Theoretical description of heavy impurity transport and its application to the modelling of tungsten in JET and ASDEX Upgrade

TL;DR

This paper develops a theoretical model for heavy impurity transport in tokamaks, emphasizing poloidal asymmetries and neoclassical effects, and applies it to tungsten behavior in JET and ASDEX Upgrade experiments.

Contribution

It introduces a comprehensive theory-based model that accounts for poloidal asymmetries and applies it to predict tungsten impurity distribution in fusion plasmas.

Findings

Poloidal asymmetries significantly influence tungsten transport.

Neoclassical transport dominates tungsten accumulation in the core.

Temperature screening can prevent impurity buildup depending on plasma conditions.

Abstract

Recent developments in theory-based modelling of core heavy impurity transport are presented, and shown to be necessary for quantitative description of present experiments in JET and ASDEX Upgrade. The treatment of heavy impurities is complicated by their large mass and charge, which result in a strong response to plasma rotation or any small background electrostatic field in the plasma, such as that generated by anisotropic external heating. These forces lead to strong poloidal asymmetries of impurity density, which have recently been added to numerical tools describing both neoclassical and turbulent transport. Modelling predictions of the steady-state two-dimensional tungsten impurity distribution are compared with experimental densities interpreted from soft X-ray diagnostics. The modelling identifies neoclassical transport enhanced by poloidal asymmetries as the dominant mechanism responsible for tungsten accumulation in the central core of the plasma. Depending on the bulk plasma profiles, neoclassical temperature screening can prevent accumulation, and can be enhanced by externally heated species, demonstrated here in ICRH plasmas.