The effect of photoionization on positive streamers in CO$_2$ studied with 2D particle-in-cell simulations
We investigate the effect of photoionization on positive streamer propagation in CO2 with 2D particle-in-cell simulations. Positive streamers require free electrons ahead of them to sustain their propagation, which in air are typically provided by photoionization. Photoionization in CO2 is much weaker than it is in air, because fewer ionizing photons are produced and their typical absorption distance is orders of magnitude smaller. Our results show that even a small amount of photoionization can sustain positive streamer propagation, but this requires a higher electric field around the streamer head than in air. Furthermore, we propose a self-sustaining criterion for streamer discharges in CO2 and we discuss the uncertainties in the photoionization model.