  In sufficiently large gaps and electric fields, discharge streamers do
branch. In [Arrayas et al., PRL 88, 174502 (2002)], we observed streamer
branching numerically within a deterministic particle density model and
explained it as a Laplacian instability of a thin space charge layer. Our
numerical results were criticized in [Kulikovsky, PRL 89, 229401 (2002)]. We
here present an adaptive grid refinement method for streamer simulations, and
we carry out the first conclusive investigation on the effect of the numerical
grid on streamer branching in different fields. On stepwise finer grids the
branching time converges, hence streamer branching is for the first time
predicted quantitatively.
