Circuit dependence of the diameter of pulsed positive streamers in air

The diam. and branching structure of pos. streamers in ambient air are investigated with a fast iCCD camera. We use different pulsed power circuits and find that they generate different spatial streamer structures. The electrodes have a point-plane geometry and a distance of 40 or 80 mm, and the peak voltages over the discharge gap are up to 60 kV. Depending on circuit and peak voltage, we observe streamers with diams. varying gradually between 0.2 and 2.5 mm. The streamer velocity increases with the diam., ranging from 0.07 to 1.5 mm ns-1, while the c.d. within the streamers stays almost const. The thicker streamers extend much further before they branch than the thinner ones. The pulsed power supplies are a switched capacitor supply with an internal resistance of 1 kW and a transmission line transformer supply with an impedance of 200 W; addnl. resistors change the impedance as well as the voltage rise time in the case of the capacitor supply. We observe that short rise times and low impedance create thick streamers close to the pointed electrode, while a longer rise time as well as a higher impedance create thinner streamers at the same peak voltage over the discharge. [on SciFinder (R)]