Numerical simulation of high-current vacuum arcs with an external axial magnetic field

Numerical simulations are presented for physical behavior and heat flux to the anode of high-current diffuse of arcs as found in vacuum interrupters. The magnetohydrodynamic approach is applied. Of importance is the consideration of energy balance. Heat flux densities to the anode are predicted in the right order of magnitude and essential physical details of the high-current vacuum arc are disclosed. Only at low or no axial magnetic field superimposed externally and low-arc currents, the anode-directed flow of plasma of diffuse arcs reveals supersonic conditions. Otherwise, subsonic conditions exist. In supersonic diffuse arcs, the anode-directed plasma flow is decelerated and highest pressures appear in front of the anode. At subsonic conditions the highest pressure prevails in the cathode region and the pressure gradient drives the flow to the anode. The transition from diffuse to diffuse columnar arc seems to occur when the evaporation rate of metal vapor from the contact surfaces approaches the emission rate of plasma from the body of cathode spots. Diffuse columnar arcs have moderate pressure variations from cathode to anode. With rising plasma density, the energy loss from the emission of electromagnetic radiation increases and can no longer be neglected.