field strength decreases, however, the arc voltage develops a large noise component and electrode melting occurs. Experiments were conducted to investigate the condition of these transition phenomena. As a result of these experiments, it was found that these two phenomena do not always occur simultaneously and that a new explanation for the mechanism of anode spot formation should be considered.     

Vacuum Arc Behavior in a Transverse Magnetic Field

Experimental measurements in a vacuum interrupter have shown that the application of a transverse magnetic field results in substantial increases in arc voltage. Photographic studies of the arc column indicate that strong magnetic fields reduce the effective anode area and may lead to severe arc constriction.     

Use of Axial Magnetic Fields to Improve High-Current Vacuum Interrupters

When an axial magnetic field is applied to a vacuum arc, the arc tends to be stabilized in its diffuse mode. A minimum arc voltage is found for a certain magnetic field. In this condition, interrupting current is significantly increased, and it is nearly proportional to the diameter of electrodes. About ten years ago, a practical axial magnetic field electrode was developed for vacuum circuit breakers. Since then, through various improvements in its structure, this electrode has been refined for practical application in vacuum circuit breaker interrupters. The application has successfully covered not only medium-voltage circuit breakers, but also high-voltage (84 kV), dc high-voltage, and high-current circuit breakers. In this paper, ten years experience in this area is described.     

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.     

Stabilization of Long Electric Arcs by Rotating Magnetic Fields of Different Configurations

The results of the investigation of the direct current electric arc under the influence on it of the rotating magnetic fields of the inductors with a different number of poles pairs are given in the article. The analysis and the comparison of the arc energetic characteristics and of its movement parameters in the rotating magnetic fields of different configurations were carried out. The comparison of the experimental data and of the results of the theoretical analysis of the arc movement in the 2-poles and 4-poles inductors fields was carried out. A number of original methods for the application of the researched type of the magnetic effect on the electric discharge in plasma techniques and technology was suggested.     

Interaction of a Plasma Beam with a Transverse Magnetic Barrier

The interaction of an energetic He plasma beam with a quasi step-like transverse magnetic barrier is reported. When the rate of flow of momentum in the incident beam is less than the pressure of the magnetic field, a collisionless electrostatic shock is observed to form in front of the barrier as a result of the drastic compression of the plasma. The initially cold plasma is substantially thermalized by the shock and the electron density is increased by a factor of four. In the opposite limit the plasma penetrates the barrier in a flute-like manner.     

Peculiarities of the Gravitational Interaction of Electrical and Magnetic Vortex Fields with Nonlinear Vortex-Free Fields

Russian Physics Journal - Within the framework of GRT, the properties of the gravitational interaction of magnetic and electric vortex fields with self-gravitating nonvortex fields are...     

Anode-Spot Formation and Motion of Vacuum Arcs

This paper reports about experimental investigations on high-current vacuum-arc phenomena, especially anode-spot formation, arc states, and motion. The presented work was stimulated by lack of information about the transition process from the diffuse low-current mode to the high-current mode characterized by anode spot(s). Optoelectronic measurements, streak photographs, high-speed movies, and correlated arc voltage/current records yielded remarkable results on power-frequency vacuum arcs. Three different high-current vacuum arc modes can be observed beyond a certain threshold current. Which mode appears depends mainly on the momentary electrode distance. The modes are characterized by different anode-spot behavior and interelectrode phenomena. The transition between different arc modes is continuous. The arc modes observed on ring electrodes producing a magnetic blast field are the same as those appearing on butt-type electrodes. Anode-spot formation is preceded by congregations of cathode spots and may be initiated by thermal overload of the anode surface opposite to these cathode-spot clusters.     

Anode Phenomena in Vacuum and Atmospheric Pressure Arcs

This paper summarizes recent experimental data related to anode phenomena in both vacuum and atmospheric pressure arcs. Currents in the range 10A to 3OkA are discussed, and particular emphasis is placed on the effect of plasma flow from the cathode. For vacuum arcs this plasma flow is the directed motion of metal ions from the cathode spots. These ions reduce the anode voltage drop, and maintain a diffuse anode termination. At atmospheric pressure the ion flow is impeded by gas-atom collisions. However, a plasma flow towards the anode can result from magnetic pinch forces at the constricted cathode termination. In the absence of plasma flow, the anode termination constricts to a vigorously evaporating anode spot. For a typical non-refractory electrode such as copper, the spot operates at a temperature close to the boiling point irrespective of the gas pressure. The spot temperature is dictated by the balance between electrical input power and evaporative losses. These anode phenomena are discussed in relation to vacuum switchgear, arc welding and arc furnaces.