which absorb and rectify the incident microwave beam. Each element consists of a half-wave dipole, an integral low-pass filter, a diode rectifier and a bypass capacitor. The dipoles are d.c.-insulated from the ground plane and appear as rf absorbers to the incoming microwaves. The collection efficiency of the array is insensitive to substantial changes in the direction of the incoming beam. Furthermore, the efficiency is independent of potentially substantial spatial variations in phase and power density of the incoming beam that could be caused by nonuniform atmospheric conditions. The amount of microwave power received in local regions of the receiving antenna can be matched to the power-handling capability of the microwave rectifiers. The rectifiers, which are Schottky barrier diodes made from gallium arsenide material, have a power-handling capability several times that required in the SPS application. Any heat resulting from inefficient rectification in the diode and its circuit can be convected by the receiving antenna to ambient air, producing atmospheric heating which will be only twice that which emanates from a typical suburban area, because only 15% of the incoming microwave radiation would be lost as waste heat. The low thermal pollution entailed in this process of rectifying incoming microwave power cannot be equaled by any known thermodynamic conversion process. The receiving antenna can be designed to be partially transparent so the area underneath it can be put to other uses. Receiving antennas can be located on land or offshore. A typical receiving antenna will be about 10 x 12 km. At least 100 potential land sites in the United States can be considered. Offshore receiving sites also are of interest because of limited land availability near major urban centers, and the proximity of these centers to coastal locations. Laser Power Transmission. Microwave power transmission is the present choice based on considerations of technical feasibility, fail-safe design and low-flux levels, but laser power transmission is an interesting alternative because of considerable advances in laser technology, and the possibility of economically delivering power in amounts as low as 100 MW to individual receiving sites on Earth. The use of high- power lasers for laser propulsion and for power transmission between satellites is being investigated. These investigations include the environmental impacts of laser power transmission from Space. Concentrated and dispersed beams generated by continuous-wave electric discharge lasers where the gas is recirculated are being considered. Gas recirculation permits removal of waste heat and minimizes consumption, thus allowing extended operation. Candidate lasers are carbon dioxide, carbon monoxide, mercury chloride, and mercury bromide to which are added atmospheric gases as diluents. Although carbon dioxide and carbon monoxide electric-discharge lasers have reached an advanced state of development, other laser concepts, including pumped free-electron lasers, are being considered. Power may be supplied to the lasers by photovoltaic conversion systems, or through their direct excitation by solar radiation. Solar optical excitation has been developed for laser communication between satellites and could be applied to advanced laser concepts for laser power transmission from the SPS to Earth. Solar energy conversion compatible with laser power transmission may be carried out in GEO or in lower orbits, particularly in a Sun-synchronous orbit. If the latter orbit is to be used, a mirror will be required in GEO to reflect the laser power to a desired receiving site on Earth. Photovoltaic cells could be used to convert laser beam radiation into power. If
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