Numerical Study of the Current Constriction in a Vacuum Arc at Large Contact Gap

A classical two-temperature magnetohydrodynamic modeling approach is used to study the influence of a large contact gap (up to 40 mm) on the behavior of a diffuse vacuum arc controlled by a 5-mT/kA uniform external axial magnetic field between two copper electrodes with a 20-mm radius. The current constriction and the energy flux density at the anode surface are more particularly analyzed, considering both supersonic and subsonic flow conditions. In the case of a supersonic arc, simulations show that the constriction of the current develops in the whole interelectrode region, but the constriction level at the anode surface does not evolve monotonically with the contact gap. The constriction is partly correlated with the radial compression of the plasma. In the case of a subsonic arc, the current constriction is related to the presence of the anode sheath. It occurs only close to the anode (from a constant distance from the anode of around 15 mm). Whereas the current constriction at the anode surface increases when the gap length goes from 10 to 20 mm, it no longer evolves when increasing the gap length from 20 to 40 mm. For both flow conditions, the evolution with the gap length of the radial profile of the energy flux density transferred by the plasma to the anode is similar as that followed by the current constriction at the anode.