Temporal and spatial characteristics of the ultra-short pulsed discharge in ambient air

Pulsed discharge plasmas are under wide investigations recently, for the purpose of industrial applications and understanding the challenging phenomena. This article describes the temporal and spatial characteristics of the pulsed discharge in ambient air. The reactor consists of a saw-blade (anode) and a plate (cathode), and the saw-blade has 4 tips with a distance of 18 mm horizontal. The gap distance between tips and the plate is 11.6 mm. The discharge was generated by a high-voltage pulse with a risetime (20-80%) of 7.7 ns, a width (FWHM) of 16 ns, and an amplitude of 56.4 kV. Experimental results show that the discharge current and energy in each channel are 50 A and 35 mJ, respectively. The streamer can bridge the gap within 2 ns, and the diameter of streamer is up to 3 mm. The streamer head is of round shape nearby the anode, and becomes sharper before the cathode. Once the streamer bridges the gap, hot spot emerges on the anode. Then the discharge transits quickly into glow-like discharge, and this phase lasts about 30 ns; while the hot spots can remain for about 130 ns. Due to the expansion of the hot spots, after the discharge elliptical shockwaves appear at the anode, and followed by those hemispherical ones at the cathode. They propagate much faster than the thermal expansion. Their averaged speeds are 867 m/s and 789 m/s, respectively at both sides (anode and cathode) 1.4 ??s after the discharge, and become 420 m/s and 410 m/s respectively 21 ??s after the discharge. The thermal effects disappear about 3 ms after the discharge.