Pulse Height and Temporal Distribution at Dielectric Surfaces under Corona

Discharges at a dielectric surface are obtained using a metal sphere separated by a distance of 0.002 inch from a 0.0005 inch thick dielectric. The magnitude and temporal distribution of the discharges is monitored by photomultiplier detection and magnetic recording. A change in the distribution of the pulse agni magnitudes is observed as the time of voltage applications is increased. There is a decrease in the number of low and high level discharges and an increase in those of intermediate magnitude. The effect is associated with the deterioration of the dielectric. This change in the magnitude distribution is more rapid as the applied electric field is increased. Also, when the metal sphere is positive there are fewer pulses and the magnitude of the pulses is less than when the sphere is negative. The fact that an equal amount of charge is not involved on alternate half cycles is surprising. It is not due to faulty instrumentation. The difference in pulse magnitude due to polarity is associated with electron trapping and/or retention of positive charge by the dielectric.