Starting Potentials of Positive and Negative Coronas with Coaxial Geometry in Pure N2, Pure O2, and Various Mixtures at Pressures from Atmospheric to 27 mm

Abstract

Consequent to an exhaustive study of coronas with coaxial cylindrical geometry in pure N₂, pure O₂, and mixtures over a considerable pressure range, it has been possible to obtain a consistent set of threshold values as a function of gas type and pressure for the principle current transitions signaling the accepted coronas. Thresholds for the positive wire corona should depend on gas characteristics only, while those of negative wires should reflect the second Townsend coefficients characteristic of the cathode material. Observations indicate that interpretation along this simple line is complicated by two factors. First, some current transitions under different conditions, which were in the past assumed to represent the same corona process, have been found to be different mechanisms. Thus, in pure N₂ positive wire corona is not a gas determined process but a cathode dependent mechanism, while in pure O₂ the corona is a gas dependent process but of the streamer type and not of the burst pulse type. Secondly, all thresholds of the characteristic negative wire corona transitions do not represent a secondary mechanisms characteristic of the cathode surface. These transitions are all preceded by a low order Townsend discharge whose threshold passes unnoticed but is characteristic of the initial cathode state. The commonly noted threshold is determined by a conditioning process which may depend on current density, duration of bombardment, and the composition of the gas. While the data yielded by the antecedent studies permits of interpretation in the present system of gases, this study also indicates that a correct interpretation of relative starting potentials of positive and negative wire coronas requires an extensive investigation of the basic nature of the coronas before it is attempted.