Thermal blooming experiments. Final report

摘要

The goals of this program were to design an experiment for determining the effect of stimulated thermal Brillouin scattering (STBS) on single pulse laser propagation and to establish the ability of both a wave optics code and of linearized theory to predict the results of the experiment accurately. The second goal is particularly important because no experimental verification of analytical tools currently in use for single pulse high power laser propagation is available. When a high power laser propagates through the atmosphere, a small fraction of the laser energy is absorbed, creating acoustic waves that may move a significant distance transverse to the propagation direction during the pulse. Such waves lead to the well-known t(sup 3)-blooming refractive-index variations. When such blooming is sufficiently strong, the induced refractive-index alters the intensity profile of the beam farther along the propagation path. This altered intensity profile induces a somewhat different refractive-index profile that may reinforce the path-integrated t(sup 3) blooming. This self-enhancement may be called near-forward stimulated thermal Brillouin scattering (STBS). The design effort described here was carried out much like the proposed experimental program, which calls for the interaction of experimental work with analytical theory and with a wave optics code, A linearized theory of STBS was developed. Results from this theory were compared to output from a wave optics propagation code for several well defined sets of operating conditions. Once good agreement between theory and code simulation was obtained for these test conditions, the theory was used to define and operating regime for a laboratory scale thermal blooming experiment that would provide information relevant to high power laser propagation. A conceptual design for this experiment was then generated and, finally, and experimental set-up, including diagnostics, was proposed.