Modelling and experimental evidence of the cathode erosion in a plasma spray torch

Abstract

The lifetime of tungsten cathodes used in plasma spray torches is limited by processes leading to a loss of cathode material. It was reported in the literature that the mechanism of their erosion is the evaporation. A model of the ionization layer of a cathode is developed to study the diffusive transport of evaporated tungsten atoms and tungsten ions produced due to ionization by electron impact in a background argon plasma. It is shown that the Stefan-Maxwell equations do not reduce to Fick law as one could expect for the transport of diluted species, which is due to significant diffusion velocities of argon ions. The ionization of tungsten atoms occurs in a distance of a few micrometers from the cathode surface and leads to a strong sink, which increases the net flux of tungsten atoms far beyond that obtained in absence of tungsten ions. This shows that the tungsten ions are driven by the electric field towards the cathode resulting in no net diffusive flux and no removal of tungsten species from the ionization layer even if convection is accounted for. A possible mechanism of removal is found by extending the model to comprise an anode. The extended model resolves the inter-electrode region and provides the plasma parameters for a current density corresponding to the value at the center of the cathode under typical arc currents of 600 A and 800 A. The presence of the anode causes a reversal of the electric field on the anode side, which pulls the ions away from the ionization layer of the cathode. The net flux of tungsten ions can be further fortified by convection. This model allows one to evaluate the loss of cathode material under realistic operating conditions in a quantitative agreement with measured values.

Research progress on numerical simulation of arc discharge plasma process

“…The properties obtained allow estimation of the lifetime and efficiency of plasma accelerators using tungsten cathode and Ar as driving medium. Baeva et al [49] established a cathode ionosphere model to investigate the diffusion transport of evaporated tungsten atoms and tungsten ions due to electron shock ionization in Ar plasma. The results exhibited that the Stefan-Maxwell equation was not reduced to Fick law due to significant diffusion velocities of Ar ions.…”

Experimental study on the life and performance of an improved DC arc plasma torch

“…A large number of numerical modelings [22,[26][27][28][29] and experimental studies [30][31][32][33][34] have been performed to analyze the mechanisms of anode erosion, and propose the methods to inhibit the anode erosion. The modeling found that the arc usually attached on the anode surface in the diffusive or constricted forms.…”

Model of non-equilibrium near-cathode plasma layers for simulation of ignition of high-pressure arcs on cold refractory cathodes

“…The unified modeling approach has been successfully applied for axially symmetric modeling of low-current arc discharges [10][11][12][13][14][15]. Unfortunately, the approach is highly computationally intense for high values of the current density, and therefore its applications to high-current arcs are limited to one dimension in space; see citations in section 3.1 of [4] and [5,6,[16][17][18][19][20][21] as further references. The arc-electrode interaction in geometries of practical interest is a multidimensional phenomenon.…”