Impurity transport in trapped electron mode driven turbulence

TL;DR

This study investigates how trapped electron mode turbulence influences impurity transport in plasma cores, revealing key factors affecting impurity peaking and conditions to prevent high-Z impurity accumulation.

Contribution

It combines gyrokinetic simulations and an analytical model to analyze impurity transport, highlighting the effects of poloidal asymmetry, magnetic shear, and collision operators.

Findings

Poloidal asymmetry and magnetic shear significantly influence impurity peaking.

Certain plasma parameters can prevent core accumulation of high-Z impurities.

Accounting for momentum conservation in collision operators improves peaking factor accuracy.

Abstract

Trapped electron mode turbulence is studied by gyrokinetic simulations with the GYRO code and an analytical model including the effect of a poloidally varying electrostatic potential. Its impact on radial transport of high-Z trace impurities close to the core is thoroughly investigated and the dependence of the zero-flux impurity density gradient (peaking factor) on local plasma parameters is presented. Parameters such as ion-to-electron temperature ratio, electron temperature gradient and main species density gradient mainly affect the impurity peaking through their impact on mode characteristics. The poloidal asymmetry, the safety factor and magnetic shear have the strongest effect on impurity peaking, and it is shown that under certain scenarios where trapped electron modes are dominant, core accumulation of high-Z impurities can be avoided. We demonstrate that accounting for the momentum conservation property of the impurity-impurity collision operator can be important for an accurate evaluation of the impurity peaking factor.