Surface Flashover Measurements on Conical Insulators Suggesting Possible Design Improvements.

摘要

Plexiglas insulators in the form of truncated cones are subjected in vacuum to high-voltage pulses having 3ns risetime. The breakdown voltage is found to depend more strongly on cone angle at angles near 0exp 0than has been reported for pulses having longer risetime. The time delay between voltage application and breakdown is measured,with nanosecond resolution,as a function of breakdown voltage and cone angle,ad the effects of electrode needles and altered insulator shape near the cathode end are determined. These measurements indicate that surface flashover can proceed by two distinct mechanisms,each sensitive to details at the appropriate insulator-electrode junction. Flashover initiated at the insulator-cathode junction involves a secondary electron emission avalanche. A time delay varying as the inverse square of electric field is required for a low-impedance discharge to develop. On the other hand,the anode-initiated flashover mechanism apparently propagates across the insulator surface at a velocity proportional to the third or fourth power of electric field. A small sample is found to break down at nearly the same electric field as a large-scale insulator when deliberate imperfections are introduced at the insulator-electrode junctions. These results suggest the performance of large-scale insulators may be improved significantly by improving the junctions. Alternative designs are proposed,including two which eliminate and make use of,respectively,leakage currents emitted from the cathode junction. Additional factors affecting breakdown are considered,including insulator surface condition,magnetic fields,and ultraviolet light. Performance under fast-pulsed conditions may be improved by semiconducting coatings or by magnetic fields having flux density greater than 0.4 T.