Electron Space-Charge-Limited Operation of Cesium Thermionic Converters

Abstract

Recent experimental results from a cesium thermionic converter reported by Houston and Gibbons show that at low emitter temperatures and high cesium pressures the output current approaches an asymptotic value as the emitter is made more and more negative with respect to the collector. With further decrease in voltage the current jumps to much higher values. The assumption was made by Houston and Gibbons that the operation of the converter under these conditions is electron space-charge-limited and that in the high current mode, collision ionization is present. In the present communication a quantitative analysis of this electron space-charge-limited operation is presented. On the basis of this assumption an expression for the limiting value of the output current is obtained. A method is outlined by means of which the difference in emitter and collector work functions can be found from an experimental current-voltage characteristic and conversely such a characteristic can be plotted if sufficient information is given. This method is applied to experimental I–V curves obtained from a parallel plate converter with a tantalum emitter. From these curves the emitter work function and the true saturation current have been estimated. These values of work function are approximately 0.5 v less than extrapolated values obtained from other experiments reported in the literature for cesium on tantalum. With the assumption that ions are introduced in the interelectrode spacing only by surface ionization, the magnitudes of the emitter and collector sheaths are computed at different points of the I–V curve.