Thermal blooming is analyzed by using a wave-propagation code to determine the speed of response of the blooming effect to changes in the transmitted laser beam. Results are developed to indicate where along the propagation path the dominant part of the thermal-blooming effects arise. It is found that most of the effect arises within about two times the depth of focus of the target, where the beam diameter is small. This suggests, and the time-dependent propagation results confirm, that the time constant descriptive of the speed of response of thermal blooming is proportional to the time it takes a parcel of air to move across the beam width in the depth-of-focus region.
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