Influence of the density gradient on turbulent heat transport at ion-scales: an inter-machine study with the gyrokinetic code stella

TL;DR

This study investigates how density gradients influence turbulent heat transport across various fusion devices using gyrokinetic simulations, revealing device-specific behaviors and the dominant micro-instability modes involved.

Contribution

It provides the first comprehensive inter-machine analysis of density gradient effects on turbulent heat transport using the stella code across multiple stellarators and tokamaks.

Findings

Ion heat flux decreases with density gradients in NCSX and W7-X.

Ion heat flux increases with density gradients in tokamaks.

Trapped-electron-modes dominate micro-instabilities in most devices.

Abstract

Efficient control of turbulent heat transport is crucial for magnetic confinement fusion reactors. This work discusses the complex interplay between density gradients and micro-instabilities, shedding light on their impact on turbulent heat transport in different fusion devices. In particular, the influence of density gradients on turbulent heat transport is investigated through an extensive inter-machine study, including various stellarators such as W7-X, LHD, TJ-II and NCSX, along with the Asdex-Upgrade tokamak and the tokamak geometry of the Cyclone Base Case (CBC). Linear and nonlinear simulations are performed employing the $\delta$f-gyrokinetic code stella across a wide range of parameters to explore the effects of density gradients, temperature gradients, and kinetic electrons. A strong reduction in ion heat flux with increasing density gradients is found in NCSX and W7-X due to the stabilization of temperature-gradient-driven modes without significantly destabilizing density-gradient-driven modes. In contrast, the tokamaks exhibit an increase in ion heat flux with density gradients. Notably, the behavior of ion heat fluxes in stellarators does not align with that of linear growth rates. Additionally, this study provides physical insights into the micro-instabilities, emphasizing the dominance of trapped-electron-modes in CBC, AUG, TJ-II, LHD and NCSX, while both the trapped-electron-mode and the passing-particle-driven universal instability contribute significantly in W7-X.