Role of electronegative gas admixtures in streamer start, propagation and branching phenomena

A critical analysis of photoionization as the major process for seed electron production ahead of the cathode-directed streamer has been made. The accumulation of O-2 ions between pulses and fast electron detachment in an electric field could be an effective source of seed electrons for repetitively pulsed discharges in electronegative gases. Measurements and two-dimensional calculations in the hydrodynamic approximation of streamer parameters in air (anode current, propagation velocity and radiative channel diameter) can only agree if one assumes uniform background pre-ionization and not by taking into account only the photoionization process generally adopted for pulsed discharges in air. The results of analytical estimates and full three-dimensional simulations confirm that the seed charge distribution in the undisturbed gap has an effect on the streamer head formation and leads to the streamer branching phenomena.