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 higher electric field than in air. 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.

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