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Abstract of GRIMG2 Project

The code GRILLIX is aimed to investigate turbulent transport of particles and heat in the edge and scrape-off layer (SOL). Owing to a field line map approach, a distinguished feature of the code is its ability to perform simulations across the separatrix of diverted devices. The validity and feasibility of the approach had been shown with a simplified Hasegawa-Wakatani model, and GRILLIX was recently extended to a full-f drift reduced Braginskii model, which is more accurate for typical edge/SOL conditions as it is based on valid assumptions. Within this model the non-linear polarization, i.e. an elliptic problem with varying coefficients, has to be solved in each time step. A geometric multigrid solver was developed in order to solve this problem in an efficient way. A major challenge arises from the fact that the boundary does in general not conform with the computational grid, and during the last cycle of this project (GRIMG, 2016, 6ppm) the existing solver was adapted successfully allowing more general boundary conditions. Moreover, after having analysed the solver in detail, several possibilities for improvements concerning computational performance, parallelisation and simplification of the solver interface have been identified. An implementation of these improvements is important in order to have a robust solver available that can cope with large problems of realistic size. Moreover, before its routine application to the full physical model in GRILLIX, a profound assessment of the final multigrid solver is necessary. The result of this project, i.e. an efficient versatile and robust solver for elliptic problems, could also be reused in many other codes and scientific fields.