The spatial growth of ionization currents in nitrogen at voltages up to 500 kV

Abstract

Accurate measurements of the spatial growth of pre-breakdown ionization currents in nitrogen under uniform field conditions were made for values of E/N in the range from 8.48*10-16 to 15.2*10-16 v cm2 and 0.825*1019<or=N<or=8.25*1019 cm-3 (E is the electric field and N the gas number density). Each set of measured currents at a constant value of E/N and N was found to satisfy the generalized Townsend equation, indicating that the spatial growth of current resulted from the action of primary and secondary ionization processes. Analysis of the measurements gave values of the primary ionization coefficient alpha and the generalized secondary ionization coefficient omega / alpha . It was found that alpha /N and omega / alpha are dependent on E/N but not on N. A quantitative analysis of the observed increase of omega / alpha with E/N led to the conclusion that the predominant secondary ionization process was that of photoelectric emission from the cathode. That cathode processes are significant was confirmed by the fact that a spark between clean electrodes led to a considerable reduction in the value of omega / alpha .