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Research on the Thermal Blooming Effect of High Energy Laser Propagation in the Atmosphere

Author: YaoYouQun
Tutor: YangRuiKe
School: Xi'an University of Electronic Science and Technology
Course: Radio Physics
Keywords: High energy lasers Thermal blooming Aerosol particles Power density
CLC: TN241
Type: Master's thesis
Year: 2009
Downloads: 246
Quote: 4
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Abstract

When the high energy lasers are propagated in the atmosphere, these can lead to a series of effects which severely limit the effective propagation. Because thermal blooming is one of most serious non-linear problems which affects laser atmospheric propagation. Thus, it is great significance that study positively thermal blooming for effective application of the high energy lasers. In order to study the phenomenon, it is studied that both epaxial light intensity and energy distribution at far field are affected by steady-state and transient thermal blooming by numerical simulation.There are five main parts in this thesis:1. The primary factors that influence laser atmospheric transmission characteristic are discussed, and the atmospheric absorption and scattering are introduced briefly. Atmospheric refractive index distribution model is introduced under the different meteorological element condition.2. Embarks from the concept of thermal blooming, systemly analyses the reason, condition, and the different form, discusses the physical essence of the thermal blooming phenomenon. Then, describes some important scaling parameters, as well as the thermal blooming effect’s threshold value is introduced. The thermal blooming involves the paraxial wave equation and density perturbation equation, which are detailed analyzed from Maxwell’s equations and hydrodynamic equations. Finally, the upper and lower limit of power threshold for thermal blooming is computed.3. According to the form of heat transfers, which obtains the analytical result approximately for steady-state thermal blooming effect under some typical conditions, to analyse the intensity distribution at far field. The results show that the bigger Nc is, the more obvious thermal blooming is. The light intensity distribution exhibits straw hatshape on a certain conditions without wind, in other word to say, the central light intensity is less than acroteric light intensity. The transverse dimension of light spot will spread, and the distribution of light spot will assumes the crescent moon shape with wind.4. The effective absorption coefficient of aerosol particles is computed by three methods. The non-dimensional distortion parameter, the beam quality factor, and the average power density of the focused beam at far-field are calculated at different levels of visibility by the relation between visibility size and absorption coefficient. The results show that the visibility is very low, aerosol particle’s absorption is strong. With visibility increase, the thermal blooming effect becomes gradually remarkable. When the visibility is large, the influence of aerosol will reduce gradually. The influence of the effective absorption coefficient of aerosol particles include laser wavelength, laser intensity, refractive index of aerosol particles, aerosol particle size distribution and its heat capacity5. According to perturbation solution of transient state thermal blooming, take the collimated Gaussian beam as the example, the single pulse laser is analyzed by numerical simulation method under atmospheric transmission.The long laser pulse is subjected to thermal blooming more than the short laser pulse, which is mainly subjected to the linear absorption.

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