DC Electric-Field Modifications Produced by Solid Insulators Bridging a Uniform-Field Vacuum Gap

The results of measurements of the electric field near the surface of stainless-steel electrodes in vacuum are described. It is found that the field is modified by a solid insulator bridging the interelectrode gap. The extrapolated cathode field is enhanced sufficiently to equal the value considered necessary to initiate break-down in a plane vacuum gap. The measured fields are greater than the applied field and consistent with the accumulation of positive surface charge on the solid insulator. Analysis of the data suggests that the surface charge results from bombardment of the surface by electrons emitted at the insulator-cathode junction.