Reignition phenomena after high-frequency current interruption withshort vacuum gaps

Based on a large number of measurements of high-frequency (HF) current interruptions in vacuum at small contact gaps (⩽600 μm), the statistical reignition behavior of vacuum switching devices after a HF current zero is investigated. Three types of reignitions can be classified. Statistical evaluation of post-arc current measurements for different parameters at current zero and different HF current ignition processes gives information about the stress of the gap as a result of the transient recovery voltage after a HF current zero (HF-TRV) and the accumulated post-arc charge. Comparing post-arc current values at the beginning of the HF-TRV and at the moment of reignition reveals a production of charge carriers during the recovery interval. Possible reasons for the different types of reignitions are discussed     

Reignitions in short vacuum gaps after interruption ofhigh-frequency currents caused by ion bombardment

The internal energy increase of the cathode material during the post arc period as a result of the high electrical field at the cathode surface and the highly energetic ions existing in the vacuum gap is studied in this paper by solving the conservation equations of mass, momentum, and energy together, with the equation of state of electrode material in the cathode region. Solving the heat conduction equation in the cathode region, the temperature distribution of the cathode material is determined. The results for copper indicate that a breakdown can occur at the cathode surface some 100 ns after current zero depending on the form of the cathode surface inhomogeneities, the steepness of the transient recovery voltage, and the initial plasma distribution in the vacuum gap. These results are in accordance with experimental measurements     

Electrical and pyrometric measurements of the decay of the anodetemperature after interruption of high-current vacuum arcs andcomparison with computations

Vacuum arcs of up to 20-kA peak current were investigated. The surface temperature of the anode area melted during the anode spot mode was determined by pyrometry and the evaluation of thermionic currents. The measurements confirm the computations of heating and cooling of the anode, taking into account heat conduction melting/solidification, and evaporation. Pyrometrically obtained temperatures agree well with theory. This gives confidence in the heat conduction model and also shows that the boiling temperature was reached during arcing. Another method evaluates currents of a milliampere value after arcs of several kiloamperes and postarc currents of amperes. Experimental observation (e.g., loading of the shield surrounding the contacts) and theoretical analysis of the interfering effects support the idea that the currents measured are due to thermionic emission     

Mechanism of ion flow generation in vacuum arcs

The ecton model of the cathode spot is used to analyze the main parameters of ion flow in vacuum arcs (ion erosion, mean charge, and velocity). It is shown that the arc plasma is formed as a result of microexplosions at the cathode surface, induced by the Joule heating by the high-density current of explosive electron emission. Ionization processes are localized in a narrow region of the order of a micrometer near the cathode and the ionization composition of the plasma subsequently remains unchanged. Under the action of the electron pressure gradient, ions acquire directional velocities of the order of 10⁶ cm/s even over small distances of the order of several micrometers.     

Experimental observation of high-voltage, low-current vacuum arcs

A poorly explored type of discharge has been investigated in high vacuum (10^-7 to 10^-6 torr), with a DC high voltage across 0.2- to 0.8-mm gaps. The discharge has been found to be quite different from other widely known types of vacuum and gas discharges by the combination of its voltage-current characteristics (hyperbola-type), source and carriers of current (mostly electrons), and spatial potential distribution (a considerable electric field across the gap and a steep potential fall near the cathode)     

Motion of high-current vacuum arcs on spiral-type contacts

Motion of vacuum arcs on spiral-type contacts is not only controlled by self-induced magnetic fields, but also by heating phenomena. An expression is derived which permits the calculation of the speed of the arc from a computation of the time needed to heat the surface up to boiling temperature. The heat flux density of the constricted arc at the anode is required as input for the calculation. Good coincidence is achieved with experimental data. The speed of the arc varies from 5 to 400 m/s, depending on experimental conditions     

The origin of current constriction in vacuum arcs

The current constriction in a diffuse vacuum arc is shown theoretically and experimentally to be caused by a pressure source, to which the charged particles and neutral particles contribute.     

