Implicit Monte Carlo diffusion - An acceleration method for Monte Carlo time-dependent radiative transfer simulations

Implicit Monte Carlo (IMC) is often employed to numerically simulate radiat ive transfer. In problems with regions that are characterized by a small me an free path, IMC can take a prohibitive amount of time, because many parti cle steps must be simulated to advance the particle through the time step. Problems containing regions with a small mean free path can frequently be a ccurately simulated much more quickly by employing the diffusion equation a s an approximation. However, the diffusion approximation is not accurate in regions of the problem where the mean free path is large. We present a method for accelerating time-dependent Monte Carlo radiative t ransfer calculations by using a discretization of the diffusion equation to calculate probabilities that are used to advance particles in regions with small mean free paths. The method is demonstrated on problems with one-and two-dimensional orthogonal grids. It results in decreases in run time of m ore than an order of magnitude on these problems, while producing answers w ith accuracy comparable to pure IMC simulations. We call the method Implici t Monte Carlo Diffusion, which we abbreviate IMD. (C) 2001 Academic Press.