High voltage static breakdown in vacuum

Abstract

A static voltage breakdown mechanism in uniform fields is proposed for widely encountered, although physically limited, situations in which prebreakdown current arises from a semiconducting layer on a cathode protrusion. Conditions are given for the simultaneous establishment of an electron reservoir and virtual cathode with an opposing ionic space charge bubble at the anode. Electron impact ionization of evaporated anode material supports a parabolic current beam through which the two electrode space charge regions interact. Ions interact electrostatically with electrons in the reservoir, the density of which determines the Debye length and, hence, the dimensions. Emission from feasibly sized protrusions is possible only from semiconducting surfaces, limiting the applicability of the model. Instability in the form of rapid beam contraction is shown to begin before reaching the anode melting point, after which the voltage collapse starts. Criteria for this runaway indicate that it is initiated for applied voltages which increase as the square root of gap separation, the proportionality being determined by various thermo-physical parameters of the anode material and its vapor. Calculation shows that the breakdown voltage for copper electrodes agrees well with the measured value obtained after high energy conditioning.