Collector Temperature Optimization Figure 8 represents the collector optimized output current density and output power density characteristics. As indicated in the figures, the emitter temperature was maintained at 1700 K, the cesium reservoir temperature was maintained at 485 K while the collector temperature was varied from 750 to 1020 K. As in the cesium reservoir temperature optimization the current densities in the positive output quadrant increased with increase in the collector temperature to about 900 K and then started dropping for high collector temperatures. The slope of the Boltzmann line however did not indicate any significant change. The variation in current and power densities over the entire collector temperature range was very small compared to the corresponding variation in the cesium reservoir temperature optimization. Significant back emission was observed for collector temperatures above 950 K. This back emission not only causes the difference in work functions between the emitter and collector to change but also reduces conduction of electrons due to the extra intermolecular/interparticle collisions. Thus there is a detrimental effect in increasing the collector temperature indefinitely in that the overall power output comes down significantly. The optimum electrode output current and power density characteristic envelopes with respect to the collector temperature are shown in Fig. 9. Both envelopes represent the best possible output parameters for a given emitter and cesium reservoir temperature. All possible operating points can only lie within the envelope regardless of how high the collector temperature could be increased. The collector optimized power density characteristic was found to peak at about 0.39 V. The variations of short circuit current density and peak power density with collector temperature are shown in Fig. 10. In this case the non-dimensional temperature was the ratio of emitter temperature to collector temperature. The short circuit current density was found to peak at a non-dimensional temperature of 2.356 while the
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