Relationship between a critical charge value and the streamer-to-spark transition

The streamer-to-spark transition in atmospheric air with a positive point-to-plane gap exhibits the well known behaviour of a delayed spark if the capacitance of the set-up is small enough. Analysis of experimental results suggests that a critical charge has to be stored in the gap for the spark to occur, both with DC positive and with impulse voltages. A critical space charge field is believed to be the spark criterion in this case. The spark develops from filaments bridging the gap. Evidence on these filaments has been found both on Schlieren frame records and on image converter streak records. The physical structure of these filaments is similar to that of the leaders in the final jump stage of large air gap discharges.