shock wave is formed in the surrounding air (61). ] Even at the threshold intensity for this phenomenon, the explosion energy is approximately 75%-80% of the total vaporization energy of the droplets. As the intensity increases, the energy required for explosion decreases. Theoretical calculations have been performed which include the effects of an increase in the imaginary part of the refractive index due to soluble impurities in actual water droplets (59). The energy required to explode such a “contaminated” droplet with radius 10 /rm is reduced further by about 40% for / = IO5 W/cm2 and X = 10.6 gm laser light. Furthermore, a train of pulses will cause repeated fragmentation with the droplets cascading into smaller and smaller radii (60). This change in the particle size distribution function also reduces the energy and time requirements for further explosive fragmentation. Thus, a train of intense, short-duration pulses can be superimposed on the main cw beam allowing a reduction in the total average power density while still achieving the conditions for boring. A repetitively pulsed laser producing ~ 100 pulses/sec with a pulse width ~ 1 /zsec and an energy density per pulse ~ 0.1 J/cm2 gives an average power density ~ 10 W/cm2. Now if the cw power-transmission component is reduced to / ~ 1-10 W/cm2, the ocular hazards from quasispecular reflection are greatly reduced and the transmission air zone would no longer be restricted to aircraft. Note that the pulse train can be turned off for clear periods and that the relative power densities of the beam components can be adjusted according to prevailing meteorological conditions to maintain a constant total average power density at the receptor. More theoretical and experimental research is needed to demonstrate the feasibility of this technique since the laser parameters suggested here are only rough estimates. CONCLUSIONS AND RECOMMENDATIONS At high elevations, atmospheric transmission windows in the wavelength region around 11 /rm provide the best combined propagation efficiency considering both molecular absorption and aerosol extinction. At low elevations, laser operation at a wavelength near 2.25 gem is perferable. If the laser wavelength is properly optimized, operation at a propagation zenith angle of 0° instead of 50° does not afford a significant improvement in the power availability and cannot be justified in terms of the increased cost and complexity of the required space hardware. Furthermore, we conclude that high-elevation receptor sites are desirable although not essential to the laser-SPS concept because of the reduction in attenuation due to haze and molecular absorption. Laser hole boring at X — 11 gm through certain types of haze, fogs, and clouds may be possible,consistent with safety and environmental concerns and without the need for weapon-quality laser beams. In particular, all but the thickest cirriform and middle clouds and all stratiform clouds with the exception of nimbostratus can be penetrated with power densities of 100-200 W/cm2. All other cloud types will require substantially higher power densities for penetration, which is unacceptable given the present safety margins. At X - 2 gm, hole boring is only feasible using combined repetitively-pulsed/cw operation. This mode of operation may be preferable, however, since the average power density can be reduced to allow unrestricted transmission air-zone access. Further research is needed to examine potential short-wavelength transmission windows for aerosols identified in this study. In addition, theoretical and experimental research on combined repetitively-pulsed/cw laser
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