Theory of free-burning arc columns including the influence of the cathode

Abstract

Previous predictions of the properties of free-burning arcs have usually required assumptions to be made of conditions at the cathode, in particular, the current density. Numerical calculations presented show that the current density at the cathode, for a 200 A arc in argon with a tungsten cathode having a 60 degrees conical tip, is determined to a good approximation by the arc column and cathode shape. The cold non-equilibrium sheath next to the cathode is very thin ( approximately 0.02 mm) and can be neglected. Predictions are made of arc column temperatures, velocities, pressure and potential by solving in two dimensions the conservation equations of mass, momentum and energy. Predictions are in fair agreement with experimental results with maximum arc temperatures of approximately 22000 K. Current densities depend strongly on the electrode radius at the cathode tip. For a radius of 0.1 mm the current density is approximately 20000 A cm-2.