Design and Scaling Calculations for the ZR Vacuum Insulator Stack

The ZR z-pinch driver is being upgraded to deliver 26 MA into a standard load, a substantial increase compared to the present Z maximum of 20 MA into the same standard load. The upgraded accelerator has basically the same foot print as the existing Z, thus bringing a number of challenges, one of the critical ones being the insulator stack. The present stack is formed by 4 levels, the top two with 5 insulators each and the bottom 2 with 6 insulators each, all of them at 5.715 cm (2.25") in height per insulator. Circuit simulations show that some levels of the new stack will need to operate at or above 5 MV, and that simply increasing the number of insulators significantly increased the inductance, thus a careful scaling and analysis needed to be done. Based on the flashover behavior/data from the Z stack, using available Rexolite data, and previous flashover experiments on Z, we scaled the JCM constant from 175 to 221. We show how this number was then further scaled by using the results of an anode field relief plug experiment; and how, with a Weibull analysis of this data, we estimated the applicable area dependence. After properly scaling the JCM equation, the multistage g-matrix approach from Ian Smith was used to project the stack reliability. A set of options were developed in which the effect of adding insulator stages vs stage voltage increase, due to the increased inductance, is compared with the reliability predictions. All calculations were done under the assumption that MFI will not make a contribution to the stack performance. The resulting stack design uses 6 insulators on the upper two levels and seven insulators on the lower two levels (6677), and uses an anode field relief plug to give a projected reliability better than the design requirement.