Influence of metal-insulator junction on surface flashover in vacuum

The influence of the metal-dielectric junction on the electric field distribution along the solid insulator-vacuum interface is analyzed in detail in two widely used electrode-insulator geometries: (1) a cylindrical solid insulator with metal inserts at both ends and (2) a cylindrical solid insulator placed in recessed electrodes at both ends. The effects of the length of the metal inserts and the angle ψ between the solid insulator and the recessed electrode on the electric field and the flashover voltage have been theoretically investigated and experimentally validated. The permittivity of the solid insulator materials used is varied from 2.1 to 100. The surface flashover voltages of Macor glass ceramic and Teflon are measured in vacuum (<10−8 Torr) in different electrode-insulator contact geometries using dc, ac (60 Hz), and 1.2/50-μs lightning impulse. The measurements show good qualitative agreement with the behavior predicted from the computation.