Onset corona fields and electrical breakdown criteria

Abstract

Predictions are made of the electric field necessary for the onset of corona in air for spheres and cylinders from the general breakdown criterion, ; α′ is the net ionization coefficient, r is the radius and Q is a constant. It is found that Q = 104 gives a fair fit to experimental results for both wires and points, and for positive and negative corona, for radii varying from 0.01 to 20 cm. By assuming a linear variation of α′ with the difference of the electric field from the critical field where ionization equals attachment, an analytic formula can be derived for the breakdown field for the onset of corona as a function of the radius of the sphere, which is similar to, but different from, Peek's formula for the onset of corona for cylinders or wires. The formulae also give the breakdown field as a function of temperature and pressure. For Townsend and Streamer breakdown mechanisms, values of Q are typically 3 × 104 and 108, respectively. Thus the value of Q derived from experimental values of the onset field for corona for both wires and points is significantly lower than would be expected from either the Townsend breakdown criterion or the Streamer breakdown criterion. It is suggested that the most likely collision process to explain the observed low value of Q is indirect ionization, for example by collisions between excited molecules.