Abrupt changes in streamer propagation velocity driven by electron velocity saturation and microscopic inhomogeneities

摘要

Streamers are elongating structures with velocities much higher than the maximum drift velocity of electrons. Streamer velocity is subject to dramatic changes. Causes of positive streamer acceleration due to electron velocity saturation at intense electric fields are investigated in this paper. Findings of this paper explain regimes of higher positive streamer velocity using a nonlinear electric field dependent electron velocity model that describes the streamer velocity more accurately compared to the experimental results in the literature. Streamer branching in inhomogeneous media is also studied as an alternative cause of sudden changes in streamer velocity magnitude and direction, since after branching, velocities of the born branches become significantly higher than the main streamer column velocity. Spatial structures of streamers are inherently three-dimensional (3-D) as they easily branch out and become axially asymmetric. Therefore, in this paper, a previously developed two-dimensional (2-D) axisymmetric streamer model [1] is extended to a fully 3-D model incorporating nonsymmetrical variations in the streamer shape. Streamer branching is traditionally thought to be driven by stochastic inhomogeneities inherited from noisy initial states, impurities, and/or charge carrier density fluctuations. Our streamer model, however, shows that the branching is an intrinsic attribute of streamers, which also has deterministic origins. Such origins of abrupt streamer acceleration in some cases make the branching inevitable depending on the shape and velocity of volume charge at the streamer head. Specifically, if the volume charge layer at the streamer head is thin and slow enough, even an infinitesimal perturbation can effectively trigger the branching. On the other hand, if the streamer head is stable, even relatively large perturbations do not grow instabilities from the streamer head. Based on the modeling results for streamers propagating in a liquid dielectric, n- mber, diameter and velocity of the just born branches are estimated, which agree quantitatively and qualitatively with experimental images of the streamer branching.