Influence mechanisms and optimization of pulsed breakdown characteristics of a megavolt class cascade self-triggered switch

Megavolt switches in large pulses typically use a cascade configuration to increase operating voltage and ensure electric field uniformity. We must reduce their jitter when we need multipulse superposition to obtain a higher voltage pulse. Moreover, in the high-altitude electromagnetic pulse simulator, the megavolt pulsed switch needs to adopt the self-triggering technique to simplify the structure and reduce weight for good mobility and hoisting requirements. However, it is difficult to carry out a large number of experiments and completely reveal the influence mechanisms of the characteristics of megavolt switches via experiments. Therefore, this paper derives a breakdown probability distribution model of a cascade switch based on the characteristic parameters of a single-stage switch, which can be obtained easier from experiments. The experiment of the cascade switch is then carried out to validate the model analysis. In the model, we assume that the rising rate of the pulse voltage is a constant and that the breakdown probability of a single-stage switch follows the Weibull distribution. It is common in cascade switches to allow one stage to break down in triggered mode while others are in self-breakdown mode due to overvoltage. When the cascade switch has n stages, and one stage is triggered, it is seen as three equivalent switches based on their breakdown sequence. This paper assumes that x self-breakdown stages first close, the triggered stage closes next, and finally, (n-x-1) self-breakdown stages close. The range of x is [0, n-1]. Then the breakdown probability distribution model of each equivalent switch can be derived using the existing calculation method. The influence mechanisms of the breakdown characteristics of a three-stage cascade switch are semiquantitatively analyzed based on the model. If the uniformity of breakdown characteristics of each triggered switch can be guaranteed and three stages are all triggered, the switch jitter is approximately equal to the jitter of a single-stage triggered switch because the latter two stages break down under high overvoltage. The mean breakdown voltage of each stage is lower than a single-stage triggered switch. When only one stage is triggered, most probably, the triggered stage will close first, followed by two self-breakdown stages under high overvoltage. The switch jitter is significantly lower than that under the self-breakdown mode, and the mean breakdown voltage of each stage is higher than that when all three stages are triggered. However, the possibility that one self-breakdown stage closes first results in the switch jitter being approximately twice when all three stages are triggered. The improvement path of the switch jitter characteristics can be inferred using the model analysis. The jitter of the triggered single-stage switch should be reduced, and it is preferable to trigger all stages if the uniformity of the breakdown characteristic of each single-stage switch can be guaranteed. In the experiment, the three-stage cascade switch operates on a pulsed voltage with a rise time of 300 ns and an operating voltage range of 0.8—2.0 MV. Self-triggered continuous preionization is adapted to eliminate the influence of trigger gap jitter on the switch jitter, ensuring the uniformity of breakdown characteristics of each single-stage switch. When one stage is triggered, the time delay jitter is 2.9—7.7 ns, and the breakdown voltage jitter is 0.51%—2.21%. When three stages are triggered, the time delay jitter is 2.3—3.6 ns, and the breakdown voltage jitter is 0.59%—0.91%. The preceding analysis is verified, and the switch characteristics are improved compared to the original one. Moreover, the self-breakdown characteristic is more stable when the gas pressure exceeds 0.3 MPa; thus, the switch jitter is approximately the same when we trigger one or three stages.