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Linear laws and parameters of ecton processes in vacuum arc
Abstract
Such arc parameters as the ion current from the cathode, number of cells in the cathode spot, energy dissipation on the cathode, force acting on the cathode, and ion mass linearly depend on the arc current. Based on these dependences, a simple theory of ecton processes in the arc with copper cathode is proposed. The interdependence of arc parameters is established based on the fact of the portion behavior of the arc. The numbers of electrons and ions in the ecton are determined. Using the Tanberg effect and Richardson-Schottky formula, the arc current density (∼108 A/cm2) and the electric field (∼107 V/cm) in the near-cathode region (∼10−6 cm) are determined.
The paper deals with the temporal structure of the explosive electron emission cell of the vacuum arc cathode spot. Essential features of the cathode spot cells operation - the ignition, burning and extinction have been considered in frames of the ecton model.
This paper presents a model for the estimation of specific parameters of vacuum arc cathode spot (CS) explosive cells. It involves the reactive force accelerating ions from the spot cell, what is known as the Tanberg effect, and the assumption that the average state of matter during the explosion is near the liquid-gas (plasma) critical point. Specifying the ions' velocity and erosion rate known from measurements, and the critical pressure, one can estimate the average current density at the CS cell to be about 100 MA/cm² for most of the materials considered.
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.
In this paper, we review the state of the art in studying the physical processes that occur in the cathode spots of vacuum arcs. The now available experimental data are interpreted in the context of the ecton mechanism of the operation of vacuum arc cathode spots. Central in this mechanism is the explosive electron emission, a phenomenon discovered by the author and his co-workers in the mid-1960s while studying high-voltage pulsed vacuum breakdown. In the light of the ecton mechanism, the cathode spot of a vacuum arc consists of individual cells which are explosive emission sites each emitting a portion of electrons termed an ecton. The cathode spot processes are cyclic in nature due to the finiteness of the ecton lifetime. It is shown that an arc is self-sustained due to the explosive emission processes initiated on the interaction of the cathode plasma either with nonmetal inclusions present in the cathode surface (first-type spots) or with liquid metal jets ejected from the zone of an active cathode spot (second-type spots). Attention is focused on the physical processes occurring during the operation of a cathode spot cell. A statistical model of a vacuum arc is used to interpret the effect of the spontaneous extinction of an arc. It is shown that an increase in the arc current is accompanied by a slight increase in the number of simultaneously operating ectons; therefore, as observed in the experiments, the parameters of a vacuum do not greatly depend on the current up to the kiloampere level.
High vacuum is a good insulator of high voltage and the processes taking place in the vacuum discharge are used in the development of various instruments. Therefore the vacuum electrical insulation and the causes leading to its breakdown are being studied with great interest. In view of this, an examination is made of the explosion mechanism of initiation and vacuum brakdown development as well as other mechanisms of the vacuum breakdown initiation.
It has been found that the cathode is the only electrode which contributes vapor for the maintenance of an electric arc under very low gas pressure. The velocity of this vapor was determined by two methods. Method 1 consisted of measuring the force of reaction of the vapor on the cathode and the rate of vaporization of the cathode material. Method 2 consisted of determining the force exerted by the vapor on a vane suspended in front of the cathode spot and the rate of vapor condensation on the vane. Both these methods gave a vapor velocity of the order of 16×105 cm/sec. A temperature of around 500,000° K results when this value for the cathode vapor velocity is substituted for c in the equation: 12mc2=3 KT2.
Ion velocities in vacuum arc plasmas have been measured for most conducting elements of the Periodic Table. The method is based on drift time measurements via the delay time between arc current modulation and ion flux modulation. A correlation has been found between the element-specific ion velocity and average ion charge state; however, differently charged ions of the same element have approximately the same velocity. These findings contradict the potential hump model but are in agreement with a gasdynamic model that describes ion acceleration as driven by pressure gradients and electron-ion friction. The differences between elements can be explained by the element-specific power density of the cathode spot plasma which in turn determines the temperature, average charge state, and ion velocity of the expanding vacuum arc plasma. © 2000 American Institute of Physics.
Measurements have been performed to determine the heat flow to the cathode during arcing in vacuum. It is shown that for a range of metals the heat flow is proportional to both current and arcing time. Depending on the cathode metal the losses vary from 25 to 34% of the total arc input. A relation between cathode heat loss and arc voltage is established. The experimental results are compared with theoretical calculations as found in literature.
