powerplant is likely to prove both technically and economically feasible by the mid-to- late 1990s for relatively short ranges (e.g. several hundred km). Over larger distances the need for large antennas, both transmitting and receiving, may prove to be limiting for beam wavelengths in the 10-cm (3 GHz) range due to physical diffraction-limited constraints on beam transmission efficiency. Submillimeter Waves. The most direct means of countering antenna size constraints on space-based central-power systems for use at moderate distances (e.g. several hundred to several thousand km) is to increase the frequency of transmission, as noted above. This involves virtually no difficulty in any of the five areas of microwave power transfer systems identified earlier other than microwave beam generation. However, modern developments aimed primarily at military uses of HPM are likely to evolve into practical hardware that is admirably suited to millimeter-wave power transmision using either gyratrons or FELs (see above) in the 100-300 GHz range. This power transmission regime, therefore, is likely to turn out to be the most practical for centralstation power distribution in space, barring the discovery and development of an effective mechanism for converting laser energy to electricity (see below). Lasers. The benefits of short wavelengths in reducing power beam diffraction, as noted above for submillimeter waves versus conventional microwaves, suggests the use of lasers, whose wavelengths are two to three orders of magnitude shorter than even submillimeter waves. Hence laser beam dispersion in space over practical distances for central-station distribution is virtually negligible, allowing the use of much smaller transmitter and receiver antennas than microwave or submillimeter wave systems. Laser power transmission, however, faces major losses as a result of the inefficiency of both converting primary power into laser power and reconversion of the laser power back into useful electricity. Jones et al. [7] point out that even with highly optimistic estimates of laser-to-thermal-power conversion of 98% and thermal-to electricity conversion of 50%, the overall system efficiency (useful power per unit of input solar energy) is only 2.6%, compared with typical microwave/millimeter-wave demonstrated efficiencies (with 20%-efficient gallium arsenide photovoltaic cells) of nearly 13%. Despite this serious drawback, however, lasers offer one important additional benefit besides their small transmission and receiving antennas: the ability to reflect the laser beam from relay mirror satellites without suffering prohibitive beam dispersion or power loss. This relieves orbital constraints considerably (see below), and allows the use of fewer costly power satellites and/or more convenient orbits. Nevertheless, early studies (e.g. Jones et al., 1978) concluded that central station power systems employing laser transmission were not cost-competitive with onboard power supplies, even for mission applications beyond 2000. There has been significant advancement in high-power laser transmission technology during the past decade, however, mainly as a result of strategic defense beamweapon research. Such research has focused primarily on two classes of laser: chemical (hydrogen fluoride) lasers at relatively long wavelength (2.8 /zm) and ground-based free-electron lasers at shorter (1 /zm) wavelengths. Although neither of these would be suitable for the long-term continuous-duty cycle of central-station powerplants (the chemical laser requires expendable chemical propellants and the free-electron laser gigawatt-level pulses of high-voltage electricity), much of the beam-control, pointing, sensing, and mirror technology developed for SDI is directly applicable to the centralstation power system option employing laser power transmission. The only practical laser sources for central-station use are those which employ either relatively low levels of electric power (e.g. up to at most a few megawatts) or
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