Methods to produce such lower plasma losses are being developed currently and include the potential use of new electrode materials; e.g., hexaborides and other semiconductors, which require lower interelectrode cesium densities, pulsed triodes using rare gases for ion production and close-spaced diodes with reduced space charge effects. 4.6.3 Radiator Design of a passive radiation cooling system for space thermionic energy converters is one of the most important tasks of this study. For electric power production, P, with a solar heat conversion efficiency, 77. the heat flux rejected, qo, is given by electric output of 4 GW, a radiator temperature 90 percent that of the collector, and a radiator emissivity of 0.9. A 24 percent electrically efficient converter with a collector at 1000 K (1340°F) requires 3.8 x 10$ m2 (4.09 x 106 ft2) of surface area for radiation cooling. According to our previous calculations, the anticipated power density at this efficiency is 12 W/cm2 (77.4 W/in2). For the geometrical tolerances required in a converter, the practical size for the electrodes is expected to be 100 cm2 (15.5 inch2) per converter with an output, therefore, of 1200W. Such dimensions will require approximately 3.4 million converters to generate 4 GW of electrical power. Consequently, the required radiating area per converter is 0.1 m2 (1.076 ft2). Since the collector material is made of nickel, it would be advantageous to have an integral radiator assembly made of similar material. Although the emissivity of nickel is low, that of oxidized nickel at 900 K (1160°F) is an attractive 0.9. Consequently, the radiating nickel surface would be oxidized to enhance emissivity. The simplest method for radiator design would be to project solid cooling fins, in the form of a cup, away from the circular edge of the collector out into space. These cups would be hexagonal so that radiators could be fitted into a "honeycomb" arrangement. The equivalent emissivity, eeq, for heat loss, qo, from a cylindrical cavity is given by (Ref. 3). Fig. 4-19. Heat Rejection as a Function of Converter Efficiency (Modified From Ref. 8) Table 4-2. Equivalent Emissivity
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