Turbulent transport of impurities and their effect on energy confinement

TL;DR

This paper uses gyrokinetic simulations to show that impurities significantly influence energy and particle transport in tokamaks, with effects depending on impurity charge, mass, and electrostatic potential variations.

Contribution

It provides new insights into how impurities modify transport scaling and introduces an approximate formula for impurity peaking factors in tokamak plasmas.

Findings

Impurities alter transport scaling with charge and mass.

Electrostatic potential variations can reduce or reverse impurity peaking.

Impurity peaking is unaffected by impurity self-collisions.

Abstract

By presenting linear and nonlinear gyrokinetic studies, based on a balanced neutral beam injection deuterium discharge from the DIII-D tokamak, we demonstrate that impurities alter the scaling of the transport on the charge and mass of the main species, and even more importantly, they can dramatically change the energy transport even in relatively small quantities. A poloidally varying equilibrium electrostatic potential can lead to a strong reduction or sign change of the impurity peaking factor due to the combined effect of the in-out impurity density asymmetry and the EXB drift of impurities. We present an approximate expression for the impurity peaking factor and demonstrate that impurity peaking is not significantly affected by impurity self-collisions.