Departures from local thermodynamic equilibrium in high-current free burning arcs in argon

A magnetohydrodynamic arc model has been combined with a kinetic plasma model to investigate departures from equilibrium in the plasma of high-current arcs. Calculations made for arcs operating in argon with various arc parameters show that the ground state levels of the neutral atoms in the region of the arc close to the cathode tip are almost always overpopulated relative to their populations under conditions of local thermodynamic equilibrium. The results suggest that this is due to the injection of a large mass flow of cold gas into the arc core under the influence of pressure gradients generated by the magnetic pinch force in the cathode region of the arc. Calculated plasma emission coefficients for the 696.55 nm Ar I line are in agreement with experimental measurements showing that the values of these coefficients decrease in the cathode region of the arc.