sions of 10 km by 13 km (1) and an area of 102 km2] and a 0.7-km-wide buffer zone (20) surrounding the rectenna (having an area of 27 km2). Similarly, each 0.6-km2 laser site includes the receiving system (a large mirror, an optical train, and an absorbing cavity) plus a buffer zone. The laser system land requirement of 6 km2 per 5000 MW(e) can be reduced somewhat if the ten sites are located together, by overlapping the buffer zones. The minimal land requirement for a laser SPS derives from the small amount of beam diffraction (spreading outside the geometrical path) that occurs as the beam propagates to Earth from geosynchronous equatorial orbit (GEO). This small amount of diffraction is, in turn, due to the smallness of the ratio of the wavelength of the laser radiation to the diameter of the beam transmitting aperture in GEO. Because laser beam receiving sites require relatively little area, they can be located close to and provide heat for existing steam power plants. Second, radiation levels outside the laser SPS buffer zone are anticipated to be virtually negligible (18,21). This assumes, of course, that the laser beam remains closely locked on the target receiving system, a situation that should be achievable with high dependability (18). With microwave transmission there is a possibility that low levels of microwave radiation outside the buffer zone will have an adverse impact on people and other biota. Third, laser beam sidelobes are not expected to interfere with communication systems or with noncommunication electromagnetic systems (22). This is in contrast to the microwave system where such interference is anticipated unless mitigating strategies are implemented. Fourth, the laser system is amenable to demonstration on a small-scale, because the short (infrared) wavelength of the laser radiation permits both a small-diameter transmitting aperture (—30 m) for the laser beam and a small-diameter receiving aperture (—30 m). In contrast, the much longer wavelength used in the microwave system requires that both the transmitting and receiving antennas be of very large diameter, even for transmission at low power levels in a demonstration program. While the laser SPS has the above-noted advantages over the microwave alternative, the laser system also has certain disadvantages. Microwaves can readily penetrate clouds and weather, microwave technology is further developed, and the conversion of microwaves to electricity is more efficient than the conversion of laser radiation to electricity. 5. ALTERNATIVE SUBSYSTEMS FOR A LASER SPS Any laser SPS will have the three components shown in Fig. 13: [1] a power satellite to convert input solar flux into one or more output laser beams, [2] the laser beam itself (one or more per power satellite) plus, if necessary, a relay satellite or satellites to reflect the beam(s), and [3] ground conversion stations to convert the beam power into electrical power (each station would receive one or more laser beams). The power satellite would convert solar radiation into laser input power and convert laser input power into one or more output laser beams. Alternative subsystems for performing each of these functions are shown in Fig. 13, along with alternative subsystems for converting, on the ground, laser beam power into electrical power. The laser alternatives have been discussed in Sec. 3, and the energy exchanger
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