Corona Inception and Electrical Breakdown in a Coaxial Cylindrical Geometry

The corona inception and the sparking potentials of dry air are measured in the pressure range 25×10-3-760 torr using two separate coaxial electrode systems. The similarity theorem for the sparking potential is obeyed when the central electrode is made to have a positive-voltage polarity over the range 10-3 < pR1 < 0.3 torr m. For the negative polarity of the inner electrode, the similarity theorem is found not to hold. The measured coronainception potential is compared with the values obtained from the theories of Werner and Townsend. It is found that the Townsend theory for calculating the corona-inception potential gives better agreement with the experimental results, as it makes no restrictive assumptions concerning the discharge parameters. At a fixed gas pressure, the sparking potential for the positive wire is higher than that for the negative polarity of the same electrode in the range 100-375 torr. As the pressure is increased further, the positive breakdown potential decreases below the corresponding value for the negative breakdown. This phenomenon is shown to be due to the role of the negative-ion space charge which is present when the central electrode is made negative.