Design criteria for the Z vacuum insulator stack

The Z (formerly PBFA Z) vacuum insulator system consists of four stacks of Rexolite rings about 11 ft. in diameter, immersed in water, that insulate four biconic vacuum power-flow regions. The power flows are combined by a post-hole convolute just outside the imploding plasma load. Each ring stack is driven by nine similar water transmission lines connected around its perimeter. The different inductances between each stack of insulators and the load lead to a different voltage magnitude and waveform across each stack; therefore two stacks consist of five 2.25 inch high insulators, and two stacks consist of six such insulators. The design criteria for the insulator stacks were developed using the analysis described by I.D Smith (Proc. 10th IEEE Int. Pulsed Power Conf., 1995). This treats the case of multiple stack modules each composed of a number of series insulators and each having a large circumferential transit time. The analysis tries to take account of the fact that when one flashover occurs at a particular location in one insulator stage, or even a number of flashovers at various locations, effective breakdown may not have occurred, because it takes time for series stages to flash locally in the same stack, while other parts of the stack are transit-time-isolated. To develop criteria for Z the analysis was first applied to Saturn, and was normalized by assuming a safe level for Saturn based on experience; it was then applied to Z. The analysis is presented, and results to date for the Z stack are described.