schemes having the same output power. With respect to (b). Rather (8) points out that the large radiator can be replaced by a much smaller one if the solar concentrator is a selective reflector. Such a reflector would reflect only those frequencies in the pumping band, letting the remaining frequencies pass through without being absorbed. Rather (8) estimates the efficiency (laser beam power out divided by solar power incident on the concentrator) of a CF3I SPL as no more than 0.54%. While he appreciates the smallness of this number, Rather suggests that a more important criterion is the ratio of the mass of the power satellite to the power of the output beam. For his CF3I SPL, Rather estimates this ratio to be 1800 kg/MW (8). However, for this same SPL, Taussig et al. (9) estimate 60,000 kg/MW. This is a fairly large discrepancy and illustrates that direct SPL technology is in its infancy. This author believes the Rather ratio is too low. For the silicon solar cell version of the SPS Reference Concept, this ratio is 7600 kg/MW. In a CF3I SPL, optically pumping the CF3I molecules causes them to break up. Therefore, a subsystem is needed to resynthesize the CF3I, and this adds a minor element of a complexity to an otherwise simple system. Rather's work (8) was published in 1978. In June 1980, Beverly (10) reported his theoretical investigation of 3 types of directly pumped lasers, viz., those employing [1] electronic-vibrational energy transfer to triatomic molecules, [2] atomic transitions in alkali metals, and [3] atomic transitions in vapor-complex rare-earth- lanthanide ions. He carried out detailed calculations only for type 1, finding heat generation to be a major problem for that type. A Rb type 2 laser and various type 3 lasers were identified as being possibilities for SPS application. In December 1980, NASA announced that two of its researchers, W. Weaver and J. Lee, had successfully operated a gas laser pumped by artificial sunlight (11) (see also Ref. 12). The lasant in the Weaver-Lee SPS is perfluoropropyliodine (C3F7I). Ultraviolet radiation in the 0.250 and 0.290 /an band causes the C3F7I to dissociate into I in an excited atomic state and C3F7. The excited iodine then lases at 1.3152 gm. Efficiency is 0.1%. The simplicity of the direct SPL is its greatest virtue. This simplicity derives from using sunlight to pump the lasant directly rather than first converting the sunlight to electricity. However, the direct SPL may be too heavy to be cost effective. 3.3 Indirect Solar Pumped Laser In this type of laser (Fig. 6), sunlight is reflected by a concentrating mirror into a blackbody cavity (9). The sunlight is absorbed by the cavity walls, which attain a temperature in the 1500-3000 K range. The lasant is optically pumped by blackbody radiation emitted by the cavity walls. Figure 7 explains how an indirect solar pumped laser works. The blackbody cavity is assumed to have a temperature of 2000 K. Radiation emitted from the cavity walls has the blackbody spectral distribution shown in the emittance vs wavelength (A) curve on the upper right. Much of the radiation at the pumping wavelength (X = 4.256 pm, if CO2 is the lasant) is absorbed by the lasant, as the radiation passes through the laser tube. Therefore, the radiation emerging from the tube is depleted at the pumping wavelength, i.e., there is a “hole” at that wavelength in the middle spectral distribution curve. The depleted radiation then propagates from the tube to the cavity wall and is absorbed by that wall. That same wall then emits new radiation having the blackbody distribution shown in the third curve. In that curve, the hole has been filled in with energy from wavelengths above and below the hole wavelength. The resultant distribution curve is identical to the original curve, because the
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