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 alumin a ceramic. The insulator is subjected to a de voltage step or to a trapezoi dal voltage in vacuum. The charging is observed by using an electrostatic p robe located at the surface of the cathode close to the triple junction whe re the insulator, cathode and vacuum meet. The probe signal shows a step in crease associated with the charging. The interval between the voltage rise and the charging, which is defined as the delay time of charging in this pa per, ranges from 10(-5) s to 10 s depending on the material and the applied voltage. II also depends on the condition of the insulator-cathode interfa ce, the electrical prestress and the surface roughness of the insulator. Am ong these factors, the surface roughness has the largest effect on the dela y time. Two dimensional Monte Carlo simulation based on the secondary emiss ion electron avalanche model has been performed to analyze the charging pro gress. During the delay period, positive charge accumulates on the surface of the insulator near the cathode, which further accelerates the charging.