DEPARTURES FROM LOCAL THERMODYNAMIC-EQUILIBRIUM IN HIGH-CURRENT FREE BURNING ARCS IN ARGON

A magnetohydrodynamic arc model has been combined with a kinetic plasm a model to investigate departures from equilibrium in the plasma of hi gh-current arcs. Calculations made for arcs operating in argon with va rious 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 alw ays overpopulated relative to their populations under conditions of lo cal thermodynamic equilibrium. The results suggest that this is due to the injection of a large mass flow of cold gas into the arc core unde r the influence of pressure gradients generated by the magnetic pinch force in the cathode region of the arc. Calculated plasma emission coe fficients for the 696.55 nm Ar I line are in agreement with experiment al measurements showing that the values of these coefficients decrease in the cathode region of the arc.