Delay Characteristics of Surface Charging on a Cylindrical Insulator in Vacuum

This paper describes the temporal characteristics of surface charging on a cylindrical insulator made of polymethylmethacrylate (PMMA) resin or alumina ceramic. The insulator is subjected to a dc voltage step or to a trapezoidal voltage in vacuum. The charging is observed by using an electrostatic probe located at the surface of the cathode close to the triple junction where the insulator, cathode and vacuum meet. The probe signal shows a step increase associated with the charging. The interval between the voltage rise and the charging, which is defined as the delay time of charging in this paper, ranges from [10.sup.-5] s to 10 s depending on the material and the applied voltage. It also depends on the condition of the insulator-cathode interface, the electrical prestress and the surface roughness of the insulator. Among these factors, the surface roughness has the largest effect on the delay time. Two-dimensional Monte Carlo simulation based on the secondary emission electron avalanche model has been performed to analyze the charging progress. During the delay period, positive charge accumulates on the surface of the insulator near the cathode, which further accelerates the charging.