Effect of Dielectric Materoals on Surface Breakdown in Vacuum and Analysis by Electron Trajectory Simulation

To study and analyze the effect of surface insulation in case of a solid insulator used in actual vacuum interrupters, prebreakdown currents, breakdown voltages and Vb-t characteristics were measured with two different dielectric materials (glass and ceramics). It has been found that the breakdown voltage with a solid insulator is lower than without it, and that this fact is closely associated with prebreakdown current. In other words, glass provides a higher surface breakdown strength than ceramics because the prebreakdown current of glass is lower than that of ceramics. Electron trajectory analysis has revealed that the above-mentioned phenomenon is caused by the presence of a critical field strength of 8.9 kV/mm at one or more points on the electrode and a prebreakdown current of the order of 10-11 A. Although the prebreakdown current for the critical field strength of 8.7 kV/mm remains too low to cause electrical breakdown, it can be predicted that the level readily increases by some million times, reaching the order of 10-4 A at the time of a breakdown. This is the result of the contribution of secondary electron avalanches on the surface of the-solid insulator.