Effects of photoionization on positive streamer propagation using EMPIRE

Photoionization is a critical aspect of positive streamer propagation. Here we study its effects on centimeter-scale gaps using EMPIRE, a Sandia National Laboratories electromagnetic/electrostatic particle-in-cell (PIC), Direct Simulation Monte Carlo (DSMC) code. Simulations of gaseous breakdown gaps between two electrodes are performed. Initial conditions are chosen such that only positive streamer propagation is allowed by controlling concentrations of electrons/ions in the initial stages of simulation. Results of 1D and 2D simulations are presented with emphasis on photoionization effect on electric field modification and propagation dynamics. Of particular interest is stochastic nature of streamer dynamics and branching, which are triggered by generation of photons in the regions of the gas gap where there are no charged particles present. To highlight the effects of these photons artificial photoionization cross-section rates are used, with changes made to photoionization to increase/decrease the rates as prescribed. This allows for a direct comparison between photoionization dominated streamers and those for which photons have limited effect. A series of studies are performed to demonstrate the most important aspects of photoionization and how existence of random photons in gaseous gap effects streamer branching and propagation behavior.