Using Eqs. 21 and 22, the minimum intensities to penetrate atmospheric areosols were calculated for representative cloud and fog parameters, a beam diameter of 100 m and wind velocities typical for the altitudes at which the various formations occur. The minimum intensity will only barely penetrate the aerosol layer and the hole will be in constant danger of closure due to convective effects. To maintain a properly cleared channel characterized by a high transmission efficiency, roughly 3 to 5 times as much intensity will be required. From an environmental and safety standpoint, the maximum cw laser intensity is probably limited to 100-200 W/cm2. For 11-gcm operation, therefore, all fogs, cirriform clouds with gi s 0.005 g/cm2, middle clouds with gi s 0.02 g/cm2, and stratiform clouds with gi s 0.03 g/cm2 can be bored at these intensities. All cumuliform cloud types and nimbostratus clouds are impenetrable except with weapon-quality (/ > 1 kW/cm2) beams. Although the environmental consequences of laser- power transmission at these intensities are probably negligible, the transmission air-zone associated with each receptor must be restricted to all aircraft due to potential ocular hazards posed by the randomly-pointing and highly-reflective aluminum aircraft skins (1). For 2-gim operation, however, substantially higher cw intensities are necessary to affect hole boring because the aerosol absorption coefficient (/3„) is much smaller at 2 gm than at 11 gm. A potentially viable and attractive solution is to combine cw laser-power transmission with pulsed laser hole boring. At I ~ 10*—105 W/cm2, the internal heat generation is so inhomogeneous and so rapid that local temperture maxima occur which satisfy explosive water boiling conditions (57-60). For an initial droplet radius ~ 10 gtm, shock boiling of the liquid in the internal regions results in fragmentation of the condensate and production of a small quantity of vapor. [At higher intensities (/a 107 W/cm2), the explosion is of a gas dynamic nature in which the flow has a clearly defined separation boundary and during which an intense
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