An experimental study of the development of gaseous ionization at ultra high frequencies

Abstract

The work is concerned with the study of ionization and related processes occurring during the transition to breakdown at ultra high frequencies. Electrons are injected into a parallel plate gap through holes in one electrode and drift towards the other under a small unidirectional field. A stronger ultra high frequency field is superimposed upon this drift field to give collision ionization in the gap. Measurements of the number of electrons crossing the gap show that as the high frequency field increases the gap current decreases initially and then rises as breakdown is approached. A theory is presented to explain the initial drop quantitatively in terms of a back diffusion current to the emitting electrode. An extention of this theory to the subsequent rise show that the electron density initially increases exponentially across the gap but as breakdown is approached this changes to a sinusoidal form. A quantitative treatment of the first case is presented. Provision is made to modulate the emitted electron stream with the object of measuring transit times. Preliminary measurements have demonstrated the feasibility of the method. It is shown that, as a consequence of diffusion, the time measured in such experiments is that for the propagation of the pulse of electrons rather than the time it would take individual electrons to drift across the gap. Instabilities and drift of gap current have been traced to the presence of partially insulating films on the electrodes. It is shown experimentally that in combined drift and ultra high frequency fields, the drift field may be progressively reduced by the charging of these films. Methods for measuring film constants and calculating the resultant drift fields are given. Firm evidence is presented that such films are built up by discharges from impurities in the gap.