Impurity transport in tokamak plasmas, theory, modelling and comparison with experiments

In this paper, the theory of collisional and turbulent transport of impurities in tokamak plasmas is reviewed. The results are presented with the aim of providing at the same time a historical reconstruction of the scientific progress and a complete description of the present theoretical knowledge, with a hopefully sufficiently complete reference to the works which have been published in the field in the last decades. After a general introduction on the physics challenges offered by the problem of impurity transport and their relevance for practical nuclear fusion energy, the theory of collisional transport is presented. Here a specific section is also dedicated to the transport parallel to the magnetic field lines. A complete review of the transport mechanisms produced by turbulence follows. The corresponding comparisons between theoretical predictions and experimental observations are also presented, highlighting the influence that the validation activities had in motivating further theoretical investigations. The paper is completed by a section on the direct interactions between collisional and turbulent transport and by a final specific review dedicated to the progress in the theory-based modelling activities. In the writing of this review paper, the main goal has been to combine readability with completeness and scientific rigour, providing a comprehensive list of references for deeper documentation on specific aspects.