The linear Boltzmann equation in optically thick systems with forward-peaked scattering

The monoenergetic linear Boltzmann equation is approximated asymptotically in the limit as (i) the mean free path tends to zero, (ii) the mean scattering cosine tends to unity, and (iii) the transport cross section is held constant. Two differential scattering kernels, the Henyey-Greenstein and screened Rutherford kernels, are treated in this limit. In both cases, the leading and first-order terms in the approximations are described by the usual Fokker Planck operator and pseudodifferential operators that can be written in closed form. These pseudodifferential operators account for "large-angle" scattering effects that are not described by the standard "small-angle" Fokker-Planck operator. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.