Investigation of the Inception Cloud of Positive Streamers in Air

We have studied the development of positive streamers in a point-plane gap in artificial air. The initial phase of such a discharge is dominated by the so-called inception cloud. At relatively low pulse voltages (e.g. 20 kV in a 160 mm gap at 100 mbar) the inception cloud initially grows during the pulse. The speed of propagation continuously decreases and finally stagnates. The size of the inception cloud is then close the theoretical maximum estimated by assuming that it is a conductive sphere with maximum radius Rmax = V/Ebr, with V the applied voltage and Ebr the breakdown field. After the inception cloud has stagnated, it can break up into one or more streamers. The timing of this break-up, its jitter and the number of streamers emitted from the inception cloud depend on the combination of applied voltage and pressure.