Numerical radiative transfer using a multigrid method

Solving the non-LTE radiative transfer problem in stellar atmospheres is computationally demanding. There is therefore an interest to investigate new methods to solve this problem effectively. A numerical method, multigrid, is a promising candidate to achieve this goal. It was originally developed to solve boundary problems in the 60s, but recently the method has been applied to several different problems. 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. I have implemented a non-linear multigrid scheme around an existing radiative transfer code, RH. This multigrid scheme achieves high convergence speed on very fine-grids, at most eight times faster than without multigrid. The results show that a non-linear multigrid method can handle various atmospheres and atoms