Torr. The optical gain at 0.5 Torr is greater than that at 2 Torr. This is probably because the optical mean-free-path at 0.5 Torr is four times greater than at 2 Torr, and suggests that at these pressures, the effect of photon mean-free-path on gain dominates. These experimental results reveal that, by optimizing the system operating conditions, the maximum overall system efficiency (i.e. laser power out/solar power in) may be on the order of 12%. 4. Conclusions and Recommendations The preliminary work on the solar-electric thermophotovoltaic (TPV) concept indicates that a total cell conversion efficiency of 50% or more is possible by using concentrated sunlight and a double-layer solar cell constructed of gallium arsenide (GaAs) and silicon (Si). Concentration of sunlight and use of the TPV system allows increased efficiency and reduced photovoltaic cell area requirements for a given power level. There are also opportunities for weight and cost reduction in solar power satellite (SPS) systems. There are, however, very significant and substantial technical developments still required to make the system economical. A space-based solar-electric TPV system deserves special attention as a viable alternative energy source to meet projected energy demands in the next century. If the TPV concept, utilizing the advantages of the space vacuum, concentrated solar radiation and efficient solar cells, can be demonstrated successfully on a small-scale experimental deployment, further development of a large-scale solar-electric TPV may be encouraged. The feasibility of blackbody optical pumping at 4.3 /zm in a CO2-He gas mixture has been experimentally demonstrated, proving the concept of solar-powered gas lasers. Optical gain with a maximum gain coefficient of 2.8 X10-3 cm-1 has been measured. Measurements of gain have been found to be sensitive to the power density of the 10.6 /zm probing beam. At pressures in the order of 1 Torr, power densities less than about 1.8 W/cm2 must be used to avoid saturation effects. Gain reduction in the Doppler-broadened region at higher pressure is attributed to a reduction in the mean- free-path of the pumping photons. Improved gain can be achieved by lowering the temperature of the gas mixture as the gain coefficient is proportional to T~5/2. This can be done by using a pre-cooled flowing gas mixture along with additional cooling of the sapphire tube, or use of two concentric sapphire tubes with a vacuum in between. ACKNOWLEDGMENTS The author wishes to express his great appreciation to Dr W. H. Christiansen for the many valuable discussions with him about solar-powered gas lasers. This portion of the work was carried out at the Aerospace and Energetics Research Laboratory of the University of Washington, Seattle, and was supported by NASA Grant NGL-48-002- 004. REFERENCES [1] Backus, C.E. (1976) Photovoltaic converters, 76 IEEE Region Six Conference, pp. 34-39. [2] Glaser, P.E. (1968) Power from the sun: its future, Science, 162, pp. 857-861. [3] WOLFE, M. (1960) Limitations and possibilities for improvement of photovoltaic solar energy converters, Part I, Consideration of Earth's Surface Operation, Proceedings of IRE, 48, pp. 1246-1263.
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