characterized by a profuse number of absorption lines which are highly pressure broadened in the lower troposphere. Windows which were at least 1.0 cm-1 wide with edges at least 1.0 cm 1 from a major absorption line were selected for detailed calculations, hitran spectral plots (53) were particularly useful in this search, although their transmission efficiencies cannot be used in the present study because they are for 10-km horizontal paths. If a known (high-power) laser line exists within a window, this wavelength was used in the LASER-code calculation; for those windows for which no laser line could be identified, the central wavelength was used. The transmission efficiencies for all windows identified in this manner are given in Table 2. For comparison, calculations are also shown for the “standard” 10.6-/zm CO2 laser line, which is totally unsuitable for space-to-Earth power beaming. Most of the absorption occurs in the lower troposphere and seasonal variations in the transmission efficiency are again pronounced. The highest annual transmission efficiency to typical receptor sites is 82.3% for the 10.916-/zm line. High-elevation operation (A = 3.0 km) increases this value to 96.3%. Indeed, power transmission in the 9 and 11 gm regions is probably limited to high-elevation sites. The examination of the 10 to 12 spectral region was exhaustive and we believe that no high-transparency window was overlooked. Therefore, laser operation at any other wavelength in this region or pressure detuning of “standard” high-power laser lines will not result in transmission efficiencies greater than those given here. HOLE BORING For a laser beam to penetrate an aerosol layer of thickness Szc moving with a lateral wind velocity v, Harney (54) showed that the aerosol vaporization time zr must satisfy the following inequality as a minimum requirement: where ZE is the time required for the droplet to shrink to some arbitrary fraction of its
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