Constraints imposed upon theories of the vacuum arc cathode region by specific ion energy measurements

Abstract

The presence of high-energy ions (with potentials greater than the overall arc voltage) in the flux from the cathode region of vacuum arcs has been well established. While much excellent theoretical work has been done on the cathode region of a vacuum arc, only a modest amount of such work has predicted the energies of the ions leaving the cathode region. Two vacuum arc cathode theories which do predict the ion energies are the potential hump theory—where a positive space charge at the cathode spot accelerates the ions electrostatically, and the gas dynamic theory—where the ions are accelerated by a flow process. Few theorists have taken advantage of the available mass spectrometric data concerning individual ion energies. In this paper I compare such data with theoretical predictions. The best confirmed ion energy data is that for a copper cathode. Additional data are presented concerning the ion flux from a vacuum arc with a ten percent Bi/Cu cathode. Comparison of the copper and bismuth/copper data with the theoretical predictions shows that the potential hump and gas dynamic theories are in rough agreement with experiment, but are definitely not correct. Modifying the potential hump and gas dynamic theories by considering time-dependent phenomena (i.e., incorporating nonequilibrium states into the theory) is shown to offer the promise of bringing either theory into better agreement with experiment. I conclude that the available data on the energies of the ions leaving the cathode region of a vacuum arc can serve as strong constraints on theoretical analyses.