with a minimum in the aerosol absorption coefficient of water based droplets. Zenith Angle. The space-to-Earth transmission efficiency for all linear attenuation mechanisms and a propagation zenith angle 8 scales as exp(-secd). If molecular absorption is strong, for example, operation at a propagation zenith angle of 0° rather than 50° results in a significant improvement in the transmission efficiency. If the laser wavelength is properly optimized, however, vertical propagation does not afford a significant improvement in the power availability (all meteorological conditions considered) and cannot be justified in terms of the increased cost and complexity of the required space hardware. Hole Boring. Using hole-boring models applicable to the range of laser-beam parameters of interest for power transmission, we have estimated the power densities necessary to affect aerosol clearing under various meteorological conditions. Laser hole boring through certain types of hazes, fogs, and clouds may be possible consistent with safety and environmental concerns. In particular, all but the thickest cirriform clouds and all stratiform clouds with the exception of nimbostratus can be penetrated without the need for weapon-quality beams. For lasers operating in the ll-/zm window, cw power densities of 100-200 W/cm2 are required. Because of the small aerosol absorption coefficient in the 2-ju.m window, hole boring at these wavelengths using a cw beam alone will be ineffective. A train of short-duration pulses superimposed on the “main” cw beam will, however, affect penetration under these circumstances. Several other physical mechanisms must be considered in conjunction with hole boring. Intensity fluctuations can be induced by temperature and water-vapor gradients within the beam path (5,6) and gross refractive bending of the beam can be enhanced under certain conditions by droplet vaporization (7-12). For laser power transmission, intensity fluctuations which do not result in significant beam spreading should be of no concern. We have examined the regime in which refractive bending occurs and found that severe distortion should be negligible for the power densities and beam diameters under consideration here. Another phenomena which has been recently considered is droplet recondensation in a laser-vaporized path. Overheating of the particles produces local supersaturation, resulting in the production of a large number of fine particles which may attenuate the beam and limit its penetration (13-15). This effect is pronounced at higher radiation intensities, for larger particles, and at lower temperatures. Again, for conditions anticipated here, this effect should not occur. Propagation Calculations—Aerosols Models. Aerosol scattering, absorption, and extinction coefficients and differential scattering cross sections were calculated for haze, advection and radiation fogs, various types of clouds, and rain and snow distributions at various precipitation rates. These calculations require detailed properties of the various aerosols, such as composition, size distribution, particle concentration, and complex index of refraction as a function of wavelength.Literature sources for distribution and concentration data are listed as results are presented. Index of refraction data are given in Ref. (16). For the calculations involving haze, absorption and scattering coefficients for the various aerosol models were taken directly from the work of Shettle and Fenn
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