Direct Solar Pumped Lasers. Candidate lasants need to be evaluated in detail. Conflicting weight estimates need to be resolved. Indirect Solar Pumped Lasers. Satisfactory optical materials need to be identified. A high-power (2= 10 MW) system should be designed and evaluated. Free Electron Lasers. A stable, efficient electron beam energy recovery system needs to be developed. An FEL SPS should be designed and evaluated, particularly with respect to total mass. Closed-Cycle Chemical Lasers. A closed-cycle chemical laser must include a subsystem for reconstituting the spent fuel. Since such a laser does not now exist, one should be designed and evaluated. Whether the reconstitution could be accomplished by direct sunlight should be determined. Magnetic Drag. Magnetic drag forces and their consequences need to be evaluated as a function of orbital altitude and satellite size. Laser Windows. Laser windows must be able to withstand high radiation fluxes. The practical limits to the fluxes that can be handled by large-area windows must be determined, for these limits will influence subsequent laser design. Maintenance of Shape of Sunlight-Collecting Mirror. If the sunlight does not focus on the solar cavity, all the sophisticated lasers and associated equipment will be of no use. Number of Independent Power Satellites. What are the upper limits for the numbers of satellites in GEO and LEO orbits? An upper limit to the number of power satellites implies a minimum output power per satellite. Biota Impacts. Will the beam affect ducks (or other flying organisms) or will they sense the heat and turn away? How many birds per year, for example, can be expected to fly into a 30-m diameter beam? Beam Scattering. The laser beams will be scattered by clouds and dust. Once the magnitude of this effect is determined, buffer zone requirements will need to be reassessed. Exposure standards will be crucial to these studies. Van Allen Belt Radiation. What are the implications of this radiation for personnel and materials? Will this radiation restrict the power satellites to certain orbits? Thermal Blooming is the distortion of the laser beam caused by spatial variations in the refractive index of the air in the beam path, these variations being due to heating of the air by molecular and aerosol absorption. For a cw beam, it is unlikely that thermal blooming will be severe (18); however, its precise extent needs to be better determined. Blooming may be particularly severe when the beam is passing through a cloud. This author's calculations, based on Ref. 34, show that a repetitively pulsed beam having a very high intensity (107 W/cm2) during each pulse and carrying the same average power as the Lockheed beam (845 MW) will not bloom. To whatever extent cw blooming occurs, it can very likely be eliminated by using a repetitively
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