A photoionization model considering lifetime of high excited states of N2 for PIC-MCC simulations of positive streamers in air

Photoionization plays an important role in the mechanism of positive streamer discharges in air. The photoelectron production rate at high pressures is significantly overestimated in the widely used quantitative photoionization model in nitrogen-oxygen mixtures. A new photoionization model is proposed to surmount this disadvantage approximately by tracking the generation and deactivation of high excited states of N-2. Implementation, validation, and discussion of the new photoionization model are presented in this paper. The photoionization coefficient, the fraction of nitrogen ionization events that leads to oxygen photoionization events without quenching, is computed in the new model. It decreases with the increasing pressure when E/p is constant, which agrees with the early experimental results and corrects the overestimation of the photoionization coefficients at high pressures to some extent. Simulations of positive streamers show that the generation and deactivation of high excited states of N-2 weaken the discharge of nitrogen and enhance the nonlocal discharge of oxygen with a time delay in the new model. The time delay slows down the positive streamer discharges in our simulations. Published by AIP Publishing.