Peaking and hollowness of low-Z impurity profiles: an interplay between ITG and TEM induced turbulent transport

Abstract

The transport of low-Z impurities in fusion plasmas is dominated, in many regimes of interest, by ion temperature gradient (ITG) and trapped electron mode (TEM) turbulence. We use a statistical test-particle approach to analyze the effects of these drift instabilities on the transport coefficients. It is found that the convection of E × B motion by the polarization drift drives an outward radial pinch via ITG and an inward pinch via TEM turbulence. Opposite radial pinches are driven by turbulent motion along magnetic field-lines. The interplay between these effects can explain the co-existence of peaked and hollow impurity density profiles observed in tokamak plasmas. The theoretical framework is validated through comparison with experimental data on the transport of Boron impurities.

Full-radius integrated modelling of ASDEX Upgrade L-modes including impurity transport and radiation

“…Finally, note that the CXRS profile at low current is hollow, but the modelling predicts no hollow profiles. The lack of predicted light impurity hollowness is a well documented missing piece of theoretical understanding [76][77][78][79], although two physical mechanisms have been recently proposed to explain it [80,81].…”

Predicting the turbulent transport of cosmic rays via neural networks

“…The magnetic vector potential, a ∥ , is assumed Gaussian, zero-averaged, and homogeneous; thus, it can be constructed numerically in each realization with a Fourier-like (harmonic) decomposition [18,27,28]:…”

Impurity outward particle flux from externally applied torque

“…As such, this new particle transport mechanism does not come from the perpendicular polarization drift, which is instead studied, with respect to the experimental observations, for example in [30], but it comes from the modification of neoclassical flows by the turbulent momentum flux, leading to a deformed particle flux, which acquires an outward component directly proportional to the applied torque.…”

“…Here we provide an estimate of the term P T = ⟨Rb θ e y • ∇ • Π T ⟩. Notice that this has been studied both in [29] using a fluid approach, and in [30] using a test particle approach. Here we…”