Atmospheric thermal blooming and beam clearing by aerosol vaporization

Abstract

Thermal blooming limits the HEL power transmitted through the atmosphere at a given beam diameter. The role of refractive index and velocity turbulence has been studied.When the absorption is due to the aerosols, one can reduce the blooming by vaporizing the particles and clearing the beam path. Atmospheric Thermal BloomingIn this section the thermal blooming theory is outlined and the quantities of interest to the atmospheric propagation of a HEL are defined. The calculation includes the effect of the finite beam size. This approach is used for the first time and it allows to evaluate the steady state when t > D/Vw. The mutual coherence function, the Strehi ratio and the critical power are evaluated for a constant wind and for an isotropic wind turbulence model.These models represent the worst and best case scenarios. In the case of the unrealistic constant wind model, one can propagate little HEL power iii a given beam aperture. The results are so pessimistic that one can not build a GBL for BMD applications. Typically, one can transmit only a few MW of HEL power with a beam director of a few meters in diameter. In the case of turbulent wind, one can propagate 100 MW at beam apertures of a few meters. The latter model of the atmosphere is more realistic and we shall base on it our estimates of the system's performance. An intermediate case of constant wind and beam slew is of interest for propagation to low earth orbits. In that case one can transmit tens of MW with beam diameters of a few meters.The wave equation for the laser field is written in the paraxial approximation1 iO2E + VE + kSnE = 0 ,where k is the laser wavenumber and 6ri is the change in refractive index, Sn 872T + STill. S?2T iS the contribution from turbulence. It is a random quantity determined by the two point correlation function with a Kolmogorov spectrum,(2) (SnT(rl)öflT(r2)) = 0.033C f 'exp{ii -F2)] . (1.2) SPIE Vol. 1221 Propagation of High-Energy Laser Beams through the Earth's Atmosphere (1990)1 91 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/23/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx