Measurements on the growth of gaseous ionization at ultra high frequencies

Abstract

An apparatus has been built to study ionization by collision in a gas under ultra high frequency fields and especially the conditions covering the transition to the conducting state. The u.h.f. field is developed between two Rogowski profiled electrodes incorporated into a tuned quarter wave transmission line which is loosely coupled to the main oscillator. The u.h.f. field has a frequency of 105 Mc/sec. and its magnitude is measured by its effect on the frequency of oscillation of a small metal disc suspended in the gap. This instrument is used as a step-over so that a rectifying circuit loosely coupled to the transmission line can be calibrated. The disc can be removed from the gap during the experiments. Electrons are emitted from a thermionic cathode situated inside one of the electrodes and diffuse out into the gap through small holes in the electrode shell where they are moved across the gap by a small unidirectional field. The large u.h.f. field is superimposed on this field so that the electrons acquire enough energy to ionize the gas by collision; the resulting electron current to the far electrode is measured by a galvanometer. With this arrangement the amplification resulting from collision ionization can "be measured and its dependence on the lifetime of the electron in the gap can be studied. The measurements can. be carried right up to breakdown conditions, and the results can be expressed in terms of the number of ionizing collisions per second in the life of one electron, a coefficient closely related to Tosnsend's a. It is shown that the lifetime of the electrons not only depends on the drift under the direct field but also on the process of diffusion out of the gap and two methods of evaluating it are given. Although only preliminary results have been obtained these are sufficient to indicate that a useful contribution to ionization studies can be made by investigations with this apparatus.