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Numerical simulation of discharge mechanism under nanosecond pulse voltage assists in the insulation performance optimization of N2-insulated switches that are widely used in the pulse power devices. This article presents a fluid dynamic model for the positive streamer propagation in N2 based on a needle-plate discharge system. The effects of the operating conditions, such as voltage amplitude, rising edge, gap width, and needle radius, are studied. Simulating results indicate that the streamer might be evoked only when the maximum electric field exceeds the critical value that is closely influenced by the needle radius and gap width, e.g., $1.3\times 10^{7}$ V/m based on a needle-to-plate electrode configuration with a 0.4-mm tip radius and 5-mm gap. The maximum electric field occurs at the head of streamer and it drops suddenly by nearly two orders of magnitude in the back of the head, which makes the streamer moves forward rapidly. Certainly, it needs a high enough voltage to ensure continuous propagation of the streamer, for example, more than 20 kV needed as the tip radius and gap set to 0.4 and 5 mm, and the electric field strength and migration speed increase with the increasing voltage amplitude. The radius of the discharge region is smaller when the rising edge time increases from 10 to 40 ns. However, if the rising edge time is too low (e.g., 2 ns), the streamer will cease before it propagates to the plate electrode and the radius of the discharge region shrink at the same time. In addition, the peak electric field and the averaged electron energy at the same position away from the needle decrease slightly with the gap width. Finally, there is an obvious decrease of the initial peak electric field as the needle radius increases. [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]

Due to its low-loss and high dielectric strength, polypropylene is widely used as electrical insulation material in pulse capacitors. In order to study its breakdown characteristics for nanosecond voltage pulses, an experimental setup was designed to generate voltage pulses with a 10%–90% rise time of 38 ns and a full width at half maximum of about 90 ns. Tests of the three polypropylene films with thicknesses of 12, 15, and $18~\mu \text{m}$ were performed. In addition, hundreds of nanoseconds pulse and dc voltage were also applied for comparison. The experimental results reveal that most of the breakdown occurs during the front edge of the pulse for tens of nanoseconds pulse, and the breakdown strengths of the three kinds of polypropylene films are almost identical for different applied voltages, which show a weak correlation between the thickness of the dielectric and the breakdown field. The expected breakdown strength of tens of nanoseconds pulse defined in this paper is not significantly higher than the mean value of the hundreds of nanoseconds pulse and the dc condition, suggesting that the breakdown of a thin polypropylene film is mainly determined by the electrical field but not the voltage-time cumulative effect. All these observations strengthen the conclusion that the breakdown of a thin polypropylene film is an intrinsic phenomenon. [ABSTRACT FROM AUTHOR]

Current transmission efficiency in a conical transition magnetically insulated transmission line (MITL) has been studied experimentally on a 1.0-MV linear transformer driver system, which has 10 identical cavities connected in series with MITL. Transmission efficiencies of anode current as high as 98.4% and 93.1% could be achieved when the MITL operates at load-limited and self-limited flows, respectively. As for the cathode current, because of the sufficient length of conical transition MITL, cathode current is also able to sustain high transmission efficiency. But as long as the diode gap is too large, many of the electrons will be launched into the anode–cathode gap and the cathode current near the diode drastically decreases. Particle-in-cell simulations were conducted to validate the conclusions. The simulation results agree with experiments. [ABSTRACT FROM PUBLISHER]