Erosion measurements on copper cathodes show the erosion rate to be dependent on different variables, such as arc current, arcing time and cathode size. The experimental results offer an explanation of the large variation in erosion rate values as found by earlier experiments. The behavior of other cathode metals proves to be similar. An analysis shows the variation in erosion to be caused by changes in the output of neutral species. It is concluded that ions and neutrals differ in origin of generation, the cathode spots being the centres of fully ionized metal vapour, while neutrals are formed in surrounding areas.
The pressure on the molten surface of a vacuum‐arc cathode due to the recoil from outward‐directed ion jets is calculated for copper arcs. The calculation uses current density data and electrode force per unit current data from experiment. In addition, the force per unit current measured by Tanberg is shown to be consistent with a calculation of the same quantity based on the energy of outgoing ions measured by Davis and Miller and the so‐called saturated positive‐ion current measured by Kimblin. The ion recoil pressure is shown to be sufficient to remove molten metal from a cathode‐spot crater in time of the order of 25–250 nsec with a velocity of 2×10<sup>3</sup>-2×10<sup>4</sup> cm sec<sup>-1</sup>. It is shown that motion of molten metal in the cathode‐spot crater must be considered as a first‐order effect in rigorous calculations of surface temperature and heat flow in the metal in contact with the cathode‐spot plasma. It is suggested that the rapid removal of metal by the plasma pressure causes molten droplets to be ejected, as has been observed experimentally, and causes a preference for the cathode to operate, after the liquid is ejected, on the rim of the crater or nearby on the surface where hotter metal may exist due to liquid‐metal overflow and spatter.
Cathode properties of arcs are investigated for ambient gas pressures of 10<sup>-3</sup>-725 Torr. Copper, silver, and carbon electrodes are used in controlled ambients of either nitrogen, argon, or helium. Experimental techniques include pressure‐dependent observations of the ionized cathode vapor detected at cylindrical collectors surrounding the electrodes, arc photography, and measurements of the cathode erosion rate for copper at 100 A using the weight loss method. At pressures approaching atmospheric, it is shown that the measured erosion rate is reduced from that in vacuum by about an order of magnitude. Increasing the ambient pressure also decreases the distance from the cathode spots at which the 8% ion current can be detected. Data extrapolation shows that cathode spots at atmospheric pressure, particularly in the lighter gases, are still associated with the fundamental ion current. Comparison between the measured reductions in both the erosion rates and the ion currents indicates that vapor and ion redeposition on the cathode contribute to the decreased erosion. It is concluded that the cathode spots of low current arcs at pressures approaching atmospheric are basically similar to vacuum arc cathode spots. The final section of the paper compares current‐dependent erosion data for copper arcs in vacuum and air, and shows the importance of collective cathode spot heating.
The net erosion rate at the cathode spots of 100‐A vacuum arcs has been determined experimentally for Cd, Zn, Ag, Cu, Cr, Fe, Ti, C, Mo, and W electrodes. Ion currents to the metal walls surrounding each of these cathode materials have also been investigated. For each material, the dependence of the wall ion current on the electrode spacing and anode geometry is consistent with an arc model which assumes predominant vapor ionization in the cathode regions, with subsequent isotropic free flight motion from these regions. Comparison of the net erosion rate with the wall ion current indicates that, for high‐vapor‐pressure materials such as Cd and Zn, ≈ 15% of the vapor leaves the cathode regions ionized. For low‐vapor‐pressure materials such as C, Mo, and W, this fractional ionization is almost 100%. The ion current magnitudes observed at long electrode spacings are similar for each material, and lie in the range 7–10% of the arc current. Ion currents of this magnitude are also predicted for Mg, Al, and Ni using experimental data by Plyutto, Ryzhkov, and Kapin. Since these 13 materials span a wide range of thermal characteristics (from Cd: bp 1038 °K to the refractory materials C, Mo, and W: bp 5973 °K), it is concluded that ion currents of this magnitude can be expected from cathode materials in general. It is also shown that the erosion rate for nonrefractory electrodes can be reasonably estimated using Ecker's theoretical values for cathode spot current densities and temperatures. Furthermore, this comparison between experiment and theory indicates that the total probability for vapor ionization in the cathode region exceeds 50%.