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     

Dielectric Recovery of Vacuum Arcs after Strong Anode Spot Activity

Recovery of dielectric strength and post-arc currents after diffuse and constricted vacuum arcs were measured for filat OFHC-Cu contacts (D = 25 mm, d = 7.5 mm) enclosed in a bakable UHV chamber. The arc current pulse had a trapezoidal shape of 5.5-ms duration with peak values up to 11 kA. In comparison with the fast recovery of diffuse arcs, the recovery of constricted arcs with gross melting is considerably retarded. Post-arc currents are simulated using the Andrews-Varey model extended to include the effects of secondary electron emission due to ion bombardment of the cathode and loss of the plasma due to thermal motion. The flow of charge carriers to the anode and the shield, which is at the anode's potential, are registered separately. The amount and decay of the residual plasma is evaluated from the measurements of post-arc current. The decay times of a few tens of a microsecond give evidence of ions with energies below 1 eV. The origin and effect of slow ions on recovery is discussed.     

Self-extraction of negatively charged carriers from anodic vacuumarcs

Measurement and magnetic analysis of self-extracted negatively charged carriers (NCCs) from anodic vacuum arcs are presented. They flow to charge collectors made of stainless steel, which are electrically connected to a cathode. When a 60-Hz sinusoidal arc current of 40 kA is burned on 20-mm-diameter copper electrodes spaced 4 mm apart, a negative current of approximately 900 A flows to a cylindrical collector surrounding the arc. The floating collector potential relative to the cathode is measured, and the mean energy of the NCCs is estimated to be greater than 40 eV. It is difficult for the NCCs to flow from anodic vacuum arcs when an anode is made of zinc or cadmium, the atoms of which have electron affinities of less than 0 eV. A magnetic filter of about 500 G, which is placed between the arc and a 30-mm-diameter circular collector, does not affect the NCC's flow from a 4-kA arc burned on copper electrodes. It is possible to extract a large amount of negative copper ions from the anodic copper vacuum arcs     

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.     

Small-Scale Anode Activity in Vacuum Arcs

Coordinated high-speed movies, streak photographs, and voltage/current oscillograms have been taken for vacuum arcs on copper-based electrodes at peak currents up to 70 kA in half-cycle pulses. These results show that small-scale transient luminous anode-spot activity is associated with the strong voltage noise that precedes the establishment of the conventional large anode spots. The characteristic dimensions of the small-scale spots go below a millimeter, and may be less than 100 ?m. Unlike cathode spots of that size, these small anode spots always move in the I × B direction. This small-scale activity is especially pronounced in experimental systems initially containing surface films of volatile matter. Good correlations have been established between bursts of anode light and corresponding bursts of arc voltage noise, both of which appear to be associated with variations in the small luminous structures. The practical importance of the small transient luminous anode activity reported here is in its clear tendency to advance the formation of electrode jets, particularly under experimental conditions favoring the evolution of gas or vapor from anode surfaces. It has theoretical significance as a precursor to the formation of the usual large anode spots and jets, and as a possible source of structure within large anode spots.     

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     

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 Time-Resolved Characterization of Erosion Products from High-Current, Copper Vacuum Arcs

A vacuum arc at high enough current can produce gross melting on electrode surfaces as a consequence of anode spot formation and other high-current electrode phenomena. Erosion from the electrodes under this condition is much more rapid than at low-current (where material loss occurs principally from the cathode) and is a process that is presently poorly understood. The present work is aimed at characterizing the erosion products from cathode and anode surfaces during high-current arcs on copper electrodes for single half cycles (60 Hz) arcs having peak currents of 30 kA. Fully open gap lengths were approximately 18 mm. Among the findings were the following. a) Erosion rate determined by electrode weight loss was approximately 8 mg/C of arcing. b) Droplets ejected from the electrodes had masses varying from a few tenths to a few tens of micrograms and velocities typically up to 40 m/s, although higher velocities are seen. c) The greatest number of droplets are produced at, or just after the current peak, and higher droplet velocities are seen in this same time interval. d) Erosion in vapor form detected in the plane of the cathode surface and moving radially is a maximum just after the peak of current and is relatively abundant. Such vapor is essentially absent in the anode plane.     

