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Simulation of the atomic and ionic densities in the ionization layer of a plasma arc with a binary cathode

A physical model was developed to study the behaviour of the cathode material evaporated from a thoriated tungsten cathode of an atmospheric-pressure argon plasma arc. The densities of tungsten and thorium atoms and ions in the ionization layer were obtained, and the influence of the different physical processes on the evaporated cathode material was established. It was found that almost all of the neutral atoms evaporated from the cathode are ionized near the beginning of the ionization layer, i.e. near the boundary between the sheath and the ionization layer. Thorium ions are concentrated in a 4 µm region near the beginning of this layer, while tungsten ions are found in a region of 9 µm. The contribution of the electric force to the velocity of ions is the dominant contribution only near the beginning of the ionization layer. At a distance from the interface between the sheath and the ionization layer greater than 3.8 µm in the case of thorium ions, and greater than 5 µm in the case of tungsten ions, the contributions of the density gradient forces and the frictional forces are more important than the electric force contribution.