Tethers. The use of space tethers as power transmission cables is perhaps the most straightforward mechanism for transmitting central-station power to customer spacecraft. An experimental evaluation of the practicality of designing, manufacturing, and operating tethers in the 50-100 km range is currently under way as a joint effort between the USA and Italy, and is expected to be flight-tested in 1993. Although this mechanism for power transmission has the lowest technical risk, it suffers from three major constraints: (a) limited tether length (due to practical limits on strength/mass ratios), (b) the need for the transmitting and receiving spacecraft to be in similar orbits and not change the distance between them materially, and (c) the requirement that power be transmitted by high-voltage AC to avoid resistance losses over the 50-100 km cable length. The latter constraint is not a serious one, but the first two are. Hence, although the technical issues for tethers are not critical, these two constraints are so restrictive as to make tethers poor candidates for central-station transmission use. Microwaves. Transmission of energy by micro waves, as distinguished from signal transmission, was carried out in the laboratory over 25 years ago, and high-power transmission (30 kWe) over a distance of 1.6 km through the atmosphere was demonstrated in 1975. Considerable research and technology advancement was conducted on long-distance transmission of 12-cm wavelength microwaves (2.45 GHz) at low receiving-antenna power densities (about 250 W/m2) during the solar power satellite concept evaluation studies conducted in the USA a decade ago. In recent years major steps in microwave power transmission technology have been taken as a consequence of military interest, both in the USA and in the USSR, in the prospect of using high-power microwaves (HPM) to disrupt enemy electronic systems and to disorient or even kill military personnel. Although the military weapon applications tend to focus on pulsed power modes rather than the continuous-wave (CW) mode which is best for useful power transmission, much of the technology is applicable. There are five steps involved in power transmission by microwaves: power conditioning of the electricity received from the generator, creation of the microwave beam, transmission, receiving of the beam, and power conditioning to convert the microwave power to the form desired by the customer. Although different customers may require different forms of power (e.g. 120 V DC, 400 Hz AC, etc.), the only element of the transmission system affected is the final stage of power conditioning, which could be designed and built to the central-station operator's specifications either by the central-station operator or by the customer. In either case, however, the cost should probably be borne by the customer. Of the five processes, efficient creation of the microwave beam involves the greatest technical challenge. Seven different mechanisms have been considered in recent years for HPM (mostly military) applications: (a) Magnetron. This device, one design of which was favored by satellite power system proponents in the late 1970s and early 1980s, was invented in 1939, operates in the 0.5-10 GHz range, and has demonstrated power density growth by a factor of 10 every decade since 1940. Peak power (pulsed) was recently reported at 5 GW. Simple magnetrons (not suitable for long-distance power transmission) are manufactured by the millions for use in microwave ovens. (b) Klystrons. These are well-proven oscillator-amplifiers, long used in radar transmitters (since the mid-1950s). Single operational klystrons in current radar systems routinely deliver CW power of 400 kWe at 2.45 GHz (JPL's Goldstone
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