Multiscale turbulence in stellarators

TL;DR

This paper presents pioneering gyrokinetic simulations of multiscale turbulence in the stellarator Wendelstein 7-X, revealing complex interactions between electron and ion-scale turbulence and their impact on plasma transport.

Contribution

First-ever gyrokinetic simulations of multiscale turbulence in a stellarator, exploring interactions between ETG, ITG, and MTM regimes with implications for stellarator transport modeling.

Findings

ETG turbulence can drive significant transport despite lacking radial streamers.

Ion-scale turbulence erodes zonal flows, increasing ion transport in ITG regimes.

MTMs persist due to limited suppression by isotropic ETG turbulence.

Abstract

We present the first gyrokinetic simulations of multiscale turbulence in a stellarator, using the magnetic geometry of Wendelstein 7-X (W7-X) and experimentally relevant parameters. A broad range of scenarios is explored, including regimes where electron-temperature-gradient (ETG) turbulence coexists with varying levels of ion-temperature-gradient (ITG) turbulence, as well as cases involving microtearing modes (MTMs) relevant to high-$\beta$ and reactor-like conditions. Notably, while ETG turbulence does not form radial streamers as in tokamaks, it can still drive significant transport and interact with ion-scale turbulence. In electrostatic ITG-dominated regimes, electron-scale fluctuations erode zonal flows, enhancing ion-scale transport, while ion-scale turbulence suppresses ETG activity. In contrast, under electromagnetic MTM conditions, the isotropic nature of ETG turbulence limits its suppressive effect, allowing MTMs to persist. These findings underscore the critical role of cross-scale effects for accurate transport predictions in W7-X and future stellarators.