Turbulent impurity transport in tokamaks

Licentiate thesis, 2013

Nuclear fusion is foreseen as one of the options for future energy production. One of the remaining scientific challenges for establishing the physics basis of future magnetic fusion reactors is 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 dominated by turbulent fluctuations. This thesis addresses the effect of poloidal asymmetries and the role of electromagnetic effects on turbulent impurity transport. Transport driven by ion temperature gradient (ITG) mode, trapped electron (TE) mode and kinetic ballooning mode (KBM) turbulence is studied through gyrokinetic modeling. It is shown that poloidal asymmetries significantly affect radial transport, and could be a contributing reason for the experimentally observed decrease of impurity core content in the presence of plasma heating through radiofrequency waves. Furthermore, we show that the value of the ratio of plasma pressure to magnetic pressure can affect the impurity peaking, which is typically found to be lower in KBM turbulence than in ITG turbulence.