Dynamic observation of needle-plane surface-discharge using the electro-optical Pockels effect

Dynamic observation of surface charge distribution is the main advantage of the electro-optical Pockels effect technique over the dust figure technique, the photographic Lichtenberg figure technique, and the static potential probe scanning method. This technique is demonstrated here to observe the surface charge distribution deposited by partial discharge during application of one period of an 8 kV sinusoidal voltage to a needle-dielectric plane electrode system with zero gap spacing. Polarity effect and backdischarge are prominent in ac surface discharges. The observed polarity effect in the charge pattern shows that if the dielectric surface is initially free of surface charge, positive 'streamer' channels burst out intermittently and radially from the needle tip resulting in a spoke-like surface charge distribution, while negative streamers expand almost uniformly in all radial directions resulting in a nearly circular surface charge distribution. This behavior results in quasi-permanent positive surface charges. Residual negative surface charge from prior discharges has a considerable influence on the trajectory of subsequent positive streamer discharges, i.e. causing deviation of positive streamer channels from the radial direction; whereas residual positive surface charge has little influence on the radial development of surface charge from subsequent negative discharges. This measurement technique has a potential for widespread application in investigating the dynamics of surface charging phenomena.