Title: Branching sparks --- the dynamics of electric breakdown
Speaker: Ute Ebert
Speaker Info: CWI Amsterdam
Brief Description:
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Abstract:
The initial phase of sparking is determined by so-called streamers. These are weakly ionized channels during their growth period. The growth is characterized by a self-induced enhancement of the electric field at the tip of the discharge channel. Streamers propagate with elocities of the order of 1000 km/sec. Recent ultrafast photography gives a new view on their dynamics. Streamer concepts are also being applied to recently discovered high altitude lightning, so-called red sprites. I will review recent observations and then explain the state of microscopic modelling, computations and theoretical concepts. Basically, already a single discharge channel has a multiscale structure with a thin ionization front surrounding a rather inert body. I will present computational results with adaptive grids, and I will discuss the properties of ionization fronts, moving boundary approximations for these fronts, and solutions of the moving boundary problem with conformal mapping methods. The result is the prediction that streamers in a sufficiently high potential can branch spontaneously due to a Laplacian instability as is also observed in computations. This quantitative prediction has to be confronted with phenomenological models for spark branching of the type of diffusion limited aggregation.Date: Friday, February 20, 2004