Experimental observations of steady anodic vacuum arcs withthermionic cathodes

Stationary plasma discharges have been investigated in a high vacuum ambient (background gas pressure <10^-2 Pa), with an externally heated cathode and a consumable hot evaporating anode. With various anode materials like chromium or copper, and electrode separations between 0.5 and 3 mm, the nonself-sustained discharge operates with DC arc currents in the range of 220 A. The waveform of the arc voltage is strongly influenced by the magnetic field of the cathode heating current, and arc voltages between a minimum of 3 V and a maximum exceeding 100 V have been observed. The voltage-current characteristics (V_CC) and the influence of the electrode separation have been measured separately for the minimum and the maximum of the arc voltages and show a different behavior. The metal plasma expands into the ambient vacuum toward the walls of the vacuum vessel and offers a macroparticle free deposition source of thin films. The arc voltage can be varied by external manipulations of the arc discharge, and the mean ion energy of the expanding metal plasma shows a linear dependence of the mean arc voltage     

Townsend-type breakdown as a criterion for high-frequency vacuumarc interruption at submillimeter gaps

In vacuum circuit breakers, multiple reignitions give rise to HF current arcing (≈500 A; ≈200 kHz). Due to the small contact distance and the very large current gradient, at every arc-current zero pressures of several tens of millibars can be expected. Very soon thereafter (≈30 ns) this gap is dielectrically stressed by the first component of the restriking voltage (≈10 MHz), originating from parasitic impedances. The combination of the associated high electric field and the relatively high neutral density may cause Townsend-type breakdown, leading to another half-sine of continued arcing. Both dielectric stress and residual neutral density are expressed as a function of di/dt, yielding values of interruptible di/dt as a function of the contact distance with the Townsend breakdown criterion. Comparison with experiments shows fair agreement in the range of di/dt of 100-1000 A/μs and distances of 0.1-0.5 mm for two different circuits     

Investigation of arc roots of constricted high current vacuum arcs

The anodic and cathodic arc roots of constricted high current vacuum arcs were investigated with a fast framing charge-coupled device camera of 1 μs exposure time. The experiments were performed with cup-shaped contacts, with sinusoidal currents of amplitudes between 20 and 100 kA, and a sine halfwave duration of 10-12 ms. The arcs were drawn by contact separation and accelerated by the Lorentz force between the arc current and the transverse magnetic field generated by the contrate contact. The anode and cathode arc roots behave reproducibility and arc scaleable within the range of currents investigated. Both types of arc roots are elliptical, with a major to minor axis ratio of 1.4. The major axis points are in the direction of arc propagation. Anodic and cathodic arc root cross-sectional areas as a function of current can both be described by a potential law with a common exponent of 0.76. For currents of 20-100 kA, mean current densities of 81-121 and 41-60 kA/cm ² were found in anode and cathode arc roots, respectively. Estimations of their temperature and vapor densities were performed. For the investigated current range T_A≈3300-3600 K, n_A ≈1.6*10¹⁹-2.2*10¹⁹cm^-3 and T _C≈3200-3400 K, n_C≈0.8*10¹⁹-1.2*10 ¹⁹ cm^-3 were found for anode and cathode, respectively     

Experimental analysis of the electrode products emitted byhigh-current vacuum arcs

The mass distribution of particles produced in the high-current vacuum arc was investigated. The experiments were concentrated on evaluating the spatial mass distribution emitted in the radial as well as in the azimuthal directions calculated from the mass deposition profile on collectors surrounding the arc discharge. The experiments were carried out in a vacuum chamber evacuated to an ambient pressure <10^-4 Pa. High-current arcs in the range from 2-7.5 kA were drawn between butt contacts of 31 and 55 mm in diameter (anode and cathode, respectively) both of a copper-chromium alloy (CuCr25). The surface mass deposited along multi-segment collectors was measured by a micro densitometer, and an inductively coupled plasma (ICP) spectrometer. Two angular mass deposit distributions were determined: the azimuthal distribution on the plane parallel to the cathode surface, and the radial distribution as a function of the angle with respect to the cathode plane. Both distributions were anisotropic and the structure of the deposition layer depended on the angle of incidence of the particles onto the substrate, the density of the particle flux and other factors. The mass deposited on the collectors consisted mostly of chromium molecules (approx. 80% of Cr and 20% Cu) for CuCr25 or CuCr40 electrodes     

