Transition from streamer to Townsend mechanisms in dielectric voids

The time dependence of the discharge processes in dielectric bounded voids of 40 mm have been studied under power frequency voltage. Fast oscilloscopic techniques were used to observe the pulse shape of single discharges or clusters of discharges. Synchronous optical registration of discharge images was performed using a highly sensitive video system. At least two distinct discharge types were registered as a function of time. A 'streamerlike' discharge regime is gradually replaced by a more diffuse 'Townsendlike' discharge regime. This transition could be recognized in the shape of the discharge pulses and in the pattern of the optical image. A key parameter governing the transition is the surface resistance of the void-dielectric interfaces. This surface resistance was seen to decrease gradually due to discharge by-products. In its turn, a low surface resistance strongly favours the transition from 'streamerlike' to 'Townsendlike' discharges.