Correlation between insulator surface and DC flashover characteristics in vacuum

The high-voltage DC flashover characteristics of a large number of solid insulating materials were investigated. These materials included high-density aluminas from different manufacturers varying in bulk composition and surface microstructure, coated aluminas to reduce surface charging, steatite, and forsterite. The results show a strong correlation between the first and conditioned breakdown strengths, the rapidity of conditioning, and the surface microstructure. Specifically, it is shown that 'generic alumina' has widely varying voltage withstand strengths depending on the bulk composition, surface microstructure, and chemical surface state. Materials with a finely granular, closely packed structure exhibited the best voltage hold-off performance. Also, nonporous materials exhibit rapid conditioning to its final value.<>