Modeling and experimental investigation of spot dynamics on graphite cathodes in dc plasma arcs at high pressure

摘要

From a model recently developed for refractory cathodes [Munoz-Serrano et al., J. Appl. Phys. 98, 093303 (2005)], the behavior of a graphite cathode spot in a dc plasma torch at atmospheric pressure was investigated. Furthermore, an experimental study of these cathodes was made guided by the results obtained from the model. The model includes the modeling of the cathode region, the solution of the heat conduction problem in the cathode, and the simulation of the cathode ablation process. As a result of the model, the values of the parameters which characterize the cathode region were obtained, and the evolution of the spot under different working conditions determined by the value of the initial voltage drop in the cathode region, U_0, was investigated. The results obtained show that the maximum spot radius diminishes when U_0 increases. Furthermore, two qualitatively different conditions for the spot dynamics were found. For U_0 ≥ 31 V, the spot radius continually grows over time until reaching a maximum value r_m, and the spot remains fixed on a point of the cathode surface. For values of U_0 less than 31 V the spot radius continues growing over time until reaching a maximum value with which it is not possible to satisfy the energy balance equation. This leads to spot extinction and to its appearance at another point of the cathode surface. Several graphite cathodes were experimentally studied, each one using different interelectrode voltage drop values U_(a-c). Before and after arcing, the cathode surface was explored by an electron microscope, and the roughness profile of that surface was determined by a perthometer. This allowed measuring the average size of the craters produced on the cathode surface by the arc. The movement of the spot attachment for different values of interelectrode voltage was observed by a digital video camera. It was experimentally found that the average crater radius diminished when the U_(a-c) potential increased. Furthermore, it was seen that for U_(a-c) values less than 32 V, the spot moved over the cathode surface and that this movement became slower when the U_(a-c) increased. The spot became immobile for U_(a-c)=32 V. These experimental results corroborate the spot behavior obtained from the theoretical model.