Distribution of electric fields in a rod to plane gap at the onset of negative corona

The distribution of electric fields in close proximity to a rod cathode at the onset of a negative corona are calculated using the charge-simulation technique. In this method, the highly stressed cathode is modelled by a number of line charges and point charges. As the potentials due to these simulated charges satisfy Laplace´s or Poisson´s equation inside the space under consideration, the solution is unique. The results are applied to calculate the onset field strength and its distribution along the axis of symmetry. The onset of the corona is defined as the transition from a nonselfsustained discharge to a selfsustained one, and it takes place when the cathode field strength reaches a critical value. The theoretical study iscarried one step further by calculating the corona-current waveshape produced predominantly by electrons. A comparison is also made between calculated and measured values near fields and current waveforms.