Light impurity transport in tokamaks: on the impact of neutral beam fast ions

Abstract

Previous studies (e.g. (Kappatou 2019 Nucl. Fusion 59 056014)) have shown that discrepancies exist between experimental and modelled light impurity peaking in specific regimes. In particular, in NBI heated plasmas, the strong hollowness of boron profiles are not captured by the modelling. In this context, a dedicated ASDEX Upgrade discharge, including boron and helium density measurements, is studied where such discrepancies appear. Emphasis is given to the impact of fast ions on the turbulent and neoclassical impurity transport computed with the codes GKW and NEO, respectively. It is shown that fast ions increase the turbulent outward impurity convection via an increase of both thermo- and roto-diffusion. This increase is correlated with the stabilisation of the ion temperature gradient driven mode (ITG). Additionally, the ratio of the turbulent impurity diffusion over the ion heat diffusion is reduced with fast ions, which in turn, increases the neoclassical contributions to the impurity peaking. Finally, neoclassical convection is also enhanced through changes in the ion density gradient due to the modification of the quasi-neutrality condition in the presence of peaked fast ion profiles. The combination of these three mechanisms induced by the fast ion population has a significant impact on the development of hollow boron profiles, up to experimental levels.