Impact of fast ions on turbulent transport in high-\b{eta} HL-2A scenarios

TL;DR

This study uses gyrokinetic simulations to explore how fast-ion pressure gradients influence turbulence in high-beta HL-2A plasmas, revealing stabilization effects, mode transitions, and the complex role of fast ions in transport regulation.

Contribution

It provides new insights into the impact of fast ions on turbulence and mode behavior in high-beta plasmas, highlighting their dual stabilizing and destabilizing roles.

Findings

Fast ions stabilize ITG modes via thermal-ion dilution.

Transition to FI-driven BAE occurs at high FI pressure gradients.

Moderate FI pressure suppresses turbulence; strong FI drive can enhance it.

Abstract

The fast-ion (FI) on turbulent transport is one of the key topics of magnetic confinement fusion. This work focus on the impact of FI pressure gradients on turbulence in a high-\b{eta} plasma scenario using gyrokinetic simulations. Linear analyses reveal that FIs strongly stabilize ion temperature gradient (ITG) modes via the thermal-ion dilution, while their influence on trapped electron modes (TEMs) is minimal. At elevated FI pressure gradients, a transition to a FI-driven BAE (FI-BAE) regime occurs, as evidenced by mode structure and frequency alignment within the Alfv\'enic gap. Electron \b{eta} scans further demonstrate the emergence of kinetic ballooning modes (KBMs) at higher \b{eta}, whereas an ITG-TEM hybrid turbulence dominates near experimental \b{eta} values. Nonlinear simulations show that moderate FI pressure suppresses transport via zonal flow (ZF) shear, whereas strong FI drive weakens ZFs and enhances transport by destabilizing FI-BAEs. These results highlight the dual role of FIs in regulating turbulence and offer insight into multiscale transport physics relevant for high-performance plasmas.