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

Abstract

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 wellconditioned, flat-surface HGI assemblies. We tested stacks with several different ( values of Ir r r r rM 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 Ir r r r rM -3 and somewhat worse for Ir r r r rM ) ) 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 Ir r r r rM -3.Index Terms -High voltage insulators, electron avalanche, vacuum surface breakdown.

Vacuum insulator development for the dielectric wall accelerator

“…The best alumina structure tested by Leopold did not exceed the breakdown strength of conventional structures until after 2500 conditioning pulses 18 , which may be partly due to a slower conditioning process than was seen by us. In fact, the conditioning procedures used by us may be too aggressive.…”

“…Conditioning was observed by Elizondo in structures formed from Lexan interleaved with thin stainless steel sheets 17 , and by Leopold in structures formed from alumina and thick Kovar rings 18 . Conditioning was not observed by Sampayan in structures formed from silica with sputtered gold electrodes 19 .…”

“…In addition, we tested four samples with metal layers that were slightly thicker than the insulator layers, as shown in Table III. This geometry was suggested by the work of Leopold, who attempted to explain the improved performance of HGIs in terms of electron deflection away from the HGI-vacuum interface 18 . This effect relies on the curvature of equipotential lines near the HGI surface, and requires relatively thick metal layers ( 3 / < M I…”

DC Characterization of High Gradient Multilayer Insulators

“…This will require either an HGI sample and more highly polished and/or greened electrodes, or construction of a higher voltage test stand. The conclusions drawn by Leopold, et.al [2] indicate a more optimal spacing of the conductors. We plan to investigate this mechanism further by fabricating and testing high gradient insulators with alternate conductor intervals under pulsed and DC conditions.…”

“…Thus, the HGI appears to behave as an ensemble of independent insulator structures [1] as shown in figure 1. Figure 1 Recent research at other laboratories both in the US and abroad have suggested that the improved performance of this topology is resultant from an alternating gradient formed by the multiple metal/dielectric intervals along the insulator surface [2]. This theory suggests electrons are deflected away from the insulator as a result of the alternating gradient thus inhibiting the production of secondary electrons leading to a cascade failure and insulator flashover.…”