Simulation of streamer-to-spark transition in short non-uniform air gaps

Propagation of positive streamers in a short non-uniform gap in dry air and the evolution of plasma channels after bridging of the gap have been simulated. The dependence of the streamer-to-spark transition time br on the mean electric field in the gap has been obtained for conditions when the transition occurs in a nanosecond time range. Calculated values of br are in a reasonable agreement with experimental data. According to the results of the simulation, the major cause of spark formation is an increase, with time, of the electron detachment rate due to the accumulation of oxygen atoms and other active particles.