TABLE 1 TRANSMITTING OPTICAL SYSTEM SPECIFICATIONS The thermodynamic cycle with dual heat exchangers affords an improvement in system performance over the cycle with a single heat exchanger, and in the discussions which follow, we assume best-case performance, i.e., = 0.234. Recent advances in non-electron-beam discharge technology in supersonic flows (14) and high efficiency, variable-geometry transsonic diffusers (6) lend credence to the possibility of achieving such a level of systems efficiency. Transmitting Optics From the standpoint of size, optical stability, and diffraction efficiency, the Cassegrain system is the best choice for a large, space based laser transmitter (15). The major disadvantage of the Cassegrain system as a transmitting telescope of high- power laser radiation is that the secondary (smaller) mirror is subject to large incident power densities, which will require some form of active cooling. The sizing of the Cassegrain laser transmitter was performed considering the effects of diameter on diffraction efficiency, beam spread, and power density loading. In the design of a large-aperture space based laser transmitter, the surface reflectivity and incident power density dictate the method of cooling. Because the average incident power density on the primary mirror of the laser-SPS transmitter is -20 W/cm2, active cooling appears necessary to maintain optical figure control. Beryllium or beryllium/copper alloys are specified because of their low density and desirable thermophysical properties (16,17). Although highly reflective at infrared wavelengths, Be or BeCu mirrors are not ideal reflectors of the solar spectrum, thus aggravating the heat loading problem. The heat loading is significantly reduced when the front surface is overcoated with UHV-deposited silver, which has a high reflectance for ultraviolet through infrared wavelengths. The best reflectivity (at 10.6
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