Blazing a fiery trail with the hounds (prebreakdown streamers)

Current research has shown that prebreakdown streamers in insulating liquids proceed in two distinct ways: a very slow mode ( approximately 200 m/s) bushlike or mushroomlike in shape, and a comparatively fast one ( approximately 2 to 20 km/s) with few filamentary branches. Starting from the hypothesis that in all cases an ionized gaseous phase is present, energetical relationships are established that show the field intensity at the streamer tips to be large (5 to 50 MV/cm), this being due in part to the mechanical energy required, which becomes dominant at supersonic speeds. It is then shown that the ionized gaseous phase must have a minimum conductivity to allow for the observed propagation. For slow streamers this conductivity level corresponds to a glow or luminescent discharge, while in the fast ones, ionization would reach arcing level. In the well-documented case of slow negative streamers, energy conversion appears to be due to free electrons from the gaseous phase. This explains why the slow mode disappears when an electron scavenger is added.