X-ray radiography of aluminum cathodes eroded in high-current vacuum arcs

The aim of the study presented in this paper was to characterize quantitatively the erosion of aluminum cathodes in high-current vacuum arcs. The experimental setup comprised two current generators. The first one, capable of generating a current of amplitude up to 350 kA, was used to produce a plasma jet, that is, the object to be investigated. The second generator was used to produce a source of probe radiation for imaging the object under investigation in soft x rays of energy ?ν ≈ 0.5–3 keV. The findings of the study are based on experimental data obtained by electrophysical and radiographic methods. It has been shown that the cathode erosion rate in a high-current vacuum arc is a function of the charge passed through the cathode.     

Plasma-assisted deposition of a three-layer structure by vacuum andgas arcs

The durability and adhesion of thin coatings often depends on the structure and properties of the layer intermediate between the coating and the substrate, especially in the case where the layer and the substrate are highly different in microhardness. With a vacuum arc and a hot-cathode arc, a process has been arranged which involves cleaning of the surface, nitration of the article, and deposition of a coating. As a result, a three-layer composition has been produced which consists of a TiN layer of thickness up to 5 μm and microhardness 20 GPa, an intermediate Fe₄N layer of thickness up to 8 μm and microhardness 7.5 GPa, and a nitrated layer of thickness up to 100 μm with a gradually varying microhardness. With the TiN layer showing high adhesion, the coating has a durability three of four times greater than that of a coating produced with the use of a conventional technology     

Observation of tungsten particles in vacuum arcs by laser inducedfluorescence

We have clarified the relation between the decay of tungsten ion density in the vicinity of current zero and vacuum arc mode in high current period by using a laser induced fluorescence method in tungsten vacuum arcs of 60 Hz sinusoidal current with the peak value of 3.3, 6.7, and 9.8 kA. In the case of 6.7 kA, the arc mode was the anode spot mode. Because of the generation of the anode spot, the tungsten ion density near the anode was higher than near the cathode and the density near the anode was about ten times as high as the case of 3.3 kA which was the diffuse mode. In the case of 9.8 kA, which was the intense arc mode, the density near the anode was not significantly different from the case of 6.7 kA. The density near the cathode was higher than near the anode and tungsten ions were observed till about 30 μs after current zero while they disappeared at current zero in the other cases     

Residence time of metal ions generated from microsecond vacuum arcs

Vacuum gaps of 1 mm with lead or copper cathode are fired by a 13 μs duration sinusoidal arc or a 10 μs duration exponentially-decaying arc, and time-of-flight (TOF) ion measurements are made at variable times after the arc ignition. At the lead cathode, Pb⁺ and Pb⁺⁺ ions are generated and the upper limit on the times for Pb⁺ ion detection are 48 μs and 46 μs from the arc ignition for the sinusoidal and exponential arcs, respectively. At the copper cathode, Cu⁺, Cu⁺⁺, and Cu⁺⁺⁺ ions are generated and detected within 15 μs and 13 μs from the arc ignition for the sinusoidal and exponential arcs, respectively. The residence time of the Pb⁺ ions in the ion acceleration region is approximately 35 μs, regardless of the waveform of the arc current. The residence time of the copper ions, described by the time constant of the time-of-flight ion current delay characteristics, is 3 μs     

Two-dimensional observation of copper vapor in vacuum arcs bylaser-induced fluorescence

