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

Abstract

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 (spot zone, thorium-depleted zone and thorium-enriched zone), and their dimensions were measured. For each current intensity, the spot temperature T during arcing was measured by one-colour pyrometry, and the spot current density was determined. Exponential growth was seen for the current density as the spot temperature increased. From the experimental data and the energy balance in the presheath, the dependence of the cathode effective work function ϕ on the arc current intensity and its dependence on T were determined. It was found that ϕ increased from ϕ = 2.64 eV when T 2900 K to ϕ = 3.06 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 ϕ. IntroductionThe investigation of cathodes is important in order to improve the performance of plasma torches due to the essential role of cathodes as electron emitters and the severe erosion of cathodes caused by the high temperatures attained on the cathode surface. Binary cathodes, formed by a tungsten matrix doped with rare-earth oxides, are of particular interest, as they combine the refractory properties of tungsten with the good electron emission characteristics of the dopant. The processes that take place in such cathodes during arcing are complex due to the interaction between the dopant and the tungsten matrix and between both the dopant and the tungsten with the plasma arc. In spite of the progress achieved in the understanding of binary cathodes, there are still fundamental aspects that are not well understood and require further experimental investigation. For instance, the dependence of the cathode work function on the spot temperature has not yet been satisfactorily established.Tungsten cathodes doped with thoria are commonly used in industrial applications of plasma torches. Our goal in the present paper is to investigate the behaviour of these cathodes over a wide range of arc current intensities. Thoriated tungsten cathodes have been experimentally studied by different authors [1][2][3][4][5][6][7][8][9][10][11], but in most of the studies, the cathodes were operated with only a few current intensities; therefore, the dependences of the cathode parameters on the arc current intensity I were not found.In the present paper, we discuss our experimental investigation of thoriated tungsten cathodes operating in the actual working conditions of an industrial plasma torch. We used arc current intensities ranging from 30 A to 200 A, which allows the investigation of cathode

Cathode fall voltage of TIG arcs from a non-equilibrium arc model

“…3 (10 mm and 50 A) in comparison with case a (6 mm and 50 A) in spite of the small changes in the heavy particle temperature near the cathode surface. These results are in good agreement with pyrometric measurements [18] which are reported, however, for slightly different parameters (tungsten electrode with 2 % ThO2 of 2.4-mm diameter and a cone tip of 68°). Tip temperatures of about 3,700 K are obtained for currents above 130 A (including 200 A).…”

“…Tip temperatures of about 3,700 K are obtained for currents above 130 A (including 200 A). The tip temperature decreases to about 3,000 K at 50 A and 2,900 K at 30 A [18].…”

Measurements of Plasma Properties Using the Floating Potential of Emissive Langmuir Probes

“…In accordance to the results of Fig. The effective work function of the metal would introduce different electron thermoionic emission departing from the classical Richarson-Dushman expression with a constant W f [13]. In addition, the operation temperatures of the emissive Langmuir probes are close to the melting point of the metallic wires.…”

Gas tungsten arc models including the physics of the cathode layer: remaining issues

“…This phenomenon, discovered by Langmuir with tungsten filaments [44], was further investigated via studies on e.g. electrode erosion [18,35,[45][46][47]. It is known that the rare earth oxides do not all behave in the same way during their diffusion.…”

“…The effect of the rare earth oxides constituting the cathode surface layer in physical state (II) on the cathode sheath remains to be investigated. Also, after some operating time more physical states are known to be formed [18,35,[45][46][47]. Their modelling has not been addressed yet.…”