Investigation Into Critical Pulsewidth and Wormhole Effect on Solid--Vacuum Interfaces

Abstract

The concept of critical pulsewidth ( $\tau _{c}$ ) refers to the pulse duration at which the surface flashover threshold ( $E_{\!f}$ ) equals the bulk breakdown threshold ( $E_{BD}$ ) in composite insulation systems, such as solid–liquid, solid–gas, or solid–vacuum interfaces. When the applied pulsewidth ( $\tau $ ) is shorter than $\tau _{c}$ , $E_{\!f}$ exceeds $E_{BD}$ , leading to bulk breakdown instead of surface flashover. This phenomenon results in a wormhole-like breakdown trace, termed the wormhole effect. In this article, the critical pulsewidth for solid–vacuum interfaces is investigated, and the conditions under which the wormhole effect occurs are calculated. It is concluded that the wormhole effect is likely to occur when: 1) the insulator thickness is too small; 2) the pulsewidth is too short; 3) the insulator configuration is too complex; or 4) the number of applied pulses is sufficiently large. These conditions are used to explain the observed wormhole breakdowns in vacuum insulators of the PITHON accelerator and the TPG700 generator.