V. CONCLUSIONS A solar-pumped Br2-CO2-He laser should lase provided the radiation is concentrated sufficiently (> 100 times) and the pressure of the gases is low. High pressures cause overheating, which results in the lower laser level being filled. A suitable choice of gas pressures for optimum inverted population would be Br2 = 0.1 Torr, CO2 = 10-3 Torr, He = 10“3 Torr; but for greater efficiency a better combination would be Br2 = 3 Torr; CO2, 10“3 to 10“2 Torr; He, IO 4 to IO'2 Torr (Fig. 4). As a power source, the laser is limited by low efficiency. The efficiency is the product of the fraction of the solar spectrum used, the fraction of radiation absorbed, a kinetic efficiency depending on the branching ratios, and a quantum efficiency. The absorption efficiency can be improved by increasing (xpd). The pressure can be increased to about 3 Torr if the cooling is enhanced by introducing fins into the gas for good heat conduction to the walls. Alternatively, a number of flat “box” lasers could be placed on top of each other to absorb different sections of the spectrum in sequence, with each producing its own output beam. With complete absorption, a 10-kW Br2-CO2-He laser would require a collector approximately 70 x 70 m, and would have a volume of about 0.5 m3; such dimensions would be feasible for space applications. The radiator has to be large to reduce the temperature, (Ac/Ar^> 1). If the temperature is high, lasing still seems possible at low concentrations of He and CO2, but at further reduced efficiency. Other systems using halogen absorbers were also examined: namely, Br2-CO2- He, Br2-H2O-He, Br2-HCN-He, and I2-HF-He. Assuming complete absorption the efficiencies were all below 5 x 10~3. A 10-kW laser with this efficiency would require a collector of several thousand m2 and a radiator of comparable size. Acknowledgments — The authors wish to thank Dr. Frank Hohl, head of the Space Technology Branch, NASA/Langley Research Center for his interest and support. The work was partially funded by NASA grant NSG 1568. The Editor wishes to thank W. H. Christiansen and John Rather for assistance in reviewing the paper. REFERENCES 1. B.F. Gordiets, L.I. Gudzenko, and V. Ya Pachenko, Pis' ma Zh Eksp. Teor. Fiz. 26, 163, 1977. 2. A.A. Passchier, J.D. Christian, and N.W. Gregory, J. Phys. Chern. 71, 937, 1967. 3. G. Herzberg, Molecular Spectra and Molecular Structure. 1: Spectra of Diatomic Molecules, p. 456, Van Nostrand, New York, 1950. 4. A.B. Peterson and C. Wittig, Appl. Phys. Lett. 27, 305, 1975. 5. R.J. Donovan and D. Husain, Chern. Rev. 70, 489, 1970. 6. H. Hofmann and S.R. Leone, Chern. Phys. Lett. 54, 314, 1978. 7. B.F. Gordiets, A.I. Osipov, E.V. Stupochenko, and L.A. Shelepin, Usp. Fiz. Nank 108, 655, 1972. 8. A. Hariri and C. Wittig, J. Chern. Phys. 67, 4454, 1977. 9. Handbook of Chemistry and Physics, 49th ed., p. E2, The Chemical Rubber Co., 1968. 10. J.R. Serry and D. Britton, J. Phys. Chern. 68, 2263, 1964. 11. J.R. Carter and H.Y. Tada, Solar Radiance Handbook. Jet Propulsion Laboratory Report 21945- 6001-RV-00, 1973. 12. J.W. Wilson and J.H. Lee, Virginia J. Sci. 31, 34, 1980.
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