Modification of ion-temperature-gradient turbulence by impurities in stellarator plasmas

TL;DR

This paper analytically and numerically investigates how impurities influence ion-temperature-gradient turbulence in stellarator plasmas, revealing key physical mechanisms and predictive scalings.

Contribution

It derives an analytical expression for impurity effects on ITG growth rates and validates it with gyrokinetic simulations, advancing understanding of impurity-turbulence interactions.

Findings

Impurities can either increase or decrease ITG growth rates.

Analytical scalings match simulation results for impurity effects.

A strong correlation exists between growth rate modifications and turbulent heat flux changes.

Abstract

Recent nonlinear gyrokinetic simulations have shown that impurities can strongly modify the turbulent heat flux in stellarator plasmas. Here, the ion-temperature-gradient (ITG) dispersion relation in a plasma containing impurities is analytically solved in certain limits and an expression for the modification of the ITG growth rate by impurities is derived. The analytical expression is the sum of three terms corresponding to three different physical causes (impurity density gradient, impurity temperature gradient and dilution) of the change in the growth rate. The scalings predicted analytically for the modification of the growth rate are shown to be reproduced by linear gyrokinetic simulations. The conditions for reduction or increase of the ITG growth by impurities are also correctly predicted by the analytical solution to the dispersion relation. Finally, a remarkable correlation is found between the analytical expression for the modification of the growth rate and the modification of the turbulent heat flux obtained from nonlinear gyrokinetic simulations.