Wavefront dependent behavior of a point-to-plane gap in SF/sub 6/

The wavefront-dependent behavior of a 5-mm point-to-plane gap in SF/sub 6/ was investigated. Some of its characteristics were determined and are explained in terms of the underlying physics. The experimental setup consists of a specially designed 60- Omega test cell which forms part of a generator capable of producing fast pulses with risetimes of a few nanoseconds and voltages up to 80 kV. Slow pulses with risetimes for a few microseconds are generated conventionally for comparison purposes. Experiments were conducted with both waveforms to determine both the effect of risetime and the impact of imposed gamma -radiation. Collimated photomultiplier measurements have permitted spatial resolution of the prebreakdown light emissions. The optical findings provide evidence that critical transitions occur in the propagation of prebreakdown events. This has been determined from spatially resolved time-lag measurements to which statistical methods have been applied. The differences and anomalies which are evident in comparing the behavior of the two waveshapes are highlighted, and the implications are discussed in terms of initiation and propagation mechanisms. Calculations are presented to indicate the role played by the critical volume in front of the point anode.<>