Some Approximations to the Radiative Transfer Equation and Their Applications

Radiative transfer theory describes the interaction of radiation with scattering and absorbing media. It has applications in neutron transport, atmospheric physics, heat transfer, molecular imaging, and so on. In steady state, the radiative transfer equation is an integro-differential equation of five independent variables. This high dimensionality and presence of integral term present a serious challenge when one tries to solve the equation numerically. Over the past 50 years, several techniques for solving the radiative transfer equation have been introduced. One among them is to use approximations of RTE. Various approximations of RTE have been proposed in the literature. These include, but are certainly not limited to, the delta-Eddington approximation, the Fokker-Planck approximation, the Boltzmann-Fokker-Planck approximation, the generalized Fokker-Planck approximation, the Fokker-Planck-Eddington approximation and the generalized Fokker-Planck-Eddington approximation. The Fokker-Planck approximation and differential approximation have received particular attention in the literature due to their relatively high accuracy, relatively low computational cost, and flexibility to potential large scale parallel computing.

In this thesis we present a well-posed result for the Fokker-Planck equation that may be used to approximate the radiative transfer equation in highly forward-peaked media. Then we study the differential approximation of radiative transfer (RT/DA) equations. Well-posedness of these approximations is studied. A convergent iteration method for the RT/DA equation is presented. Then we turn to a study of RT/DA based inverse problems. The inverse problems are ill-posed and regularization is needed in solving the inverse problems. We present an existence result for solutions of regularized formulations of the inverse problems. Finally examples are included to illustrate numerical results in solving the inverse problems.