Double pulse streamer experiments

It is well known that streamer discharges are influenced by background ionization and other effects of previous discharges1. Also externally applied background ionization can have a great influence on streamer propagation and morphology2. In a newly designed experiment we have studied the influence of repeating positive streamer discharges by applying two subsequent high voltage pulses with a variable interval between them. The pulse-to-pulse interval was varied between 200 ns and 40 ms so that a large range of effects could be observed for three different gases: artificial air, pure nitrogen and pure argon. The discharges are studied with two ICCD cameras which image the same area by means of a half-mirror. Each camera is gated so that it images the streamer propagation during either the first or the second voltage pulse. Experiments have been performed in a 103 mm point-plane gap at a pressure of 133 mbar. We have found a range of phenomena that depend on the inter-pulse time ¿t. These phenomena occur in all three different gasses, although their timing. For small values of ¿t, (roughly below 1 µs for air and nitrogen and below 15 µs for argon) we observe that the streamers just continue their old paths. If they did not cross the gap during the first pulse, they will travel further where they stopped during the first pulse. At larger values of ¿t the conductivity of the old channels has decreased too much and streamers no longer continue on the old paths. However, parts of the old paths do glow up again like secondary streamers. At still larger values of ¿t (roughly above 2.5 µs for air and 30 µs for nitrogen, for argon this is less determined) new streamer channels appear. At first they avoid the entire area of the previous discharge; next they follow the edges of the old channels; then they start to follow the old channels exactly and finally (with ¿t in the order of (tens of) milliseconds) they become fully independent of the old paths. These experiments give insight in the effects of leftover ionization and other species on subsequent discharges. 1. S. Nijdam, G. Wormeester, E.M. van Veldhuizen and U. Ebert, "Probing background ionization: positive streamers with varying pulse repetition rate and with a radioactive admixture", J. Phys. D: Appl. Phys., 2011, 44, pp. 455201. 2. E. Takahashi, S. Kato, A. Sasaki, Y. Kishimoto and H. Furutani, "Controlling branching in streamer discharge by laser background ionization", J. Phys. D: Appl. Phys., 2011, 44, pp. 075204