Interpretation of Experimental Characteristics of Cesium Thermionic Converters

Abstract

Experimental V–I curves have been obtained from a 150-w cesium thermionic converter. Two different kinds of V–I curves can be clearly distinguished. One, corresponding to a collision-free type operation, is obtained when the cesium mean free path is of the order of three or more times the interelectrode spacing. The other, corresponding to a sheath-type operation, is obtained when the spacing is of the order of 30 cesium mean free paths. In the first case the log I vs V plot has the familiar shape observed by Hernqvist et al. [K. G. Hernqvist, M. Kaneisky, and F. H. Norman, RCA Rev. 19, 244 (1958)]; it consists of two straight lines meeting at the breakoff point. From such an experimental curve the difference in emitter and collector work functions and the emitter temperature can be readily obtained. In the second case the V–I curve has the standard shape of a sheath curve which is smooth without breakoff point and which shows saturation in the current for both the high positive and high negative values of the voltage. In the present work a method is outlined by means of which the difference in emitter and collector work functions, as well as the emitter temperature, can be determined from an experimental V–I curve of the second kind. With this method experimental data from the above-mentioned converter in the sheath-type operation has been successfully analyzed, and good agreement has been obtained between experimental results and theoretical predictions.