Influence of solid dielectric on the impulse discharge behaviour in a needle-to-plane air gap

The structure of the streamer zone of an impulse discharge of positive polarity in a needle-to-plane air gap is observed with high spatial resolution for two cases: for gaps with and without a thin dielectric plate (barrier) placed along or at a certain distance to the gap axis. The experiments are carried out with two amplitudes and shapes of the applied voltage pulse (impulse length about 800 and 360 ns). The discharge structure is analysed using a CCD camera and dust figure technique. It is shown that the discharge characteristics such as shape and amplitude of the discharge current, the streamer zone length and structure depend on the permittivity of the barrier, its thickness, its surface state and its position relative to the needle electrode. With a closer position the influence of the barrier is more pronounced, but in all cases of the barrier present in the gap, including the case when the barrier touches the needle electrode, a volume part of the discharge is detected in addition to the surface discharge. The structure of the streamer zone of the discharge at different conditions is analysed.