Study of the impact of fast ions on core turbulence at rational surfaces via global gyrokinetic simulations

TL;DR

This paper investigates how fast ions influence core turbulence at rational surfaces using global gyrokinetic simulations, revealing mechanisms that can significantly suppress turbulence and affect plasma stability.

Contribution

It introduces a comprehensive analysis of fast ions' effects on turbulence, including suppression mechanisms and interactions with fishbone modes, via advanced global gyrokinetic simulations.

Findings

Fast ions reduce turbulence by up to 90% in certain regions.

Fast ions lower the destabilization threshold for zonal modes.

Fishbone modes can further decrease turbulence when thermal profiles are not flattened.

Abstract

In this work, the interplay between fast ions and safety factor rational surfaces is studied in a turbulent plasma via global nonlinear gyrokinetic simulations. Initially, the fast particles-induced enhancement of shearing structures from turbulence self-interaction is analyzed. Our study takes into account the competition between this mechanism and other fast ions effects, i.e. thermal profiles dilution and quasi-resonant interaction. We find the fast ions-induced reduction of destabilization threshold for the zonal modes to be a very efficient way to suppress turbulence. Indeed, it leads to the formation of regions where turbulent transport is reduced by 90\% of its original value. Furthermore, an $n=m=1$ fishbone is driven unstable inside the plasma and its interaction with turbulence is studied. We find the beat-driven zonal structure generate by this mode to further reduce turbulence when its presence does not drastically flatten the thermal profiles.