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     

Macroparticle Dynamics during Multi-Cathode-Spot Vacuum Arcs

Macroparticle dynamics in multi-cathode-spot (MCS) vacuum arcs were studied by utilizing laser Doppler anemometry (LDA) methods for in situ measurement of the cathodic macroparticle velocities and relative emission rates. Arc current pulses having peak values of 1-2 kA at either 6 or 1 ms after arc initiation were investigated. Systematic dependence of the macroparticle dynamics (i.e., speed and direction of flight) on cathodic thermophysical properties, location of the measurement probe in the interelectrode region, instantaneous value of the arc current, arc current waveform, and macroparticle size was determined. It was found that the macroparticle velocity increased with the melting temperature of the cathode metal, distance from the cathode surface, and the instantaneous value of the arc current, and decreased with macroparticle size and the rise time of the current waveform. All the above dependencies may be understood as direct indications of the plasma-macroparticle interaction during the discharge. The measured instantaneous relative emission rates were found to peak later than the arc current but before the peak average cathode surface temperature, which was estimated using a semi-empirical model. This result may be an indication of the dependence of cathodic erosion in the form of molten metal droplets on the average cathode surface temperature.     

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     

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.     

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     

Separation of spiral contacts and the motion of vacuum arcs at highAC currents

A framing camera is used to photograph the vacuum arc between separating spiral-petal vacuum interrupter contacts. The rupture of the molten bridge between the contacts first leads to a high-pressure, transient arc column. This arc motion can become constricted for several milliseconds before it goes diffuse as the current decreases to zero. The current through the spiral contacts produces a magnetic field perpendicular to the arc column, which forces the arc to move outward and run along the periphery of the petals. Several vacuum arc modes occur during the half-cycle of high current arcing. Movies, gap-current curves, and arc voltage traces are used to study the development of the arc motion and how it is affected by the contact structure. This information is used to generate arc appearance diagrams in which the arc form and motion are correlated to instantaneous values of current and gap for a wide range of peak currents. Appearance diagrams are shown for two ranges of opening delay from current onset     

The Local Electrical Properties of Gas Blast Arcs near Current Zero

Results are reported of experiments performed on a model air-blast circuit breaker to determine the distribution of voltage along the axis of the arc gap during the current zero period, following half sinusoidal current pulses of frequency 85 Hz. The air-blast was sustained by a reservoir pressure of 6.89 × 105 N/m2. Measurements were made following peak currents of 3 and 8 kA, with copper and carbon upstream cathodes, different separations of upstream cathode from nozzle inlet and different conditions downstream of the nozzle throat. The results have enabled the conductance decays at various axial positions to be determined and these have been related to the improved circuit breaking performance observed by other authors when the nozzle and arc gap geometries are optimized. Although a detailed theoretical analysis awaits the measurement of other fundamental plasma properties, an approximate evaluation of the role played by various basic processes has been made.     

脉冲HF化学激光体系中的放电模式分析

采用面阵滑闪火花预电离诱导的平行板放电结构,研究了SF6-C2H6混合气体中的脉冲放电模式,确认了不同放电条件下存在自持体放电(SSVD)和电弧放电两种模式,且两种模式可以互相转化。SSVD模式放电电流波形主要由电容电压和气压决定,气体混合比和储能电容值对其影响很小。SSVD电流峰值随电容电压增加基本线性增加,随气压增加线性减小,分析表明这一特性是因为SSVD是由多个同步电子雪崩在时空上叠加而成,属于a过程主导的电子雪崩放电。     

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     

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     

Der freibrennende Störlichtbogen

Long high current arcs (I ? 60 kA, f = 50 Hz, t ? 200 ms, l = 1 m) appear after discharges at electric power systems. The arc must be localized in a separate volume in order to protect the equipment from the high temperature. The arc is stabilizing by the plasma-beam starting at its bottom. The properties of the plasma-beam are described: general phenomena, the length as a function of current and time, the behaviour in a half-cycle and the acoustic emission. Several conclusions for the construction of the protective armatures are presented.