Generation of jets and drops by the cathode spot of a vacuum arc
The explosion of liquid-metal jets produced during the formation of a crater on the cathode of a vacuum arc is the basic mechanism of the initiation of individual cells in the cathode spot. To analyze the processes involved, a two-dimensional axisymmetric model is proposed that describes the generation of quasi-one-dimensional liquid-metal jets during the growth of azimuthal instability of the liquid-metal rim of a cathode microcrater. The simulation input data are the jet diameter, propagation velocity, and temperature, which are calculated using a previously developed semiphenomenological model of crater formation. In the context of the proposed model, the jet formation has been simulated for different modes of melt splashing from a crater without considering the jet interaction with the cathode spot plasma. It has been shown that in the “active splashing” mode, a jet of almost constant diameter, longitudinal velocity, and temperature is formed. The head of the propagating jet transforms into a spherical drop due to surface tension. This is accompanied by the excitation of a capillary wave, which propagates to the jet base and harmonically modulates its diameter. In the “inertial splashing” mode with the melt velocity near the jet origin decreasing with time, a jet with a longitudinal velocity gradient is formed. The velocity gradient acts to elongate the jet (reduce its diameter) and causes the drop-shaped head to separate away from the jet and the entire jet to break away from the crater. As the jet further propagates, it breaks into drops. It has been demonstrated that the jet breakup and its breakaway from the cathode are due to the Rayleigh-Plateau instability that occurs when the length of the modulated jet becomes equal to a capillary wavelength satisfying the instability criterion.