Optimizing the performance of flat-surface, high-gradient vacuum insulators

High-gradient insulators (HGI) are periodic assemblies of conducting and insulating layers that have been shown to withstand higher pulsed voltages in vacuum than homogeneous insulators of the same length. We carried out calculations and experimental studies to understand the effect of geometry on the performance of well-conditioned, flat-surface HGI assemblies. We tested stacks with several different values of I/M (where I is the axial length of an insulating layer and M is the length of a metal layer). The experiments showed that HGI performance was substantially better than conventional insulators for I/M<3 and somewhat worse for I/M>3. Numerical calculations of electron orbits showed: 1) that the electric fields in HGI assemblies may have the favorable property of sweeping charged particles away from the surface and 2) that electron multiplication on the surface is suppressed when I/M<3.