Streamer dynamics under repetitive nanosecond pulse (RNP) is of great importance to understand the streamer essence and breakdown phenomenon in non-equilibrium-plasma-based applications. However, the evolution of streamer dynamics, discharge mode transition, and influential mechanisms in high-pressure gas under long-term RNP were not sufficiently clear and required further investigations. We presented the pulse-sequence resolved analysis on streamer dynamics, discharge mode transition, and polarity effect in high-pressure nitrogen under longterm RNP of pulse width from 15 to 800 ns and RNP superimposed by DC sweep voltage. Under positive RNP, the corona discharge probability in a pulse train was affected by the pulse repetition frequency (PRF) and a transformation from the intermittent mode to continuous mode appeared under high PRF. The envelope curve of the number of applied pulses before breakdown followed the shape transformation from capital 'L' to 'U' with pressure from 0.1 to 0.4 MPa. The streamer channel contraction and the spatial shift of streamer initiation position under high PRF were observed. Under negative RNP, the corona discharge would be in the continuous mode from a low PRF. The average inception voltage and phase of following corona discharges were lower and earlier than those of the first one, respectively. The auxiliary sweep voltage illustrated dramatic and nonlinear effects on discharge characteristics under RNP, which was determined by the sweep voltage amplitude, pulse polarity, and PRF. The inception oscillation stage and steady-state stage were distinct under short-width RNP. An influential mechanism in high-pressure nitrogen was proposed with emphasis on the spatial electric field distortion caused by remaining space charges, which was affected by the pressure-dependent nonlinear diffusion and time-dependent space charge drift under a sweep voltage. The evolution of streamer dynamics and discharge mode transition would help in better understanding of the streamer essence and insulation capability criteria under long-term RNP.

Evolution of the surface streamer and the discharge mode transition from the corona discharge to the surface flashover were investigated by pulse sequence resolved electrical and optical measurements under long-term repetitive nanosecond pulses (RNP). The test sample was a cylindrical epoxy resin insulator attached with a needle electrode in 0.1-0.4 MPa nitrogen. Under positive RNP, the inception phase of subsequent streamer discharges decreases with increasing the pulse repetition frequency (PRF), and a periodical streamer stagnation phenomenon appears at high gas pressure. Under negative RNP, the surface streamer does not illustrate symbolic decreasing tendencies in light intensity and corona inception phase in gas gap. The dependence of the evolution of surface streamer velocity on PRF is correlated with the repulsive force from surface charges and the accessibility of seed electrons. Statistical characteristics of back discharges under negative RNP are qualitatively explained by the local electric field around the triple junction. The reversal phenomenon of polarity effect of the allowable repetitive working coefficient is probably resulted from the corona stabilization effect, the surface electron detrapping process, and the assistance of background electric field. Principal results qualitatively support that behaviors of surface streamer and flashover are dominated by the electric field distribution, volume and surface memory effects as well as their interactions and bidirectional transformations, which are different from the monotonically facilitative tendency predicted by the metastable-species-dominated memory effect mechanism.