Adaptive grid simulations of negative streamers in nitrogen in under- and overvolted gaps

A local uniform grid refinement strategy is implemented for the simulation of negative anode-directed streamers. The results are shown for an undervolted gap filled with nitrogen at 300 K, with a uniform background electric field of 30 kV/cm. The build up of space charge during the electron avalanche is sped up by a net inflow of electrons at the cathode. The streamer then propagates in the self-induced field enhancement of the space charge layer. Eventually, the streamer branches. The results are compared to those in an overvolted gap (background field of 100 kV/cm). In this case the propagation has the same qualitative features as in the undervolted gap, but the streamer velocity is a factor four larger, while the time to branching is sped up with roughly a factor hundred