Evolutions of repetitively pulsed positive streamer discharge in electronegative gas mixtures at high pressure

Electronegative gas components and gas pressure significantly change residual charge dynamics, which are critical for pulse-periodic streamer discharge behaviors. Evolutions of repetitively pulsed positive streamer discharge and the streamer-to-spark transitions were investigated at high pressures and compared between typical weak (O-2) and strong (SF6) electronegative gas mixtures. Pulse-sequence resolved electrical and optical diagnostics were implemented to capture discharge evolutions in long pulse trains. We observe that streamer inception and propagation under subsequent pulses in N-2 and N-2-O-2 mixtures are similar, including the earlier inception of the primary streamer and the accelerated propagation of the secondary streamer. The repetitively pulsed breakdown is extended to the low pulse repetition frequency region with the addition of O-2. Discharge evolutions are unexpectedly different in N-2-SF6 mixtures. Subsequent discharge channels prefer to propagate around the periphery of the inception cloud region with large radial deviations. Another difference is the precursor channel identified besides multiple streamer channels. Effects of electronegative gas on streamer evolutions under positive repetitive pulses have been qualitatively analyzed. Collisional electron detachment and photo-ionization are crucial in N-2-O-2 mixtures. With the presence of strong electronegative gas (SF6), the leader formation is probably induced by the earlier corona inception and longer voltage stressing period under following pulses in a pulse train, which are caused by the strong electronegativity of SF6 and the small ion mobility.