The density distribution and the velocity of copper neutral atoms emitted from a single cathode spot in 40 A vacuum arc were measured by the two-dimensional (2-D) laser-induced fluorescence method. The density was calibrated from the two-dimensional fluorescence image observed by a CCD camera. The gap space was almost filled with the copper atoms, and the density reached 5×10¹⁹/m³. We varied the wavelength of the laser light and measured the velocity of the copper atoms emitted from the cathode spot using the effect of Doppler shift. The velocity of the copper atoms was about 10 km/s     

Investigation of ion behavior associated with anode-spot modetransition in vacuum arcs

The ion behavior phenomenon associated with transitions of the anode discharge mode to the anode-spot mode is studied by measuring the wall ion current and by spectroscopic observation in vacuum arcs. The anode mode transfers when the wall ion current attains a certain magnitude that is independent of the cathode, but dependent on the anode. The ion-current function to the arc current increases when the arc current increases in the diffuse arc. Spectral-line intensity of Cu III emitted from the plasma in the anode region increases with an instantaneous arc current of a 5-kA peak (kAp) sinusoidal half-wave. These findings suggest an idea for the mode transition, that an ion generation region appears, and that an increase in the ion density produces a positive potential hump near the anode, which results in the negative anode voltage drop triggering the mode transition. After the mode transition, an arc current is found to reduce the ion current near the crest of a sinusoidal current in a copper arc. This appears to be significant for the arc on a small anode. The decrease in the ion current is attributed to the recombination of ions decelerated by anode vapor with electrons emitted from the hot spot on the anode     

On the ion energy distribution of high current arcs in vacuum

With the experiments presented in this paper, applications of a retarding field analyzer (RFA) for the measurement of the ion energy E _i in a vacuum arc plasma are discussed. The examined plasma was produced by a sinusoidal half-wave vacuum arc current. The experiments were concentrated on evaluating the plasma parameters at the last three milliseconds before current zero. In a current range from 300 A_rms to 10 kA_rms, the ion energy distributions and their peak values were evaluated. With the increase of the arc current, a decrease of the ion energy was found. By additional investigations of the angular distribution of the ion energies, a transition from a collision dominated interelectrode plasma to a freely expanding plasma was observed, depending on the arc current     

The behavior of vacuum arcs between spiral contacts with small gaps

The application of small gaps in high-current vacuum interrupters highlights the interdependence of the contact design, the contact gap, and the arc behavior. In this investigation, a framing camera was used to record the appearance and motion of drawn vacuum arcs between spiral-petal contacts with final gaps of 2 to 3 mm. After the rupture of the molten metal bridge, a high-pressure arc column formed and expanded across the width of the spiral arm. With a single arc column for the duration of the half-cycle, an intense anode spot formed if the peak current exceeded ~15 kA. Compared to results previously obtained at larger gaps, the arc motion was greatly reduced, and severe contact damage was observed at lower currents     

Dielectric recovery of copper chromium vacuum interrupter contactsafter short-circuit interruption

The recovery of a vacuum interrupter gap after short-circuit interruption was measured by application of an overshooting transient recovery voltage (TRV) several tens of microseconds after current zero. Copper chromium contact materials were employed varying in composition (25 and 50% chromium content), gas content, and production method. The gap failure was either pure dielectric or it was dominated by a significant postarc current. Therefore, postarc current phenomena were experimentally investigated focused on the relationship among the postarc current, the power frequency current amplitude, and the gap length. It was found that two postarc current maxima exist: the first strongly dependent on the power frequency current, and the second on the field strength. A correlation among postarc current facilitated failures, the ultimately dielectric recovery, and the erosion rate of the material was found. Strong indication is given that all of these effects are dominated by the metal vapor pressure rise given by the constricted rotating arc. A significant influence of the material properties can be drawn from these experiments, allowing a good estimation of the capability for short-circuit current interruption, thus providing a useful tool for material development