Introduction. Muiti-megawatt lasers appear to be technologically feasible for space power transmission in the 1990s time frame. Solar driven lasers based on conventional gas dynamic and electric discharge laser concepts (GDLs and EDLs) have been investigated to determine the feasibility of using existing laser technology for this application. With conventional solar photocells as the power source the GDL and EDL lasers do not appear to be as efficient as microwaves in transmitting power back to earth. However, for relatively new and untested laser concepts such as the solar optically pumped laser (OPL) and free electron laser (FEL) much higher laser efficiencies may be achievable, leading to a laser SPS system competitive with the microwave SPS. Because of their compact transmitters and receivers, lasers may have an advantage in better economies of scale for smaller SPS sizes than the microwave SPS. Further, laster light wavelengths longer than 2.5 microns may be considerably safer and have less impact on the earth's environment (e.g., communications) than microwaves. For these reasons one should consider the laser as a serious power transmission option. Results from a brief survey of solar powered, space-based lasers are given below to gain some perspective on the types of lasers reasonable for power transmission. A preliminary selection of candidate lasers for SPS application was made on the basis of scalability to high powers (1 MW and greater), relative weights, efficiency (better than 1%), and safety (wavelengths greater than 2.5y). The preliminary list includes CO and C02 EDLs, direct optically pumped lasers (e.g., CF3I, etc.), indirect optically pumped lasers (e.g., C0/C02 mixing laser), and free electron lasers (FELs). Electric Discharge Lasers. Several previous studies have focused on solar powered, closed cycle EDLs for power transmission. The EDL requires electric power both to drive the high voltage discharge which pumps the laser medium to an excited state before it lases, and to circulate the lasant through a cooling loop which extracts the waste heat and returns it to its original state. Either photocells or perhaps a more efficient solar thermal power cycle can be used to produce the electricity. Table 1 summarizes the characteristics of the EDL and other 1 MW cw solar powered lasers, where electricity in each case is assumed to be produced by a 25% efficient solar thermal Brayton cycle power system. Monson has shown how to optimize the closed cycle flow conditions for minimum flow loop compressor power per unit laser power output. His results are employed to keep the total laser system weight small. Direct Optically Pumped Lasers. A preliminary consideration of the direct optically pumped lasers in a previous study suggested that the only class of direct optically pumped lasers with reasonable efficiency capable of high power operation are those utilizing CF3I, C3F7I, C^Fgl, and (CF3)2AsI. These molecules photodisscoiate in the near UV of sunlight. With proper filtering, only the solar wavelengths appropriate for pumping the molecules need to be focused on the lasant. Concentration ratios on the order of 100 appear adequate to pump the heavier versions of these iodine molecules leaving an excited I* which lases at 1.315p. Unfortunately, this wavelength is smaller than the safety limit of 2.5y for retinal damage. The dissociated fragments of these molecules also polymerize so that in a closed cycle lasant for space, a substantial amount of SOLAR DRIVEN LASERS FOR POWER SATELLITE APPLICATIONS R. Taussig, P. Cassady and E. Klosterman MSNW, Inc., 2755 Northrup Way, Bellevue, Washington 98004
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