Numerical radiative transfer using a multigrid method

Solving the 3D non-LTE radiative transfer problem in stellar atmospheres is computationally demanding. We have implemented a non-linear multigrid scheme in the radiative transfer code MULTI3D. The important advantages of the multigrid scheme are the very high convergence rate, and that the convergence speed does not deteriorate when the discretization is refined. Both properties are highly desirable for solving the radiative transfer problem for high-resolution atmosphere models produced by MHD codes. We present timing tests and example calculations based on atmosphere models computed with the Bifrost code.