High efficiency requirements also dictate the band of microwave frequencies that can be considered. The effect of molecular absorption shown in Figure 3 limits frequencies to below 10 GHz to 16 GHz. The upper limit reduces further if brownouts in light rain (5 MM/HR) are to be excluded, and the avoidance of brownouts in heavy rain and severe thunderstorms, for which attenuations are shown in Figure 4, would place an upper limit not far above 3 GHz. The severe rain conditions are experienced even in the desert locations that are prime candidates for the ground receiving antenna location. Having introduced scale and frequency considerations, we proceed to examine the technical and cost aspects of the major systems building blocks, then discuss the transportation and assembly of the orbital elements, present a selection of recommended system parameters based on overall economic and technical considerations, and finally summarize the areas of critical technology requiring priority attention in the future. A 30 year useful life is taken as a design goal for candidate configurations. 2. DC TO RF CONVERSION The study examined two generic types of devices for converting dc power to rf power at microwave frequencies: the amplitron, or crossed field amplifier (CFA), and the klystron, a linear beam device. In current usage the amplitron is characterized by high efficiency and low gain; the klystron is known for moderately high efficiency, high gain and low noise. A cross section view of an amplitron designed for the MPTS application is shown in Figure 5. Special features of the design include open construction for low weight and reliability, and a platinum metal cathode operating on the principle of secondary emission to achieve an essentially infinite cathode life. Tube de voltage input is 20 kV. Samarium Cobalt magnets provide a very low specific weight, and pyrolytic graphite with low density and high emmissivity radiates waste heat to space. The only element with a potential wearout mechanism is the movable magnet shunt designed for regulating the output as input voltage fluctuates. A regulating concept eliminating moving parts by using an impulse magnetic control is proposed as an a1tern a tive approach. Specific weight, specific cost and efficiency trends with frequency as a variable are shown in Figure 6. These favor a selection near 2.45 GHz which is in the center of the industrial microwave band of 2.40 GHz - 2.50 GHz. An output power level selection at 2.45 GHz should be near 5 kW as indicated in the weight and cost trends of Figure 7. The dominance of the thermal radiator in overall weight is indicated in the breakdown of Figure 8 for a 5 kW, 2.45 GHz amplitron. The pyrolytic graphite radiator is sized by an 85% tube efficiency and by the maximum temperature (350°C) allowed for the Samarium Cobalt magnet. Power budget for the MPTS amplitron is given in Figure 9. Improvement to 90% efficiency is believed a reasonable development goal since amplitrons already have reached 85% [Brown, 1974].
RkJQdWJsaXNoZXIy MTU5NjU0Mg==