Development of positive streamers in air

Summary form only given. The rough development of streamers has been studied by many and is reported well in literature. Here we will show the development of a point-plane streamer in artificial air (a mixture of 20% pure oxygen in pure nitrogen) in much better detail. This is done in a 16 cm point plane gap at pressures between 100 and 200 mbar. Positive high voltage pulses with amplitudes of 20 to 40 kV, rise times of 10 to 30 ns and 0.5 to 4 Hz repetition frequency are applied to the pointed tip. We have studied these discharges with an ICCD camera of which the gate timing with respect to the voltage pulse was varied between discharges. This was done in such a way that the gate closing was spread over the full time-domain between the start of the voltage pulse and the moment that the first streamers crossed the gap. By doing this for hundreds of images, a complete set of pictures of the streamer development as function of time was made. From each of these pictures the longest streamer channel (measured from the tip) was selected and measured by an automated computer script. This results in a streak-camera-like graph of the streamer length as function of time, similar to figure 4 in Clevis et al., albeit with a huge increase in accuracy and number of discharges (as well as a change from pure nitrogen to artificial air). Compared to a streak camera, our method has the advantage that also streamers outside the symmetry axis are taken into account. With this method we have studied the development of discharges for a variety of conditions. The early stages of the discharge are dominated by the formation of a so-called inception cloud. At higher pressure or lower voltages this inception cloud stagnates and in most cases breaks up into separate streamer channels. One of the observations we can make is that the speed of formation of the inception cloud scales with the pulse risetime, but that at a faster voltage-rise, the cloud breaks up later. Furthermore, the size of - he inception cloud is very close to its theoretical maximum predicted by the applied voltage and the breakdown field.