cavity absorber. Waste heat is rejected through a gas-to-liquid heat exchanger to a liquid metal cooling loop. The ac output is then distributed to the microwave generator for transmission. (c) Thermionic/Brayton Cycle Conversion In this "cascaded" system the waste heat from the thermionic diodes is used as input to the Brayton cycle turbomachine. The diodes are cooled by the helium flow and the Brayton loop is cooled by a liquid metal radiator. The output voltages from the two systems are reconciled and combined for distribution to the microwave subsystem. (3) The Nuclear Concept This concept, rather than using solar radiation as its primary source, uses an orbiting molten salt breeder reactor (MSBR). Sixteen 1 GW reactors will be combined in geosynchronous orbit to provide the necessary power. The heat produced by the reactors will be converted to electricity by two methods: (a) Thermionic Conversion In the thermionic system each generating module is located in the center of a 2300 ft (700 m) square radiating panel. Eight panels are joined linearly and connected to the 0.6 mi. (1 km) transmitting antenna between the fourth and fifth modules. The other eight panels are similarly assembled and oriented 90° from the first set to minimize the mutual view factor. Distributing the panels in this fashion increases the distance over which electrical distribution occurs, but the resultant mass penalty is less than the radiator manifold penalty which would occur if the module were clustered together. The most recent Boeing studies (Ref. A17) indicate that nuclear thermionic conversion is not feasible with 1985 technology and the MSBR, since the temperature differential which can be obtained across the diodes is not great enough for efficient operation. (b) Closed Brayton Cycle Conversion In the closed Brayton cycle system, the salt mixture is circulated out of the reactor core through a heat exchanger which
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