Propagation of positive streamers over insulating surfaces in air

The positive streamer in air has been shown to exhibit reproducible properties which can be measured precisely in a uniform electric field; examples are the field required for its propagation and its velocity as a function of field. It is thus an excellent tool for assessing the influence of the material upon discharge propagation over insulating surfaces. The paper compares streamer properties in air alone with those when propagated over a surface in a uniform field. In the latter case, streamers are usually found to propagate in two modes, namely, a "fast" component on the surface and a slower one in the ambient air. Materials studied include ceramic, PTFE and silicone rubber; each material is shown to have its own "signature" in terms of light output, minimum field for propagation and velocity over the surface.