The Dependence of the Impurity Transport on the Dominant Turbulent Regime in ELM-y H-mode Discharges

TL;DR

This study investigates how impurity transport in ELM-y H-mode discharges varies with the dominant turbulent regime, revealing significant differences in impurity diffusion and confinement between electron and ion heating conditions, and comparing experimental results with gyrokinetic modeling.

Contribution

It provides detailed experimental measurements of impurity transport under different turbulent regimes and evaluates the accuracy of gyrokinetic modeling in predicting these transport variations.

Findings

Impurity diffusion increases 30-fold with electron heating.

Impurity confinement time drops threefold with electron heating.

Gyrokinetic model underpredicts the magnitude of impurity transport variation.

Abstract

Laser blow-off injections of aluminum and tungsten have been performed on the DIII-D tokamak to investigate the variation of impurity transport in a set of dedicated ion and electron heating scans with a fixed value of the external torque. The particle transport is quantified via the Bayesian inference method, which, constrained by a combination of a charge exchange recombination spectroscopy, soft X-ray measurements, and VUV spectroscopy provides a detailed uncertainty quantification of the transport coefficients. Contrasting discharge phases with a dominant electron and ion heating reveal a factor of 30 increase in midradius impurity diffusion and a 3-fold drop in the impurity confinement time when additional electron heating is applied. Further, the calculated stationary aluminum density profiles reverse from peaked in electron heated to hollow in the ion heated case, following a similar trend as electron and carbon density profiles. Comparable values of a core diffusion have been observed for W and Al ions, while differences in the propagation dynamics of these impurities are attributed to pedestal and edge transport. Modeling of the core transport with non-linear gyrokinetics code CGYRO [J. Candy and E. Belly J. Comput. Phys. 324,73 (2016)], significantly underpredicts the magnitude of the variation in Al transport. The experiment demonstrates a 3-times steeper increase of impurity diffusion with additional electron heat flux and 10-times lower diffusion in ion heated case than predicted by the modeling. However, the CGYRO model correctly predicts that the Al diffusion dramatically increases below the linear threshold for the transition from the ion temperature gradient (ITG) to trapped electron mode (TEM).