e. Solar Cell Radiation Degradation There is general agreement that array degradation from radiation damage, particularly ultraviolet but also particulate and micrometeorite, over the 30 year SPS life poses problems. Ref. A5 points out the potential for such damage in the transit from LEO through the Van Allen belts to GEO. It also states that the exposure in GEO alone would result in a power degradation of 20-25% for present solar cells protected by 150 micron glass cover slides. For the SPS, Ref. A5 states that the cost and mass penalties to provide adequate shielding would be unacceptable. Ref. A4 reports that multi-junction, edge-illuminated silicon solar cell concepts have been pursued to some degree by the Air Force, LeRC, and the USSR. The close spacing of the junctions in this approach improves the cell radiation tolerance. Ref. All states that, with regard to radiation degradation, the objective must be 6 percent over 5 years. To achieve this, it states that improvements in materials, radiation spectral tailoring, high voltage plasma protection, meteorite hardening, and annealing techniques are required. It also points out that improvement in radiation damage resistance is key to the solar photovoltaic SPS feasibility. f. Solar-Thermal, Reflectors and Cavity/Absorbers The overall energy conversion process in a solar-thermal SPS would consist of mirrors/reflecting surfaces which focus into cavity absorbers. Thermionic diodes or Brayton cycle turbo/generators would, in turn, utilize this heat inpuc for electrical power generation (see Section III) . The mirror concentrators must achieve efficiencies on the order of 74-80%, and the absorbers about 89% (Ref. A9). Concentration ratios of one thousand or more are required (Ref. A12). Ref. A4 states that rough order of magnitude efficiencies of 88% for the concentrator and 55% for the cavity absorber approximately represent today's capabilities. The mirror concentrator would consist of individually steerable, active-controlled metallized, plastic film mirrors. This reflecting
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