Local thermodynamic equilibrium in the cathode region of a free burning arc in argon

Measurements of normalized plasma emission coefficients for the 696.55 nm Ar I line for free burning argon arcs at 1 atm are presented for various are parameters. These coefficients show variations in the cathode region of the are, depending upon the are current, the cathode composition and the mode of operation of the are, and suggest departures from local thermodynamic equilibrium in the plasma near the cathode tip. In this paper, these results are discussed and an explanation is proposed based on the overpopulation of the ground state level of the neutral atoms. This overpopulation is due to the injection of a large mass flow of cold gas into the are core under the influence of pressure gradients resulting from the magnetic pinch force in the cathode region.