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Progress in Plasma Processing of Materials, 1997

ISBN Imprimer: 1-56700-093-2



The present paper deals with an experimental and theoretical study of the influence of the anode material (copper, iron or steel) on the properties of an argon plasma. This study was carried out on a transferred arc burning in pure argon at atmospheric pressure. Our working conditions were 18mm arc length; 70,90 and 110 A current intensities and a gas flow rate of 8splm
It is noteworthy that depending on operating conditions, two stable arc configurations near the anode corresponding to pure argon plasma or argon-metallic vapour (Ar-Cu or Ar-Fe) mixtures were obtained. When the metallic vapours were present above the anode, the contaminated zone was of about 2 to 3 mm in thickness.
The experimental characteristics of the arc (temperature assuming LTE, electron and excited atom densities, total metal concentrations) were determined by emission spectroscopy. Even with a strong cooling leading to the absence of metallic vapours in the plasma, the nature of the anode material had an influence on the temperature field of the arc. The temperatures obtained with a copper anode in pure argon where higher than those obtained with iron or steel anode. The presence of metal vapours led to a lowering of the plasma temperature; temperature over the whole section was lower in Ar-Fe than in Ar-Cu plasmas. Along the axis of the discharge, the mean value of the mole fraction (metal/argon) deduced from the measurements was about 10−3, corresponding to a cooling of about 1500K.
A modelling based on the conservation equations and taking into account the arc-anode interaction, was developed. The theoretical predictions are in good agreement with the experimental results. The model allows to interpret the main observed features. In the absence of metallic vapour the arc behaviour was explained by the material properties of the solid metal anode. When the arc was seeded by metallic vapours at equal concentration, the cooling due to iron vapour was more important than that due to copper one. This effect was mainly due to the important radiative losses of a plasma containing iron.
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