Prebreakdown conduction in vacuum gaps under 50 Hz excitation

Prebreakdown currents have been measured under 50 Hz a.c. excitation in a vaccum environment close to 10⁻⁶Torr in a system using liquid nitrogen trap to eliminate contamination. The currents were at least two orders higher than under comparable d.c. voltages. The current peak occurred later than the voltage peak and the current waveform was asymmetrical about its own peak. It is attempted to explain these features on the basis of the I-F emission proposed by Porotnikov and Rodnevich, the ions being generated in the anode vapours produced by electron beam impact. The I-F emission model has been modified to take into account the 50 Hz time-varying voltage. The agreement between the model and experimental results appear to be satisfactory. Certain experimental results appear to favour ‘non-metallic’ type of emission processes as proposed by Latham et al or Hurley rather than a pure metallic type of field emission proposed by Fowler and Nordheim.