Time-Resolved Measurements of Surface Flashover of Conical Insulators.

摘要

The voltage at which an insulator in vacuum flashes over depends on the angle between the insulator-vacuum interface and the electric field. Test samples of Plexiglas (PMMA) in the form of truncated cones 0.6 cm thick were subjected to high-voltage pulses in vacuum (10 exp -5 to 10 exp -6 torr pressure). The conical surfaces of the samples were of uniform roughness, achieved by circumferential polishing with 400 grit silicon carbide paper. The samples were cemented to stainless steel electrodes of larger diameter without visible gaps at the insulator-electrode junctions. Flat-topped, high-voltage pulses with 3 ns risetime were generated by collecting a pulsed electron beam on the cathode electrode of the sample, where the charge was stored prior to breakdown. The applied voltage and time delay prior to breakdown were obtained from waveforms recorded with the aid of a capacitive voltage divider. The time resolution was better than a nanosecond. As a result, true flashover breakdowns were easily distinguished from leakage currents, sometimes as large as several kA, which often occur for insulators having positive cone angles (base of the frustum at the cathode electrode). Such currents appeared to be emission across the vacuum gap from the insulator-cathode junction. Surface flashovers on the PMMA samples were characterized by an abrupt collapse of insulator impedance, typically falling through greater than 10 exp 3 omega to a fraction of an ohm within a nanosecond. This collapse launches an underdamped oscillatory current in the breakdown channel. The delay prior to breakdown was measured from the point in time the high-voltage pulse attains 90 percent of its maximum value.