Perturbation of the static voltage breakdown mechanism in vacuum by a weak magnetic field

Abstract

The theory of static high voltage breakdown initiated by field emission defines a weak magnetic field as one with insufficient strength to deflect significantly the space charge beam associated with a virtual cathode. Experimental data with uniform field copper electrodes are presented, showing that the breakdown voltage is increased when weak magnetic fields below about 0.01 T are applied, but it is progressively reduced beyond this value. The functional variation is moreover dependent upon whether the cathode is pretreated by bakeout in hydrogen or vacuum. Magneto-transport of electrons through the surface layer of oxide is shown analytically to be capable of modifying the field emission from a protrusion and so to influence the Fowler–Nordheim equation. The effect is significantly influenced by the relative magnitude of the oxide work function and that of the inner potential barrier at the underlying metallic junction. Experimental data of the magnetic perturbation of the breakdown voltage are fitted to a corresponding family of analytical curves from which a value is derived for the electron density in the oxide layer. Consistency is achieved between theory and experiment, whilst the phenomena attendant upon pretreatment and conditioning are accounted for by shifting of the Fermi level.