Abstract
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 arc parameters. These coefficients show variations in the cathode region of the arc, depending upon the arc current, the cathode composition and the mode of operation of the arc, 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 arc core under the influence of pressure gradients resulting from the magnetic pinch force in the cathode region.
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