Operation of a five-stage 40000-cm2-area insulator stack at 158 kV/cm

We have demonstrated operation of a 3.35-m-diameter insulator stack at 158 kV/cm with no total-stack flashovers on five consecutive Z-accelerator shots. The stack consisted of five +45/spl deg/-profile 5.715-cm-thick crosslinked-polystyrene (Rexolite-1422) insulator rings, and four anodized-aluminum grading rings shaped to reduce the field at cathode triple junctions. The width of the voltage pulse at 89% of peak was 32 ns. We compare this result to a new empirical flashover relation developed from previous small-insulator experiments conducted with flat unanodized electrodes. The relation predicts a 50% flashover probability for a Rexolite insulator during an applied voltage pulse when E/sub max/e/sup -0.27/d/(t/sub eff/C)/sup 1/10/=224, where E/sub max/ is the peak mean electric field (kV/cm), d is the insulator thickness (cm), t/sub eff/ is the effective pulse width (/spl mu/s), and C is the insulator circumference (cm). We find the Z stack can be operated at a stress at least 19% higher than predicted. This result, together with previous experiments conducted by Vogtlin, suggest anodized electrodes with geometries that reduce the field at both anode and cathode triple junctions would improve the flashover strength of multi-stage insulator stacks.