A research diode with polycrystalline molybdenum electrodes is described. Voltage-current characteristics are presented as a function of the cesium reservoir temperature. A power density of 4 W/cm2 is obtained at an emitter temperature of 1400¿°C. The influence of the temperatures of the emitter, collector, and cesium reservoir and of the interelectrode distance is experimentally investigated. Physical explanations for the various effects are given. The work function of the polycrystalline molybdenum emitter in a cesium atmosphere is evaluated as 2.5–2.7 eV depending on the reduced emitter temperature (i.e., the emitter temperature divided by the cesium reservoir temperature). A barrier index Vb =2.0 eV and a cesium plasma drop Vd =0.4 eV are found.