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For nanosecond pulsed gas discharge under hundreds of kV/cm in sulfur hexafluoride (SF6), the electric field emission (EFE) effect may be the mechanism of the initial electrons. Electrode surface micro-protrusions (ESMPs) generated after discharging, would affect the discharge process under a high electric field. To investigate the effect of ESMPs on the breakdown time delay and jitter, the breakdown characteristics of different electrode materials and surface roughness are determined in this paper. Three types of electrode materials include stainless 304L, EDM-AF5 graphite, and CuW70 W-Cu alloy. The influences of stainless 304L electrodes with surface roughness coefficients of 1.6 and 0.8 on the breakdown delay and jitter are compared. After 700 discharge shots, the morphology characteristics and distributions of ESMPs are observed by a contourgraph. Based on the statistical results of ESMPs and EFE theory, the relationship between the electric field of EFE and ESMPs can be calculated. In addition, the effective electron rates of negative ion detachment (NID) are estimated. The experimental and calculation results indicate that the initial electrons are mainly produced by the EFE effect. When the pressure is below 0.5 MPa, the NID effect is limited and the breakdown time delay and jitter are dependent on the height to radius ratio and distributions of ESMPs. Once the pressure exceeds 0.5 MPa, the NID effect will be significant and the influence of ESMPs can be neglected. [ABSTRACT FROM AUTHOR]

In the practical operation of an electromagnetic pulse (EMP) simulator, the flashover phenomenon on the edge of polypropylene dielectrics dominates the insulation failure mode of a peaking capacitor. To analyze the characteristics of polypropylene film flashover voltage under high-pressure SF6, an experimental platform was established, and the pulse generator could output a pulse with a rise time of approximately 27 ns. The experiments were performed on finger electrodes, and the SF6 pressure was set from 0.1 to 1.0 MPa. The experimental results show that the relationship of flashover voltage and gas pressure could be divided into two regions: in low-pressure regions, the flashover voltage increases with the gas pressure linearly and in high-pressure regions, the growth of flashover voltage gradually slows down and approaches a saturation value. As the gap distance increases, the saturation point of gas pressure decreases, and even when close to 0.4 MPa, with a 9-mm gap distance, this unconventional phenomenon cannot be ignored, particularly in practical EMP simulators. When the high surface resistivity and the charge trap distribution on the polypropylene film are considered, the accumulation and emission of surface charges on the film might be the primary reason for the saturated phenomena. [ABSTRACT FROM AUTHOR]