solar energy upon the cell and/or by providing cooling. Series strings of cells may be used to build to the 20,000 vdc (or 40,000 vdc), nominal, required for the microwave transmitter. Lower voltage arrays may be required if low orbit operation is required (such as for self-powered transfer) (Fig. 3-5). Fig. 3-5. Silicon Photovoltaic System The solar cells employed in this concept are the "conventional" silicon type, except they are power economics dictates that they be only approximately one-half as thick as are currently used. 3.7 GALLIUM ARSENIDE PHOTOVOLTAIC (CONCEPT 6) A photovoltaic, or solar, cell directly converts solar energy to electric power. Performance may be augmented, within certain limits, by concentrating solar energy upon the cell and/or by providing cooling. Series strings of cells may be used to build to the 20,000 vdc (or 40,000 vdc), nominal, required for the microwave transmitter. Lower voltage arrays may be required if low orbit operation is required (such as for self-powered transfer) (Fig. 3-6). The cells employed in this concept are the gallium aluminum arsenide/gallium arsenide type. This multilayer "heterojunction" cell has the apparent potential for high efficiency at elevated temperatures; it is also more radiation resistant. 3.8 NUCLEAR THERMIONIC (CONCEPT 7) The energy source in this system is nuclear; a molten salt breeder reactor (MSBR) is used. The Fig. 3-6. Gallium Arsenide Photovoltaic System salt mixture contains both fissile fuel, the energy source, and fertile fuel, which breeds to become fuel for subsequent use. The salt mixture is circulated out of the reactor core through a heat exchanger which transfers energy to a sodium loop. The sodium loop is used since there is insufficient salt flow for the diode emitter area. A small secondary salt flow is continuously passed through a fuel process system. This system removes the protactinium and wastes which would "poison" the reactor by excessive neutron capture. The fuel process system introduces fertile fuel and removes bred fuel. The MSBR is an unique breeder concept in that a single liquid fuel mixture contains both fissile and fertile fuels, and that processing of solid fuel elements is not required. The diode collectors are cooled by a liquid metal radiator loop. The low voltage DC output of the collectors is stepped-up and converted to AC by rotary converters/transformers. An AC to DC converter is used to provide the DC necessary to energize the transmitter. The nuclear thermionic system is shown in Figure 3-7. Fig. 3-7. Nuclear Thermionic System
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