is possible near the start of a pumping pulse, but continuous lasing may not occur. The reason is that early in the pulse the Br created from Br* by quenching and stimulated emission is sufficiently removed by exchange reactions to maintain inversion. Eventually, however, the IBr decreases and the Br density increases to cut off the lasing. The dominating mechanisms are the source terms, quenching by IBr and I, two body exchange reactions and stimulated emission. With 200 /zs pumping pulses the behavior of the light output can be represented using only these reactions. However, if long pulses are to be simulated, then all the reactions must be included. Our computer results showed that for 3 torr of IBr, steady depletion occurred and the pulse lasted for about 350 with C = 5000, L =1 m. At 13 torr the pulses were many tens of ms for the same conditions. The exchange reactions can overcome the effect of high quenching cross sections, and high quenching need not necessarily indicate that a material is unsuitable for solar pumped lasing. Heating will reduce quenching. It will also cause dissociation and depletion of IBr with the formation of I, I2, Br and Br2 all of which quench Br*. The exchange reaction rates will be reduced causing the population inversion to disappear, thus killing the laser. If the path length L is small, requiring the solar concentration C to be high for threshold to be reached, then the pressure has to be kept low to prevent heating (<50 torr). The overall efficiency r/ is then 10~4/? (torr) or 0.1% at 10 torr. Such a 10 kW laser would require a collector of 85 x 85 m2. If L is large, so that overheating does not occur, and p is high enough to absorb all the photons, then tq could reach 1.2% — a 10 kW laser collector would be 30 x 30 m2. The above efficiencies can be compared to the calculated efficiencies of a Br2-CO2-He laser and a C3F7I laser, each of which was around 0.1%. The effective length L can be increased by multiple passes in a flat “box type” laser. IfL = 10 m, then lasing should be possible withC = 300. The temperature rise would then be 700 °K even if p = 50 torr, with a depth of 0.5 cm, Tw = 300 and no admixture of a cooling gas. The receiving area of the laser would be 400/300 = 1.4 m2 and an effective length L of 10 m could be achieved in about 8 passes. The possibility of steady-state working seems feasible if a continuous flow system is used. Acknowledgements — The authors acknowledge useful discussions with Drs. J.W. Wilson, S. Raju, and L. Zapata. The work was partially supported by grant NSG 1568 from the National Aeronautics and Space Administration/Langley Research Center, Hampton, Virginia. REFERENCES 1. J.F. Coneybear, The Use of Lasers for the Transmission of Power. Radiation Energy Conversion in Space, K.H. Billman, ed., Program Astronaut. Aeronaut. 61, 279, 1979. 2. B.F. Gyordiets, L.I. Gudzenko, and V. Ya Pachenko, Pis’ ma Zh Eksp Tear. Fiz. 26, 163, 1977. 3. W.L. Harries and J.W. Wilson, Space Solar Power Rev. 2, 367, 1981. 4. L. Zapata, Private communication. 5. J.H. Lee and W.R. Weaver, Appl. Phys. Lett. 39, 137, 1981. 6. W.R. Weaver and J.H. Lee, Proceedings of the 16th Intersociety Energy Conversion Engineering Conference, Atlanta, Georgia, p. 84, 1981. 7. J.W. Wilson and J.H. Lee, Proc. Virginia Acad. Sci. 31, 34, 1980. 8. J.R. Carter and H.Y. Tada, Solar Cell Radiation Handbook. Jet Propulsion Laboratory, California Institute of Technology, Report No. 21945-6001, RV OU 1973, Table 2.1, p. 2-2.
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