Charges and vacuum flashover

It is stated that the increasing importance of vacuum insulation in technology, and the recognition of the disruptive effects of surface discharges on satellites, make a better understanding of the flashover effect important. Solid insulators are a source of gas under vacuum conditions, particularly under bombardment, and can also cause local enhancement of electric stress due to the acquisition of surface charges. The work of de Tourreil and Srivastava in this field (Proc. 5th Int. Symp. on Discharge and Electrical Insulation in Vacuo, p. 295, 1972) is mentioned, and also the experiments described by the present author in Nature 262:277 (1976), in which electrons of energy up to 3 keV were directed through a counter electrode on to insulator specimens and the charges produced measured. Investigations on fused SiO2 and on polycarbonates and polyimides are here described. The observations suggest that the straightforward secondary emission theory of insulator charging, leading subsequently to breakdown, may be inadequate, and some aspects requiring further investigation are indicated. (U.K.)