An Ionization Process in a Low-Energy Cesium Plasma

Abstract

The mechanisms of ion formation in cesium plasmas at relatively low electron temperatures (3500°K) and pressures of a few Torr are investigated. It is concluded that most of the ions are molecular and that they are formed by collisions between cesium atoms in the first excited state. The excited atoms are predominantly produced by inelastic electron collisions. The excitation cross section is computed by means of the impact parameter method. It has a maximum value of 100×10−16 cm2 at 7 eV. The de-excitation cross section is computed and found to be approximately constant at 50×10−16 cm2. Radiation trapping and diffusion of excited atoms are also studied. The ionization cross section is calculated by means of statistical thermodynamics from the measured recombination coefficient for the inverse of the ionizing process. It is found to be 1450×10−16 cm2. The plasma is uniform throughout its volume except in the immediate vicinity of the bounding electrodes. The proposed mechanism results in performance characteristics for cesium thermionic converters, operating in the ``ignited mode,'' which are in qualitative and quantitative agreement with experimental data.