Cell Types and Operating Wavelength Silicon cells showed better response than GaAs cells to the induction format pulses, and it was suggested that, in the near term, the pulse format problem should probably be solved by simply going to silicon cells. The feeling was, it works and it's available. Silicon was also desirable since it is already flying, although it was noted that cells now flying are relatively old designs which have been further radiation damaged, and thus will not have significant response to 1.06 micron radiation. A wavelength of 1.06 micron has been suggested 1111 as being considerably better than the 840 nm under consideration from the point of view of laser technology and atmospheric transparency and compensation. The atmosphere is known to be extremely transparent at some specific wavelengths near 1.06 microns. It is important to start testing cells at this wavelength. Cell possibilities include both silicon and InGaAs cells. It is noteworthy that, while silicon has low optical absorption at 1.06 microns at room temperature, the absorption constant rapidly increases at elevated temperatures. For Si cells at 1.06 microns, it may be desirable to operate at high temperature. It was agreed that it would be possible to operate photovoltaic cells that respond in the "eye-safe" wavelength range of 1.5 to 1.7 microns, but that this will result in a very large loss in performance due to the lower efficiency, and that these cells would not be able to operate at high laser intensities due to the adverse temperature coefficient. It is important to know just how- advantageous operation at this wavelength is. Operation in the eye-safe wavelength range may be required if a relay mirror is used, since an error in the mirror pointing would direct the beam back toward the ground On "exotic" cell types, production will be a big problem. For anything except silicon or conventional GaAs cells, the capability for production of large (square meters) arrays is nonexistent. In many cases technologies such as cell to cell interconnections have not been addressed. Cells that have only been produced on a laboratory scale will take considerable time and effort to bring to production readiness and spacc-qualify. Conclusions The possibilities for laser power beaming engendered a lively discussion, and it was agreed that there were likely to be many applications that have not yet been thought of. The idea of an early technology demonstration to stimulate interest in the technology, was particularly well received. It was cautioned that, despite the cutting- edge nature of the technology, mundane solar array considerations such as space qualification and manufacturing readiness cannot be ignored. References [1] Conway, E. J., (1986) "Solar Pumped Laser Technology Options for Space Power Transmission," paper 869423, Proc. 21st Intersociety Energy Conversion Engineering Conference, Vol. 3, 1862-1868, American Chemical Society [2] Walker, G.H., Williams, M.D., Schuster, G.L., and Iles, P.A., (1991) "Potential Converter for Laser-Power Beaming Using Diode Lasers," Space Photovoltaic Research and Technology 1991, NASA Conference Publication 3121, 25-1 to 25-5
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