GdfidL: a finite difference program for arbitrarily small perturbations in rectangular geometries

A FD-program for calculating the resonant fields in a perturbed chain of rectangular cavities is presented. The unperturbed cavity chain can be discretized exactly by standard FD-programs that operate on regular grids. Discretizing a perturbed chain exactly with such a grid would lead to a computation time inversely proportional to the perturbation. GdfidL does not represent the grid as a matrix but as a linked list. This allows discretizing arbitrarily small perturbations of a certain class without increasing the time for the solution. As a nice side-effect, only interesting volumes are discretized, reducing the computation time further