DEPARTURES FROM EQUILIBRIUM IN THE CATHODE REGION OF AN ARGON TRANSFERRED ARC AT ATMOSPHERIC PRESSURE
Centre de Physique des Plasmas et et de leurs Applications de Toulouse (CPAT) ESA CNRS N° 5002, Université Paul Sabatier 118 route de Narbonne, F31062 Toulouse cedex, France
Centre de Physique des Plasmas et de leurs Applications de Toulouse, UMR 5002, Universite Paul Sabatier, 118 route de Narbonne F31062, Tolouse cedex 4, France
Centre de Physique des Plasmas et de leurs Applications de Toulouse (CPAT) UMR n° 5002 − Université Paul Sabatier, 118 Route de Narbonne F31062 Toulouse Cedex 4 - France
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.