For conversion of direct current power to microwave radiation, klystron tubes have been included in the SPS reference system. Alternatives to this type of tube include another tube-type converter, the magnetron, or solid-state microwave amplifiers. Magnetrons may have advantages in cost and industrial experience, while solid-state microwave amplifiers are attractive because of their inherent reliability. As shown in Fig. 7, solid-state devices were studied in two configurations. In one concept, the reference system klystrons were replaced with solid-state devices. In the other, solar reflectors were used to concentrate sunlight on a solar-cell array, which is mounted back-to-back with the solid-state devices. Because solid-state devices require lower operating temperatures, system design considerations limit the power output range over which these systems can be optimized. Laser power transmission was briefly evaluated. With a laser, energy can be transmitted to comparatively small Earth receivers (tens of meters in diameter). This factor could reduce the capital costs for an SPS ground receiving system. Smaller blocks of power may be economical with a laser SPS. In addition to reducing the land requirement, laser power transmission offers several other advantages: [1] the radiation levels outside the receiving sites probably would be negligible, [2] there is no interference with conventional electromagnetic communications or other electromagnetic systems, and |3J smaller-size satellite units make small-scale demonstrations feasible. Two significant disadvantages of lasers are [1] high-efficiency laser technology is relatively undeveloped at this time and 12] the laser beam is attenuated
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