Modeling of the increase of the electric field in the encounter between a negative and a positive streamer

Abstract:

The encounter between negative and positive streamers has recently been suggested to be a plausible mechanism explaining the production of X-rays by discharges in air [Cooray et al., JASTP, 71,1890, 2009; Kochkin et al., J. Phys. D: Appl. Phys., 45, 425202, 2012]. Moreover, encounters between streamers of different polarities are very common. Indeed, during the formation of a new leader step, the negative streamer zone around the tip of a negative leader and the positive streamers initiated from the positive part of a bidirectional space leader strongly interact. In laboratory experiments, when streamers are approaching a sharp electrode, streamers with the opposite polarity are initiated from the electrode and collide with the former streamers. Given that positive and negative streamers respectively carry positive and negative charges at their fronts, it is reasonable to consider that the electric field between a negative and a positive streamer would increase as the two streamers are approaching each other. However, this is a non-linear problem that depends on the dynamics of both streamers. Indeed, an increase of the electric field above the conventional breakdown threshold field will increase the electron density, and therefore the conductivity, at this location, which in turn, will tend to reduce the electric field. In this work, we will simulate numerically encounters between negative and positive streamers in order to examine the behavior of the maximum electric field that can be reached. We will quantify whether and how such collisions between streamers can increase the electric field up to sufficient magnitudes to produce thermal runaway electrons and the associated X-rays.