Numerical simulations and conformal analysis of growing and branching negative discharge streamers

The dynamics of an anode-directed streamer can be described by advection-diffusion equations for the charged particles, including a local field-dependent impact ionization term, and coupled to the Poisson equation for the electric field. We present the results of new simulations that use a local uniform grid refinement strategy. Even on very fine grids, provided the electric field is high enough, the streamer appears to branch spontaneously. These results are supported by new analytical solutions based on a moving boundary approximation.