Discharges due to separation of a corona-charged insulating sheet from a grounded metal cylinder

An experimental study shows that when a corona-charged PMMA (polymethyl methacrylate) sheet, 0.5 mm thick, is separated from a grounded metal cylinder, discharges occur in the wedge-shaped air gap. These discharges serve as a model for discharges that occur between dielectric films and transferring rollers. Two modes of discharges are found: weak microdischarges and stronger major discharges. The microdischarges are very weak, consisting of small light spots, but it is possible to observe them through two cascaded image intensifiers. Charges are deposited on the PMMA sheet by the discharges; these charges are visualized by means of bipolar electrographic powder. The microdischarges occur at the position where the air gap length is ≈100 μm, and yield striped powder figures. The occurrence can be explained by means of the partial capacitance model and Paschen´s law. At high electrification levels, both microdischarges and major discharges occur; the major discharges appear at longer gaps, since some part of the initial charge cannot be neutralized by the microdischarges. Powder figures of major discharges are tree-like with negative electrification, and round-moon-like with positive electrification. The major discharges with negative electrification are hazardous, since they yield large and clear tracks