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Progress in Plasma Processing of Materials, 1997

ISBN Print: 1-56700-093-2

DEPARTURES FROM EQUILIBRIUM IN THE CATHODE REGION OF AN ARGON TRANSFERRED ARC AT ATMOSPHERIC PRESSURE

Abstract

Several kinds of experimental diagnostics of the plasma in the cathode region of an argon transferred arc at atmospheric pressure have been performed. Previous studies based on emission spectroscopy in the stationary arc have shown that the plasma is not in equilibrium in this region: 1) the maximum intensity of ArI lines does not remain constant; 2) near the cathode, the values of the electron number density obtained by two methods are not consistent with each other; 3) the ratio of the excited level populations do not correspond to that of an equilibrium composition.
First, we present measurements of a stationary arc, at low current. The results are similar to those obtained at higher currents, showing that the departures from local thermodynamic equilibrium (LTE) are not explained by the classical criteria and must be associated with the convection of cold gas in the cathode region. Second, measurements have been performed during the arc extinction. They show that the thermal non-equilibrium (difference between the electron temperature and the gas temperature) is not detectable. Finally an analysis based on relaxation times of several mechanisms is proposed; some departures from equilibrium could be explained by an excitation non-equilibrium.
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