Effects of spark conditioning, insulator angle and length on surface flashover in vacuum

A flashover test was conducted in a vacuum gap, bridged by an insulator in the shape of a frustum of a cone. The test gap was subjected to a lightning impulse voltage of 2/50 mu s. The flashover voltages were observed for various cone angles and insulator lengths. Traces on the cathode due to flashovers were observed. A spark conditioning using voltage pulses of reverse polarity is shown to reduce the number of shots needed to obtain a stable flashover voltage. The flashover voltages depend strongly on the cone angle even in the case of the lightning impulse. The flashover fields depend strongly on the insulator length. The traces are distributed not only at the triple junction but also over a wide area of the cathode surface when the gap distance is shorter than 15 mm. This fact indicates the existence of two different flashover processes.>