Paraxial corona discharge. I. Review, instrumentation and experiments

This paper is the first part of a two-part inaugural study into the characteristics of DC corona discharges between circular cylindrical electrodes with parallel axes (a paraxial system). This geometry is intermediate between those of wire plane and coaxial cylindrical systems, which are two limiting cases of practical importance. A review of the literature on these limiting cases is followed by a description of recently developed instrumentation for field and current measurements on the passive outer electrode. Experimental results are reported for positive discharges in ambient air with various overvoltages for fixed diameter electrodes (100 mm and 0.56 mm) and various offsets of the inner electrode. The measurements confirm a quadratic law dependence of total current on voltage which is typical of the limiting cases, and an empirical expression has been developed relating this law to the paraxial offset. The current density at each point on the outer cylinder also shows this quadratic dependence on voltage at all positions and for all offsets. Similarly, the electric field distribution is shown to behave approximately linearly with voltage for all offsets. However, when normalised using their maximum values, both the current and field distributions show a remarkable independence of the overall voltage. These normalised distributions have been shown to be related by a simple power law where the power is independent of the corona wire diameter. The interrelated data of current density and electric field will be used for numerical simulation of corona spatial characteristics in the second part of the study