On the role of nonmetal inclusions at the cathode surface in the initiation and self-sustaining of a vacuum discharge

Emission processes are treated for the case where dielectric and semiconductor microinclusions are present on the surface of a metal cathode. It has been shown that the emission current-voltage characteristic plotted in the Fowler-Nordheim coordinates is close to a straight line corresponding to a high field enhancement factor. A stationary problem on the heating of a nonmetal microprotrusion by the field emission current has been solved. Calculations have shown that for explosive emission processes to be initiated in this case, lower critical currents and lower Joule energy spatial densities are required then in the case of metal microprotrusions on a cathode. This is the reason why the initiation of a vacuum discharge and its operation at the early stage occur at those sites where nonmetal inclusions are present. For a vacuum arc discharge, the required electric field strength is achieved through the charging of the surface of a nonmetal inclusion by the ion current from the arc plasma.