requirement Tx < 100 °K. In this thermodynamic cycle, Tm = 360 °K and = 3.00. The calculated radiator area (1.58 km2) is consistent with the space allowed within the rotating yoke assembly on the current Rockwell microwave-based SPS design (4). ANCILLARY ISSUES Laser Beam Spreading In the sections which follow, beam spreading and required receptor size are calculated for two analytically tractable laser beam intensity distributions which bound the range of expected profiles. These intensity distributions are the uniform or constant-intensity profile and the Gaussian profile. It is assumed that the receptor axis coincides with the laser beam axis such that the minimum focal spot size is intercepted. Hence, the receptor views the laser source at a zenith angle (0) of 50°. Diffraction effects are considered initially, and the effects of pointing inaccuracies and turbulence are then calculated. Although the angular divergence attributed to turbulence is much larger than the divergence due to diffraction and pointing inaccuracy, turbulence induced spreading only occurs during the final 30 km of beam path and can be neglected for spot diameters > 1 m. If laser line selection is employed, then molecular and aerosol absorption is weak and thermal blooming is not a problem. Note that for earth-to-space laser power transmission, however, small beam perturbations attributable to turbulence and nonlinear effects near the transmitter produce significant beam wandering at the target because of the long optical “lever arm” involved. Due to the proximity of these effects to the receptor, beam spreading is much less severe for space-to-earth propagation. Uniformly Illuminated Transimitter Aperture. If the primary mirror of the Cassegrain optical transmitter, an annular aperture, is uniformly illuminated by the laser, then the normalized intensity at the receptor due to diffraction only is (45) The fractional power intercepted within a radius r„ is given by
RkJQdWJsaXNoZXIy MTU5NjU0Mg==