Mesh generation and preliminary Findif calculations for limiter W7-X configuration

Findif is a 3-D finite difference, multi-fluid code devised mainly for edge plasma simulations. Intrinsically and irreducibly 3-dimensional problems, such as stellarator or ergodic divertor simulations, are a natural playground for Findif.

The mesh for Findif is produced in B field line tracing process and thus it directly takes into account magnetic field structures. Lines in the core area, magnetic islands, ergodic region and region of open field lines, need to be carefully chosen so they cover space quite uniformly. Computational mesh nodes lie where field lines cross poloidal cuts or material surfaces, but on the latter ones, boundary conditions are imposed. While parallel transport happens along field lines, perpendicular one is present on the Poincaré plots. Irregular mesh there, precludes using simple derivatives computation methods; non-conservative 'free point method' is currently used instead.

Strong parallel transport is explicitly separated in the equations solved from weak, perpendicular one, which eliminates an important source of numerical errors. As for now, the code self-consistently solves 4 equations (plasma density and momentum, electron and ion temperatures). The code presented is currently being applied to limiter configuration of Wendelstein device. Preliminary results of such simulations will be presented.