Study On Repetitive Nanosecond-pulse Breakdown In Gases

Research on breakdown characteristics becomes more and more attention under some extreme conditions, i.e. nanosecond pulses, strong electric field strength etc. Investigation about nanosecond-pulse breakdown comes from the fast development of compact repetitive pulsed power system, and the published data are relatively scarce. Electrical breakdown studies of gas dielectrics due to repetitive nanosecond-pulses are of considerable importance in theory and practical application, which would enrich research and development of gas discharge.A nanosecond SOS-based pulse generator, which produces an output voltage range of 0 to -200 kV with a rise time of about 10 ns, a full width at half maximum of 20-30 ns, and a repetition-rate up to 2 kHz, was used in the experimental investigation. The experimental chamber and relevant measuring systems have been developed and tested, and the main measured parameters about breakdown characteristics are applied voltage, discharge current, breakdown electric field strength, breakdown time lag, repetitively stressing time, and the number of applied pulses to breakdown. Under the different conditions of applied voltage (-60, 80, 100, 120 kV), plan-plane gap distances (0.5-2 cm) and point-plane gap distances (0.5-3 cm), and repetition-rates (single pulse to 1 kHz) respectively, repetitive nanosecond-pulse breakdown characteristics have been systematically investigated on dry air and nitrogen. The experimental results present that breakdown electric field under repetitive nanosecond-pulses is much lower than that under single pulse, and breakdown electric field of higher repetition rates is adjacent to DC breakdown electric strength. Polarity dependence in repetitive nanosecond-pulse breakdown is not distinct and weakened. The variations involved in gas density, breakdown time lag, and breakdown electric filed strength are concluded, and the modified empirical formula is presented.In the range of nanosecond pulse, both Townsend theory and the traditional streamer model are in question, and the runaway breakdown model induced by high-energy fast electrons is promising. Based on the relations between the electron energy and effective retarding force, the evolution of injected electron energy as a function of distance away from the avalanche head was simulated. The simulation results show that the higher applied electric field strength is, the lower the runaway energy threshold is and the more fast electrons can runaway, and gas pressures also affect the runaway process of fast electrons greatly. Memory effect has been described qualitatively about the appearance of the effective initial electrons and the influence to the streamer development. Repetitive nanosecond-pulse breakdown is characterized by runaway behavior of fast electrons and memory effect of metastables and residual ions.