Investigations of the performance of grooved electrodes thermionic converters at collector temperatures up to 1023 K

The performances of grooved emitter (GE), grooved collector (GC) and grooved electrodes (GEL) thermionic converters are investigated, and the results are compared with those of an identical converter with smooth electrodes (SEL), which is tested at the same conditions. These converters, with planar polycrystalline molybdenum electrodes and a 0.5 mm inter-electrode gap, are tested at emitter temperatures, TE=1473-1673 K, collector temperatures, TC=773-1023 K and Cs pressures, PCs=10-500 Pa. The grooved electrodes have concentric macro-grooves 0.5 mm wide, 0.5 mm deep and 1.0 mm apart. Measured and calculated performance parameters include ignition voltage, VIG, barrier index, VB, electric power density, PD, and conversion efficiency, η, as functions of the cesium pressure and electrodes temperatures. VIG of the GC converter is smaller than that of the SEL converter by up to ∼0.9 V for PCs=20-100 Pa. VB for the SEL converter is always lowest, except at TE=1673 K and TC=1023 K, indicating that grooved electrode(s) are effective in reducing VB only at high electrodes temperatures. The GE converter has the lowest PD and η, followed by the GEL converter, GC converter and, finally, the SEL converter, except for TE=1473 K and TC=773 K and for TE=1673 K and TC=1023 K. PD and η for the SEL converter at the optimum TC (873 K) are 3.74 We/cm2 and 17.0%, respectively, and they decrease to 1.61 We/cm2 and 10.4%, respectively, as TC increases by 150 to 1023 K. The corresponding PD and η for the GC converter decrease from 2.56 We/cm2 and 14.7% at TC=873 K to 2.38 We/cm2 and 14.3%, respectively, at TC=1023 K. Those for the GE converter decrease from 2.18 We/cm2 and 12.9% at TC=873 K to 1.56 We/cm2 and 11.0%, respectively, at TC=1023 K. For the GEL converter, PD and η decrease from 1.86 We/cm2 and 11.2% at TC=873 K to 1.44 We/cm2 and 9.7%, respectively, at TC=1023 K. At TE=1673 K and TC=1023 K, the converter with a GC has the lowest VB and operates at the highest PD (2.38 We/cm2) and η (14.3%)