SOLPS-ITER modelling of the ASDEX-Upgrade tokamak with a liquid Tin module

In the frame of a continually increasing energy demand, nuclear energy has the potential to play a major role in creating a sustainable, reliable, safe and carbon-free energy mix, capable of satisfying the energy demand while limiting the CO2 production, thus contributing to containing the temperature raising. While many fission power plants are currently operating around the world, the energy production at industrial level from fusion is still missing and a big part of nuclear research investigates this topic. The difficulty in reproducing the fusion process on earth in a controlled way lays in the high densities and temperatures needed in order to activate the process. To do so, the magnetic confinement approach in machines called tokamaks, where different systems of magnets are devoted to confining the plasma, currently represents the most studied configuration. The international scientific community is devoting a huge effort to the realization of the ITER project, the largest tokamak ever built, which has the aim of demonstrating the feasibility of the production of energy from fusion at industrial level. Nevertheless, the present thesis work is related to a smaller tokamak, located at the Institute of Plasma Physics (IPP) in Garching, ASDEX Upgrade, where many experiments were performed since the '90s. One of the big challenges in building and operating the tokamaks is the problem of the power exhaust. In order to understand and try to solve this problem the physics of the edge plasma, called scrape-off layer region, needs to be studied and modeled with proper computational tools. The problem lies in the very high heat fluxes incoming on the divertor targets, the elements of the machine designed to be in contact with the plasma. The current design solution consists in using tungsten monoblocks as plasma facing components, but at present the fluxes are close to the tolerability limit in steady state, and the resilience to transient events may be insufficient. Therefore, alternative solutions are being investigated, including liquid metals divertors. The advantage lies in the self-healing mechanism: once the plasma impacts on the targets, it causes the erosion of the liquid metal in the form of evaporation and sputtering, but this is compensated by the replenishment with new liquid metal. The use of liquid metals as plasma facing components was proposed in the '90s and different experiments confirmed the relevance of this different approach to face the power exhaust problem. The present thesis is located in this research field. The aim of the work is the development of a 2D model of the edge plasma, with the SOLPS-ITER code, in a tokamak that adopts the liquid metal divertor solution, taking into account the consequent introduction of impurities. We intend to apply the model to the simulation of recent experiments carried out in the ASDEX Upgrade tokamak, in which a part of the divertor was replaced with an actively refrigerated module covered by a capillary porous structure soaked in liquid tin. The results showed the tin vapor remained in the divertor area, where the source is located, consistently with the experiments, with some traces found in the rest of the machine. Lower charge states are located near the targets, higher charge states near the core. Regarding the high concentration of liquid metal in the core region noticed during the experiments, this behavior was not detected in the simulations, suggesting that this has to be imputed to the leakage from the CPS edges.