means that the aggregate beam would have a “footprint” on the ground almost as large as that from a microwave system. Therefore, this system fails to exploit the important potential laser SPS advantage of greatly reduced land requirements for the ground receiving/conversion stations. The overall (sunlight-to-busbar) efficiency using the 9.114 p,m line of l2C18O2 is 10% (based on Ref. 15) for transmission to 0.5 km elevation, with the laser beam at 50° from the zenith. The MSNW flowing-CO system requires an auxiliary power generating unit to drive the compressors that maintain the flow. This unit converts solar energy into power by means of solar panels or by a mirror-solar-cavity/Bray ton-cycle system. Cooling takes place at low temperatures, necessitating a large radiator. Before entering the laser cavity, the lasant is optically pumped as it flows through transparent tubes inside the blackbody. Assuming Beverly's line-selected CO spectrum #1 (best case) (18), the sunlight-to-busbar efficiency is 5% for transmission to 0.5 km elevation, with the laser beam at 50° from the zenith. Of the systems listed in Table 4, the MSNW flowing-CO concept appears, in the author's view, to be the most attractive. As noted in Sec. 3, the BDM system developed by Rather (8) utilizes a filter mirror. Because most of the sunlight incident on the filter mirror simply passes through that mirror, the efficiency for the conversion of the incident sunlight power into laser output power is low, at most 0.5% (8). Assuming an efficiency of 40% for the conversion of laser output radiation into electric power at the busbar (this efficiency is not specified in Rather's paper), the overall incident-sunlight-to-busbar efficiency of the BDM system is estimated to be at most 0.2%. The efficiency for converting the captured sunlight (i.e., sunlight reflected by the mirror into the blackbody cavity) to power at the busbar is at most 8% 8. KEY ISSUES FOR FURTHER RESEARCH Key issues for further laser SPS research are discussed below, with the issue having the highest priority (in the author's view) considered first, etc. Degradation of the Laser Beam by Clouds. Cloud cover not only reduces the beam power reaching the ground. Due to time variations in cloud thickness, it also causes fluctuations in that power. Fortunately, the issue of beam degradation by clouds has already been investigated by Beverly (30), who found that [1] due to clouds, laser beam transmission will not be practical in the U.S. (except in the Southwest) if only a single receiving station is available per laser beam, and [2] if 2 to 3 stations are available per beam and there is switching between the stations, then laser beam transmission can meet or exceed minimum requirements for practicality in all geographic regions of the U.S. Further research on this issue is necessary, and should be directed, according to Beverly (30), to obtaining better estimates of joint power availability for multiple sites. One way to reduce beam degradation by clouds is to use a laser beam which is repetitively pulsed, that is, a beam consisting of a series of brief pulses, the intensity (W/cm2) during each pulse being quite high. [Heretofore, in referring to a “laser beam” we have had in mind a continuous wave (cw) beam. All of the SPS concepts discussed in Sec. 7 utilize cw lasers. Henceforth we use “laser beam” to mean either a cw or a repetitively pulsed beam.] There are two ways to use repetitive pulsing to effect hole boring through clouds. One, suggested by Beverly (30), is to use a pulsed beam superimposed on the main
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