How density fluctuations influence the motion of streamers

In an electric gas discharge, free electrons and photons create additional electrons and photons through impact- and photoionization of the neutral constituents. The probability of ionization within a distance Δz is 1-e ^Δz·σ _j ^·n, where σ_j is the cross section of the jth ionization process and n the density of ambient air molecules. The density is therefore an important parameter of the discharge process, with temporal and spatial parameters scaling with the density [1]. Commonly, streamers are modelled in a homogenous gas with a few exceptions of streamers in the mesosphere as in sprites, with the positive polarity streamer propagating towards increasing density [2]. However, here we discuss gradients in a more general geometrical configuration. With a 2.5D cylinder symmetric particle code we have simulated streamer dynamics in the atmosphere with density perturbations around n_o=2.54·10²³ m^-3 typical for the atmospheric density at sea level. Fig. 1 shows one example of a spatially inhomogeneous density distribution along the symmetry axis z. The ambient field pointing towards -z with a magnitude 1.5 times the conventional breakdown field. Fig. 2 shows the electron density of a
streamer after 1.65 ns. The left half shows the simulation with a constant air density of no and the right half with the density as shown in Fig. 1. In this configuration, there is already a visible effect. We will present how differing density distributions changes the velocity of the streamer and the electric field evolution.