Effect of Protrusions on Cathodic-Arc-Attachment Mode in High-Pressure Arc Discharges

Abstract—It is shown via computer simulations that a protru-sion on a thermionic arc cathode may dramatically change the pattern of steady-state modes of current transfer. This happens through a bifurcation of a special type occurring at certain geom-etry of the cathode and may play a role in operation of electrodes of compact high-intensity discharge lamps. Index Terms—Arc attachment, arc cathode, bifurcation of current transfer, bifurcation theory, high-pressure arc. CONSIDERABLE advances in theoretical and experimen-tal investigation of the plasma–cathode interaction in high-pressure arc discharges have been achieved during the last decade [1]. Simulation methods, available at present and validated experimentally, allow one to reliably describe many aspects of operation of thermionic cathodes. A challenging task is to simulate operation of cathodes of complex geometries, which are of practical interest, in particu-lar, those with protrusions [2], [3]. Results of such simulations are reported in this paper. Steady-state modes of current transfer have been calculated with the use of the commercial software COMSOL Multi-physics, as described in [4]. Stability of each of the steady-state modes has been investigated, as described in [5]. In Fig. 1, current–voltage characteristics U(I) are shown of axially symmetric steady-state modes of current transfer to an axially symmetric tungsten cathode in the form of a rod of radius R = 750 µm and a height of 12 mm (here, U is the near-cathode voltage drop, and I is the arc current). The cathode has a hemispherical tip. A spherical protrusion with a radius of 200 µm is located at the top of the tip. The arc operates in argon under the pressure of 2 bar. Also shown is the dependence of the maximum temperature of the cathode surface on the arc current. Stable and unstable sections of each mode are shown by solid or dotted lines, respectively