inflatable desiging is also optimistic. In U.H. Kurzweg’s (University of Florida) report, (Analysis of 10 Megawatt Space-Based Solar-Pumped Liquid Neodymium Laser System, NASA Report CR 3174, January 1984), the possibility of designing a large-scale, liquid, active, medium laser with direct solar pumping is considered. There are foundation of an output laser power of 10 MW, using a concentrator with a diameter of 434 m, a ring laser cell with a diameter of about 1 m and with a length of 6 m filled by liquid neodymium lasant. Total system mass is estimated as 20 tons. Glass could also be considered in such projects, but it has less thermal conductivity and less thermal strength than crystal. For this reason crystal solid state lasers are preferable as a base for L-SPS. Let us consider only the principal parameters of a laser for L-SPS. At the experimental testing stage of a ground based model of a laser with solar pumping for L-SPS, one could obtain all parameters in detail. The total volume of active medium 1100 dm3 is necessary to convert the energy of the solar radiation collected by the concentrator, which has an area of 0.3 km2. Output laser power is about 32 MW. Taking into consideration that as much as 7-9% efficiency of solid-state lasers with wide-band optical pumping has been achieved, using an electrical power supply (the efficiency is the ratio of laser output power to electrical input power), and that part of the energy is lost as a result of the conversion of electrical energy, then let us assume that it is possible to achieve an efficiency of 8%. The total area of output surface of the laser crystals (and hence the total area of laser beam before entering the optical system) is defined by an acceptable power density for crystal safety. For a continuous-wave regime this value is about 1 kW * cm'2. Then the total diameter of the laser beam is about 2m, whilst the length of the output amplifying stage is about 30 cm. For stationary, ground-based consumers the diameter of the receiving aperture could be used to achieve optimum on-board optics. For example, the lower limit of an on-board laser beam defines the diameter of on-board optics as 2m, and taking into consideration L = 40,000 km and =. 1 micron, one has a receiving diameter of 50 m. Photovoltaic converters have an advantage compared to others: the highest efficiency (as much as 70% is predicted even for wide-band solar radiation, an efficiency of 52% for the conversion of laser energy to electricity has been achieved experimentally, and it is quite probable that this efficiency will be increased in the near future) with suitable maintenance. Cooling problems, caused by a low semiconductor operating temperature, have simple solutions in ground-based power plant. Energy transmission from space to the surface is extremely important and a critical point for all SPS problems. I believe that the present level of science and technology allows us to solve this problem in a refined manner and within the means of mankind. This possibility is concluded in diffraction-limited laser beams, which could be produced by phase conjugation techniques — phase conjugating mirrors (PCM).
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