On the spatial scale of streamers

We calculate the streamer velocity, which is determined by the streamer mechanisms which include electron drift, electron diffusion and photoionization, in a 1D model that also includes the curvature of the ionization front. In particular, we show that the electron drift may contribute only to the propagation of negative streamers, and the effect of photoionization on the streamer velocity is mostly determined by the photoionization length. The results are used to modify the fractal model of [1]. The transverse size of streamers, as well as the fractal dimension of the streamer structure emerges from the front velocity dependence on electric field and the front curvature. In particular, when the photoionization is the main mechanism which determines the streamer propagation, the transverse size of the streamers is of the order of the photoionization length.