3D simulations of streamer branching in air

3D simulations have been performed on the streamer branching process for a quasi-uniform electric field configuration. The propagation of the individual streamer branches is simulated by a series of electrostatic calculations and a physical and deterministic criterion is used for the streamer branching. The step length assigned to each branch is proportional to the electric field magnitude in the space around the tip, which in effect makes the step length a measure of the local speed on each filament. The simulations have been focused on positive streamer discharges propagating in air and in air along a dielectric surface. In the case when the streamer propagates in air, we have investigated the effect of the background field, the streamer radius and the branching angle. It was only possible to simulate a few branching steps due to high demand on required computer power and memory. Still, several interesting features were deduced from the simulations using the accomplished steps. It was observed that the multiplication process was intense in the early stage of the streamer development. In the later stage, when the streamer passed the high field region, the existing branches continued as separate almost straight filaments. In the case when the background field was increased, the branching was found to be substantially higher. Also, in this case the length of the individual filaments was significantly longer which indicates that the streamer speed was increased. Concerning the streamer radius, we found that an increasing streamer diameter will inhibit the branching process. The streamer propagated for some distance in the gap as a single filament and then started to multiply. Finally, when a dielectric surface was introduced parallel to the electrode gap, we could observe a distinguished preference direction taken by the individual branches. Some of them were attracted to the surface and had a more pronounced multiplication. This indicates that the field enhancement caused by the dielectric constant can be a considerable factor when a streamer propagates along an insulator surface. (C) 2003 Elsevier B.V. All rights reserved.