Electro-optic field measurement at a needle tip and streamer initiation in nitrobenezene

In consideration of initiation of prebreakdown phenomena in liquids one assumes that charge injection processes heat and vaporize the liquid giving rise to an ionized gas or plasma phase. The plasma then acts like a conductor and a process continues whereby the streamer grows across the gap. This paper serves to support this initiation picture in that it provides an estimate of the electric field, amount of injected charge, and joule heating in the vicinity of a needle tip which is the cathode in a needle-sphere electrode geometry. Kerr-effect electro-optical observations of the impulse field in the vicinity of the tip show a distortion from the geometrical field due to charge injection from the tip prior to any streamer development. The injected charge is not distributed evenly over the surface of the tip, but appears to reside mainly in a relatively narrow channel. Estimates reveal that the injected charge densities are on the order of 100 μC/cm3. The streamer initiates where this charge-injection channel touches the needle tip.