The physics of heavy impurity accumulation and accumulation avoidance illuminated by recent progress in theory and modelling

In magnetic fusion devices, the need of adopting highly charged metallic materials as plasma facing components provides challenges on the discharge scenario and on the plasma physics standpoints. Stable discharge operation requires very low central concentrations of highly radiating impurities. Furthermore, high heat loads to the walls can be mitigated by seeding radiating impurities, with the goal of producing optimized profiles of radiated power density, without reducing the global confinement of the plasma. The identification of means by which radiated power density profiles can be controlled is therefore one of the current priorities. Like in other fields of plasma physics, this research largely benefits from a combined effort from both the experimental and the theoretical standpoints. Even in axisymmetric devices like tokamaks, the computation of the transport of heavy, highly charged, impurities is confronted with additional complexities, not only because both collisional (neoclassical) and turbulent transport can occur at comparable levels, but also because it has to take into account the impact of density inhomogeneity on the magnetic flux surfaces. Poloidal inhomogeneity of the heavy impurity density can arise from the centrifugal force and as a consequence of temperature anisotropies of particle species which are heated by auxiliary systems. It can significantly modify the size and direction of neoclassical transport. Furthermore, the turbulent transport of highly charged impurities is predicted to strongly depend on the fraction of electron to ion heating of the plasma. In this talk, recent theoretical advances in the understanding of neoclassical and turbulent transport of heavy impurities in tokamak plasmas are presented, with emphasis on the underlying physical mechanisms. Recent achievements and current challenges in the modelling of experimental observations in present devices like ASDEX Upgrade and JET are described. Finally, lessons learned towards the prediction of density profiles of heavy impurities in a future tokamak reactor are discussed.