Corona discharge and the visualisation of electric fields

A new technique is presented that employs an ultraviolet excited phosphor to image the spatial dynamics of electric fields, which result from the electrostatic charging of insulating films. This technique has an important advantage over traditional methods because it is able to produce two-dimensional dynamic images in the form of changing patterns of light in the phosphor as it responds to changing fields. A preliminary study of phosphor composition is presented in an attempt to identify the important elements which promote electromodulated photoluminescence. This type of luminescent phenomenon is more commonly referred to as the Gudden-Pohl effect (GPE). This new technique has allowed the observation of dynamic effects which have not been previously reported. In the point-to-plane electrode configuration the phosphor forms part of a thin insulating layer covering the plane electrode. Under corona conditions, the accumulation of charge on the dielectric surface is revealed by the phosphor as an expanding light ring. A mathematical model of the dynamic field situation in point-to-insulated-plane geometry is developed. This provides a description of the light ring effect in terms of the time-varying field across the dielectric layer, and has enabled the light ring dynamics to be predicted from basic electrostatic principles. In addition current density measurements at the plane electrode are compared with brightness profiles using the model to provide a theoretical basis for interpreting the results. It is concluded that the phosphor technique can display useful information about the dynamics of the charge accumulation zone at an air-dielectric boundary. Although in its present form, the technique cannot provide absolute measurements, the two-dimensional brightness information provides an approximate representation of the current density variations across the dielectric surface. (DX84536)