Operating characteristics of two thermionic converters having rhenium-nickel and tungsten-nickel electrodes

Abstract

Two parallel plane thermionic converters were tested. One had a rhenium emitter and a nickel collector surrounded by a nickel guard ring. The other had a tungsten emitter and nickel collector and guard. Both had interelectrode spacings of 0·005 in. The collectors and guard rings could be kept at the same temperature and potential. Both converters were capable of delivering large currents. The WNi converter at T_E = 2235° K developed over 40 W/cm² at 0·6 V) at the electrodes. At each emitter temperature the ReNi converter had maximum output at a lower optimum cesium pressure than that for the WNi converter. At equal emitter temperatures and current densities the ReNi converter had higher output voltages. It is believed this is a result of the lower cesium pressure which enabled the collector work function to be lower. The ReNi converter at 2150°K had a usable output at 15 A/cm² of 16·8 W/cm² and a calculated efficiency of 19·8 per cent. An analysis based on the assumption that at each current density the current is limited by a maximum negative potential in the system, suggests that the variation of output voltage with current is primarily a result of a variation in the maximum potential point. This analysis also suggests that there is a double sheath at the emitter, i.e., negative sheath next to the emitter followed by a positive ion-rich sheath.