Impurity transport in magnetically confined fusion plasmas

Doctoral thesis, 2015

Nuclear fusion is foreseen as one of the options for future energy production. In establishing the physics basis of future magnetic fusion reactors it is necessary to find scenarios where the impurity content in the core can be kept low. High concentration of impurities leads to dilution and radiative energy losses and is detrimental for fusion reactivity. Therefore the understanding and control of impurity transport is of critical importance for the success of fusion. Impurity transport in fusion plasmas is often dominated by turbulent fluctuations, but in certain scenarios, and in particular for 3D magnetic configurations, the collisional contribution can be significant. This thesis addresses the effect of poloidal asymmetries on turbulent impurity transport in axisymmetric devices (tokamaks), and collisional impurity transport in 3D configurations (stellarators). In tokamaks, transport driven by ion temperature gradient mode and trapped electron mode turbulence is studied through gyrokinetic modelling. It is shown that poloidal asymmetries can significantly affect radial transport, and could contribute in reducing the impurity core content. The theoretical predictions are compared to experimental data from the Alcator C-Mod tokamak, where poloidal asymmetries are induced by radiofrequency heating. Furthermore, we show that collisional transport in stellarators can cause very strong impurity accumulation, highlighting this as one of the major concerns for 3D devices as a reactor concept. We also investigate how the bootstrap current is affected by the presence of impurities.