On the Natural Decay of Corona Charged Insulating Sheets

The surface potential of insulating sheets – and Teflon FEP in particular – charged by a negative corona decays faster than the potential of samples charged to the same level in vacuo by an electron beam. This motivated the reinvestigation of the charge decay in air. The samples are corona charged on a grounded metal substrate, then transferred on their substrate into an open Faraday cup, the sample facing an electrode connected to the input of an electrometer. This configuration – which deliberately maintained a high field in the air gap – favors the decay of charge by ionic conduction in air between the charged sample and the measuring electrode. This extrinsic conduction process is shown to dominate the early phase of the decay. However, the transition from the circulating air configuration to the confined air configuration happens to be much longer than the transit time of the ions in the gap. This is tentatively explained by a model involving the possible displacement of charges below the sample surface, while shining some light on the absorptive character of dielectric.