Model of liquid-metal splashing in the cathode spot of a vacuum arc discharge
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TICAL, NONLINEAR, AND SOFT MATTER PHYSICS
Model of Liquid-Metal Splashing in the Cathode Spot of a Vacuum Arc Discharge M. A. Gashkova, N. M. Zubareva,b*, O. V. Zubarevaa, G. A. Mesyatsa,b, and I. V. Uimanova a Institute
of Electrophysics, Ural Branch, Russian Academy of Sciences, Yekaterinburg, 620016 Russia Lebedev Physical Institute, Russian Academy of Sciences, Moscow, 119991 Russia *e-mail: [email protected]
b
Received October 7, 2015
Abstract—The formation of microjets is studied during the extrusion of a melted metal by the plasma pressure from craters formed on a cathode in a burning vacuum arc. An analytic model of liquid-metal splashing that includes two stages is proposed. At the first stage, the liquid motion has the axial symmetry and a liquid-metal wall surrounding the crater is formed. At the second stage, the axial symmetry is broken due to the development of the Plateau–Rayleigh instability in the upper part of the wall. The wall breakup process is shown to have a threshold. The minimal plasma pressure and the minimal electric current flowing through the crater required for obtaining the liquid-metal splashing regime are found. The basic spatial and temporal characteristics of the jet formation process are found using the analytic model. DOI: 10.1134/S1063776116040051
1. INTRODUCTION The main properties of a vacuum arc discharge are determined by processes in a small bright luminous region on a cathode through which the current transfer with an interelectrode gap occurs. This region (cathode spot) includes the active part of the cathode surface heated to temperatures exceeding the melting point and a near-cathode plasma. According to [1], the cathode spot of a vacuum arc consists of separate fragments or cells. The electric current that flows through a cell is limited (a few amperes), while the total current in the arc consists of currents from individual cells. This concept forms the basis for the ecton model of the cathode spot [2–4], which also assumes the cyclic character of cell operation. Cells in the cathode spot have the characteristic micron spatial scale and the lifetime of a few tens of nanoseconds. Indeed, discharge burning leads to the formation of traces on the cathode surface that have substructures in the form of individual craters. It was found that the most probable diameter of the crater in the range of arc currents from a few to a few dozen amperes weakly depends on current and is several micrometers for copper [5] and tungsten [6] cathodes. The authors of [6] studied the formation time of craters for currents close to a threshold below which the discharge spontaneously quenches. The directly recorded oscillograms of the arc current showed that the current flowed through a single crater for 25–50 ns. These results are consistent with in situ observations of the cathode spot emission with the nanosecond time resolution and micron spatial resolution [7, 8], which confirms the
presence of the quasi-periodic cellular structure of the cathode spot. Craters are formed due to the e
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