Hydrodynamics of the Molten Metal During the Crater Formation on the Cathode Surface in a Vacuum Arc

Abstract

2-D axially symmetric hydrodynamic model has been developed to describe the formation of a crater and liquid-metal jets on a vacuum arc cathode using Navier-Stokes equations for an incompressible viscous fluid with a free surface and a heat conduction equation taking convective heat transfer into account. The formation of an elemental crater on a copper cathode during the operation of a cathode spot cell has been numerically simulated by varying the heat flux and the pressure produced by the cathode spot plasma. Based on the simulation results, we can distinguish three different modes of the crater formation process: 1) no splashing; 2) inertial splashing; and 3) active splashing. It has been shown that a crater with metal jets forced away can be formed within 30 ns of plasma action if the heat flux density is above 10 12 W/m 2 and the pressure is above 10 8 Pa.

Mechanism of vacuum breakdown in radio-frequency accelerating structures

“…5(c) represents some illustrative hypothesis. Nevertheless, the calculations allow the conclusion that the estimated crater formation times and crater dimensions are consistent with the simulations of the process of crater formation in vacuum arcs [43][44][45]. This is not surprising if one takes into account the common mechanism of crater formation that involves high pressure and the existence of a dense plasma having a temperature of several electron volts at the metal-plasma interface.…”

An integrated approach to understanding RF vacuum arcs

“…Unipolar arcs leave a variety of characteristic damage structures on materials 30 , 31 , 37 , 41 . The primary mechanisms by which the plasma affects the (presumably molten) metallic surface are plasma pressure caused by ions leaving the plasma and hitting the surface, electrostatic Maxwell stresses pulling on the surface, and surface tension, which tends to locally flatten the surface 38 , 41 . The interplay between these mechanisms is geometry dependent.…”

“…The process occurs in four stages: (1) local surface fields, measured from field emission, are high enough so that Maxwell stresses can be comparable to tensile strength causing surface failure 9,15,22 , (2) field emission ionizes the fragments of surface material, producing a positively charged ion cloud near a field emitter that will increase the field on the emitter [30][31][32] , (3) an unstable. non-Debye plasma is maintained by field emission and self-sputtering [33][34][35][36][37] , and, (4) surface damage is caused by Maxwell stresses, thermal gradients, and surface tension on the liquid metal surface 18,38,39,41 .…”

On the current density at the cathode of a vacuum arc

“…The boundary of the autograph (crater periphery) is the melting point (MP) isotherm. This implies that the liquid layer inside the cavity is substantially thinner than the radius of the crater, the assumption often made and proven by hydrodynamic simulations [6][7][8][9][10]. Note that the shape of the crater after shutting the arc off could differ from its shape during arc functioning: some molten metal could flow back into the crater from the rim once the arc stops functioning.…”