An experimental study of thoriated tungsten cathodes operating at different current intensities in an atmospheric-pressure plasma torch

Abstract

Thoriated tungsten cathodes operating in an open-air plasma torch at current intensities between 30 and 200 A were experimentally studied. The morphology and composition of the cathode tip after arcing were investigated by scanning electron microscopy and energy-dispersive x-ray spectroscopy. Three relevant zones were found on the cathode tip (high-erosion zone, thorium-depleted zone and thorium-enriched zone), and their dimensions were measured. For each current intensity, the cathode tip temperature T during arcing was measured by one-colour pyrometry, and the spot current density was determined. From the experimental data and the energy balance in the presheath, the dependence of the cathode work function on the arc current intensity and its dependence on T were evaluated. It was found that in the absence of an electric field, the work function evaluated with the theoretical value of the Richardson constant increased from 3.2 eV when T = 2900 K to 4.0 eV when T = 3700 K. These values are consistent with the presence in the spot of a layer of thorium atoms on the tungsten matrix during arcing. The atomic surface density of the layer determines the value of the work function.