Investigation of positive streamers in CO₂: experiments and 3D particle-in-cell simulations

We investigate the propagation of positive streamers in CO₂ through 3D particle-in-cell simulations, which are qualitatively compared against experimental results at 50 mbar. The experiments show that CO₂ streamers are much more stochastic than air streamers at the same applied voltage, indicating that few electrons are available in front of the streamer head. In the simulations, we include a photoionization model for CO₂. The computational results show that even a small amount of photoionization can sustain positive streamer propagation, but this requires a background electric field close to the critical field. When we compare streamers in CO₂ and in air at the same applied voltage, the electric field at the streamer head and the electron density in the streamer channel are higher in CO₂. We discuss the uncertainties in CO₂ photoionization and provide an estimate for the quenching pressure, which is based on the radiative lifetime of emitting states and the collision frequency of the gas. Furthermore, a criterion for self-sustained streamer growth in CO₂ is presented and compared against simulation results.