direct solar energy. Nuclear-pumped lasers have been demonstrated, but are extremely inefficient and are in the very early stages of research. For high efficiency, gallium arsenide (GaAs) diode-pumped solid-state lasers are the best choice. Use of a number of high-power (e.g., 1 kW) slab YAG:Nd lasers phased to produce a multi-kilowatt or megawatt-level beam is certainly feasible; whether or not it is commercially viable has yet to be determined. Cooling is a problem, as with the electric power conversion subsystem; roughly half the electric power generated, even with the most efficient lasers, must be radiated to space. A real benefit in using solid-state lasers, however, is their very short wavelength. Some of the YAG:Nd laser employing frequency doubling or tripling operate very close to the ultraviolet, thereby reducing transmitter and receiver diameters to the order of a meter or two for distances up to 40 000 km. A possible new development, the solar-pumped laser, has been conceived but not yet tested. The only lasant considered to date is trifluoromethyl iodide (CF3I), which could theoretically operate at a peak efficiency of only 2.5%. Indirect solar-pumped lasers, which use solar energy to heat a blackbody cavity whose radiated heat creates a population inversion in the lasant, have been conceived theoretically but have not yet operated experimentally, even in the laboratory. Laser beam transmission, pointing, stability, and control are state-of-the-art, as demonstrated by SDI ground-based experiments. Space qualification of these processes will soon be established by the US STARLAB and Zenith Star flight demonstration/ validation programs. Technologies used in these tests include both segmented and deformable mirror phasing controlled by narrow field-of-view wavefront sensors, acquisition and tracking using low-energy lasers, cooled mirrors, heat-resistant mirror coatings, and others. The main unresolved technical issue in laser power transmission, other than the low conversion efficiency of electric to laser power in the laser source, is the reconversion of laser energy into electricity at the customer's spacecraft. The two most practical methods for doing so appear to be GaAs concentrator photocells, which could be as efficient as 45% at the short YAG:Nd wavelengths and operate at intensities up to and perhaps beyond 500 suns (about 700 kW/m2), and wave energy exchangers used with Brayton-cycle gas-turbine power conversion systems. The wave energy exchanger is a laboratory device which uses nonsteady gasdynamic compression and expansion to transfer energy from a hot gas (heated by the incident laser in a cavity absorber) to a cooler gas (the Brayton-cycle working fluid). Although both these systems could theoretically offer overall reconversion efficiencies in the 40-50% range, they must both be demonstrated and tested for lifetime, reliability, and performance in orbit. System Aspects The central-station power system must be able to supply customers in various orbits. There are four primary Earth-orbit regions which are likely to contain most of the potential customers. Lunar and planetary power requirements are not likely to be subject to power delivery from Earth orbiting power stations, but, should development there proceed rapidly, might be amenable to supply from lunar or Martian orbiting substations. The four Earth orbit classes of greatest interest are low-inclination, low-altitude orbits (space station, microgravity research laboratories, space factories, many science payloads), geostationary equatorial orbits (communications and remote-sensing plat-
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