The critical propagation distance increases with the square root of wind velocity; the convective dissipation of heated air zones within the beam path is promoted by high wind velocities. To insure the absence of thermal blooming, we require zc > h at all points along the beam path. The largest absorption (without cloud cover) occurs for the Midlatitude Summer model under hazy conditions and a low surface wind velocity. Velocity-distribution data, taken from Ref. (8), correspond to the curve specified by ^max = 33 m/sec at h = 12 km. Figure 8 thus shows the dependence of the critical propagation distance on altitude for the CO-laser spectrum under worst-case meteorological conditions. For a receptor located at an elevation of 0.5 km, thermal blooming begins about 1 km above the receptor, but little additional beam spreading occurs due to the proximity of the absorbing medium to the receptor. Under more typical meteorological conditions or for receptor sites at higher elevations, thermal blooming can be completely ignored. As a corollary to this conclusion, it is evident that line selection at the laser transmitter becomes less critical for high-elevation receptor sites since most of the molecular and aerosol absorption occurs at altitudes less than 3 km. Optical phase corrections at the Cassegrain laser transmitter will only be able to correct for gross beam wander due to steady-state thermal lensing. Microscopic beam fluctuations (boiling) due to either turbulence or thermal blooming cannot be corrected since they occur on a time scale shorter than the adaptive system response time. Since we are interested in power rather than information transmission, this should pose no problem. Safety and Security The transmission air zone associated with all receptor sites must be restricted to all private, commercial, and military aircraft of all types. High-speed jet aircraft probably will not suffer any damage in traversing the beam, with the possible exception of the canopy. The dwell time within the beam and the high infrared reflectivity of aluminum skins combine to yield absorbed fluence levels (J/cm2) far below those required for damage. Slower, less reflective aircraft could succumb to damage. The principal reason for restricting all air traffic is the ocular hazard presented by an uncontrolled and randomly pointing reflection surface traversing a 100-MW laser beam. Coherent optical adaptive techniques (COAT) have been suggested as a viable solution to sense intruding aircraft and affect beam defocus. The system response time is limited by the round-trip light propagation time, 0.285 sec in this case. In this time, most high-speed jet aircraft would have completely passed through the beam, and to defocus would be useless. Intruding aircraft should be detected by radar as they enter the restricted zone to allow sufficient time for defocus. Sensing the unintentional loss of pointing accuracy would also be limited by the system response time. The ground distance slewed by the beam in traveling away from the receptor site before complete laser shutdown is a function of the maximum plausible rate of angular beam deflection. Due to the immense size of the proposed
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