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Positive streamer discharges have been studied and modelled extensively inair. Here we study positive streamers in CO$_2$ with and without oxygenadmixtures; they are relevant for current high voltage technology as well asfor discharges in the atmosphere of Venus. We discuss that no efficientphotoionization mechanism is known for gases with a large CO$_2$ fraction, asphotons in the relevant energy range are rapidly absorbed. Hence positivestreamers can propagate only due to some other source of free electrons aheadof the ionization front. Therefore we study positive streamer propagation inCO$_2$ with different levels of background ionization to provide these freeelectrons. The effect of replacing photoionization by background ionization isstudied with simulations in air. Simulating streamers in background fields of16 to 20~kV/cm at standard temperature and pressure within a gap of 6.4~cm, wefind that streamer propagation is rather insensitive to the level ofphotoionization or background ionization. We also discuss that the resultsdepend not only on the value of breakdown field and applied electric field, andon preionization or photoionization, but also on the electron mobility $\mu(E)$and the effective ionization coefficient $\alpha_{\rm eff}(E)$, that aregas-dependent functions of the electron energy or the electric field.

Simulation of positive streamers in CO$_2$ and in air: the role of photoionization or other electron sources