respect to the cesium reservoir temperature. These are called optimum characteristics because, for given fixed parameters and electrode output voltage, the characteristics yield the maximum output current density or power density that can be achieved by varying the cesium reservoir temperature. The cesium pressure at each point of the optimized characteristic equals that of the envelope at the point in question. For relatively low voltages, the points of tangency are on the ignited mode branches of the output current density characteristics. This implies that at these voltages, the performance achieved by the unignited mode is better than that of the ignited mode. The cesium optimized power density characteristic was found to peak at 0.395 V. The significance of this peak is that from a design point of view, if this diminiode were to operate at an emitter temperature of 1700 K and a collector temperature of 900 K, the best output can only be obtained by operating the diminiode at the optimum cesium reservoir temperature and an output voltage of 0.395 V. In order to narrow down the range in which the optimum cesium reservoir temperature occurs, the short circuit current density and the peak power density for each cesium reservoir temperature were plotted as a function of a non-dimensional temperature. These plots are given in Fig. 7 where the non-dimensional temperature was chosen as the ratio of emitter temperature to the cesium reservoir temperature. As seen in the figure the short circuit current density plot peaks at a non-dimensional temperature of 6.8 whereas the peak power density plot peaks at 7.2. Thus, there is a small bandwidth in the cesium reservoir temperature variation that translates to an optimum cesium reservoir temperature of 485 ±10 K. If the criterion for maximum performance is peak power density, then the diminiode should be operated with the cesium reservoir temperature at the higher level. Empirical Relationship Between Power Density and Output Voltage for Cesium Reservoir Temperature Optimization The peak power densities for the plot in Fig. 7 were obtained by simulating curve fits for each one of the power density characteristics in Fig. 5 over a voltage range of 0.00
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