Numerical modeling of plasma behavior and heat flux to contacts of vacuum arcs with and without external axial magnetic field (AMF)

Abstract

General subject is the physics of the different modes of arcs of vacuum interrupters (VI). Numerically simulations applying the MHD approach are performed. Consideration of energy balance is decisive. Heat flux densities to anode are predicted in the right order of magnitude, and essential physical details could be disclosed. Only at low or without axial magnetic field (AMF) externally applied and low arc currents, the anode-directed plasma of diffuse arcs reveals supersonic conditions. At stronger AMF and higher currents the diffuse arc exhibits sub-sonic conditions. Transition from diffuse to diffuse columnar arc occurs when the evaporation rate of metal vapor from the contact surfaces approaches that of the cathode spots. With rising current an increasing part of the dissipated energy is lost by emission of electromagnetic radiation. In super-sonic diffuse arcs the anode-directed plasma flow is decelerated and highest pressures appear in front of the anode. In sub-sonic diffuse arcs highest pressure prevail in cathode region and the flow to the anode is accelerated by the pressure gradient. Diffuse columnar arcs exhibit moderate pressure variations along the way from cathode to anode. The present results are promising. Experimental data are urgently needed for validation and further refinement of the physical model.