THE FOKKER-PLANCK OPERATOR AS AN ASYMPTOTIC LIMIT

It is shown that the Fokker-Planck operator describing a highly peaked scattering process in the linear transport equation is a formal asymptotic limit of the exact integral operator. It is also shown that such peaking is a necessary, but not sufficient, condition for the Fokker-Planck operator to be a legitimate description of such scattering. In particular, the widely used Henyey-Greenstein scattering kernel does not possess a Fokker-Planck limit.