will have to be producible at rates and in volumes consistent with an SPS deployment schedule. To extend the lifetime of the solar cells, annealing methods could be utilized to eliminate or reduce the degrading effects of accumulated radiation exposure. Advanced photovoltaic materials could be used to form thin-film solar cells which would significantly reduce material requirements and the mass of solar cells and permit mass production at rates far greater than could be achieved by producing single-crystal solar cells. Technology Options for Power Transmission to Earth (11,12,13). To transmit the power generated in the SPS to Earth, there are two optional transmitting methods: • A microwave beam, or • A laser beam. Microwave Power Transmission. Free-space transmission of power by microwaves is not a new technology. In recent years, it has advanced rapidly and system efficiencies of 55%, including the interconversion between d.c. power and microwave power at both terminals of the system, are being obtained. The application of new technology is projected to raise this efficiency to almost 70%. The devices which are being considered for converting d.c. voltage to rf power at microwave frequencies in the SPS are crossfield amplifiers (amplitrons) and linear beam devices (klystrons). Microwave solid-state power transistors also are being investigated, as it appears feasible to combine them with solar cells in a sandwich power panel to form a resonant cavity feeding to waveguides. Considerations of mass, cost, and efficiency at specific frequencies have led to the selection of a frequency within the industrial microwave band of 2.40 to 2.50 GHz for the SPS reference system. The transmitting antenna for the SPS reference system is designed as a circular, planar, active, phased array having a diameter of about one kilometer. Space is an ideal medium for the transmission of microwaves; a transmission efficiency of 99.6% is projected after the beam has been launched at the transmitting antenna and before it passes through the upper atmosphere. To achieve the desired high efficiency for the transmission system while minimizing the cost, the geometric relationships between the transmitting and receiving antennas indicate that the transmitting antenna should be about one kilometer in diameter; the receiving antenna should be about ten kilometers in diameter. The power density at the receiving antenna will be maximum at the middle and will decrease with distance from the center of the receiver. The exact size of the receiving antenna will be determined by the radius at which the collection and rectification of the power becomes marginally economical. The transmitting antenna is divided into a large number of subarrays. A closed- loop, retrodirective-array, phase-front control could be used with these subarrays to achieve the high efficiency, pointing accuracy, and safety essential for the microwave beam operation. In the retrodirective-array design, a reference beam is launched from the center of the receiving antenna and is received at a phase comparator at the center of each subarray and also at the reference subarray in the center of the transmitting antenna. The receiving antenna is designed to intercept, collect, and rectify with high efficiency the microwave beam into a d.c. output. The d.c. output can be designed to either interface with high-voltage d.c. transmission networks or be converted into 60-Hz alternating current. The receiving antenna consists of an array of elements
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