Fig. 6. Solar Power Satellite reference system highly detailed analyses, and the efficiency of this link is now expected to exceed 60%. The transmitter employs high-power klystron tubes. About 100,000 such tubes are required for a transmitting antenna of a 5000-MW satellite. As in all elements of the Solar Power Satellite concept, a variety of technical approaches is available. A system employing klystron dc-to-RF converters has been described. Other converters such as amplitrons and magnetrons should continue to receive attention. Solid-state converters are also receiving serious consideration. They would offer the great advantage of very low failure rates and thereby eliminate a major maintenance consideration. In addition, it appears that a solid-state system would “optimize” at a lower power level, on the order of 2500 MW, with an attendant reduction in Earth rectenna size. It may be that such a power level and size would offer more flexibility in integration with commercial utility grids. The parameters of the microwave beam were established by: Thermal limitations of the 22-kW/m2 output of the transmitter array Peak power in the ionosphere of 23 mW/cm2 to preclude nonlinear heating of the ionosphere RF power levels incident on the rectenna which are sufficient for efficient reception Power levels at the edge of the rectenna not to exceed 1 mW/cm2 The characteristics of the microwave beam are illustrated in Fig. 7. The 1-mW/cm2 level at the edge of the rectenna is an order of magnitude below the U.S. continuous-exposure standard of 10 mW/cm2. For comparison, microwave oven door seals are permitted external leakage as great as 5 mW/cm2. If a total failure occurs within the Solar Power Satellite phase control system, the total beam will be defocused and the power level reduced to 0.003 mW/cm2. The
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