strong function of aperture (D) and a weaker function of output power. It is seen that a 100 MW laser with a 30 m primary will have a mass of about 500 T, one percent of the power satellite mass of 5 x 10^4 T. The volume of the system is primarily due to the laser device, primary mirror, and the heat rejection equipment to cool the recirculating laser gas. Higher gas temperatures will result in more efficient heat radiation systems and therefore smaller size. This rejection system would be located on the shaded side of the solar array and may not add any overall SPS volume. The total volume of the laser system is plotted in Figure C.3-3. C.3.2 Performance Assessment Based on standard laser performance equations, estimates were made for laser system capabilities for the year 2000, including both SPS-based lasers and ground- based lasers. Two SPS-based laser projections were made, for a "nominal” and an "optimistic" capability. The path length for all of these calculations was the distance from geosynchronous orbit to the Earth. Table C.3-1 contains "nominal" calculations for a SPS-based laser focused at low Earth orbit (LEO) altitude for the year 2000. Considerable extension of the current state-of-the-art is necessary to achieve some of these values. The values also represent irradiance at the Earth's surface, except at wavelengths at which the atmosphere absorbs strongly (2.7 pm and 0.2 pm). Table C.3-2 is also for the year 2000, but is called "optimistic," since it assumes a greater advance of the state-of-the-art in laser power, mirror diameter, and jitter accuracy. It might also be regarded as a nominal projection for the year 2020. Irradiances are ten times greater than the nominal performance, except for the 0.2 pm wavelength. The ground-based laser is considered as a possible threat to the SPS system. Projected performance of such a laser for the year 2000 is displayed in Table C.3-3. Since the path is through the Earth’s atmosphere, certain changes have been made to the SPS nominal performance. First, the atmospheric absorption by water vapor has caused a shift in the chemical laser from HF at 2.7 pm to DF at 3.8 pm. Also, the transmittance at 10.6 pm and 5pm is assumed to be 0.5. The UV laser at 0.2 pm is assumed to be entirely attenuated by ozone absorption and scattering. Residual atmospheric turbulence which is uncorrected by adaptive
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