Effect of in situ heat treatment on surface flashover characteristic and surface condition of alumina in vacuum

Abstract

Flashover along the surface of a solid insulator (surface flashover) prevents minimizing the size of vacuum devices It has been considered that the emission ofelectrons from the cathode hiple junction won is the first step of initiating the surface flashover, subsequent gas desorption from insulator surface stimulated by elechon impact is the second stop and finally the surface flashover occurs in the desorbed gases In addition it has been recognized that conditioning treatment by repetitive surface flashover cleans insulator surface and brings higher flashover voltage. Therefore removal of adsorbed gas= and surface contaminants may improve surface flashover voltage. In this expetiment, vacuum heat treatment (about 465 degrees Celsius) for Alumina samples was employed as a means of removal of adsorbed gases and surface contaminants. AAer heat treatment, repetitive flashover test was conducted in vacuum without the expsure.of sample to atmosphere Flashover voltages with and without heat treatment were compared. As a mull, flashover voltage for the sample with heat treatment was higher than that for without heat treatment In particular, an appreciable improvement of first flashover voltage was confirmed for heat treatment samples significant conditioning eNwt for the sample with heat treatment was obtained as well. In addition the dependence of the flashover voltage on the purity of aluminas with heat treatment was clalified.