cw beam. The pulses shatter the cloud droplets into much smaller droplets. The smaller droplets cause negligible scattering of the beam and the shattering process requires less energy than complete evaporation. This approach needs to be more thoroughly evaluated. The second approach is to use only a repetitively pulsed beam, one which takes advantage of the fact that such a beam will suffer less degradation than a cw beam of the same average power when the interval between pulses is large relative to the beam diameter divided by wind speed. In this limit, the initial part of a sufficiently powerful pulse “punches” a hole through the cloud, the remaining part of the pulse then suffers little attenuation in passing through. With hole punching, a much smaller volume of condensed water needs to be vaporized than is the case with a cw beam, which vaporizes a continuous channel through the cloud. This is why the pulsed beam suffers less degradation. As an example, we compare the degradation of the Lockheed beam (a cw beam with 845 MW of power, a diameter of 42 m, and a wavelength of 10.6 ^m) with a repetitively pulsed beam having the same diameter and wavelength and an average power of 845 MW. Using Sutton's solution (31), we find that the cw beam cannot bore through the thickest possible cloud cover [one with 3 gm/cm2 of liquid water in a vertical path (32,33)], but the pulsed beam can. Indeed the pulsed beam will get 89% of its power through the 3 gm/cm2 cloud cover. This assumes a maximally energetic pulse. For a pulsed beam having the Lockheed parameters, such a pulse will last 1.22 x IO 2 sec (a longer pulse will cause too much thermal blooming, specifically r3 blooming, see Ref. 34) and will have an intensity, during the pulse, of 107 W/cm2 (a higher intensity may cause aerosol breakdown). This pulse duration and intensity result in a maximally energetic pulse having a fluence of Since, by the Sutton theory, only 13,800 J/cm2 is needed to bore through a 3 g/cm2 cloud cover, 122,000 — 13,800 = 108,200 J/cm2 (89% of 122,000 J/cm2), would be transmitted through the cloud. Note that 122,000 J/cm2 is an extremely high fluence. It would raise the temperature of a person standing in the beam by several hundred °F, i.e., it is a very lethal beam. Further, a single pulse would vaporize a surface layer of an airplane's aluminum skin. It is precisely this high fluence/pulse which makes this repetitive beam so effective in boring through clouds. Clearly, pulsing the beam can do much to eliminate the problem of beam degradation by clouds. However, the use of a pulsed beam introduces new problems. With both approaches discussed above, a more massive and complex laser system is needed on the power satellite. With the second approach a more complex system is needed on the ground to store energy between pulses (the interpulse interval is 33 min for the pulsed version of the Lockheed beam discussed above), and the high fluence/pulse will significantly exacerbate safety problems. The question of a cw versus a pulsed laser SPS needs to be further addressed. Induced Clouding Above a Receiving Station would be due to updrafts resulting from heat rejected by the receiving station and heat from beam absorption in the lower troposphere. To what extent will such clouding occur and how deleterious will it be? Perception of a Laser SPS as a Weapon. A laser SPS might be perceived as a weapon or as readily convertible to a weapon. How likely is this perception and what are its implications?
RkJQdWJsaXNoZXIy MTU5NjU0Mg==