Some Aspects of Breakdown Streamers

Photomultiplier and oscillographic study of positive breakdown streamers yields values of current and distance of tip advance as a function of time. From these the relatively constant number n of excess positive ions left behind by the streamer per cm advance and the tip velocity vₜ are obtained. These are related to the linear electron density nc per cm of streamer channel plasma and the electron drift velocity vc in the channel by ncvc=nvₜ, with vₜ>vc. Since vc is related to the gradient Xc in the channel by well-known drift velocity data, it is possible to limit the range of values of nc and vc. The limiting conditions are that nc>n, ∫0ˣXcdx<Vₓ, where Vₓ is the applied potential, and finally that Xc must be sufficiently large to maintain the channel conducting despite dissociative recombination and dissociative attachment losses. In longer channels heating cannot be neglected. The dense positive space charge left behind by the electron current in the ionized channel of radius R causes a radial expansion of the excess space charge to a radius R^' by incoming electron avalanches leading to a broad transient luminosity for some distance behind the tip. The tip and the conducting channel taking the return stroke, however, retain their small radius R.