First principles of modelling the stabilization of microturbulence by fast ions

TL;DR

This paper develops a reduced gyrokinetic model to explain how fast ions stabilize microturbulence in fusion plasmas, highlighting the roles of fast ion class, zonal flows, and energetic limits, supported by simulations.

Contribution

It introduces a new physically-transparent model extending the dilution approach to fast ion kinetics, providing qualitative predictions on stabilization mechanisms.

Findings

Fast ion classes have different stabilizing effects.

Zonal flows are crucial for stabilization.

Highly energetic fast ions behave like the dilution model.

Abstract

The observation that fast ions stabilize ion-temperature-gradient-driven microturbulence has profound implications for future fusion reactors. It is also important in optimizing the performance of present-day devices. In this work, we examine in detail the phenomenology of fast ion stabilization and present a reduced model which describes this effect. This model is derived from the high-energy limit of the gyrokinetic equation and extends the existing "dilution" model to account for nontrivial fast ion kinetics. Our model provides a physically-transparent explanation for the observed stabilization and makes several key qualitative predictions. Firstly, that different classes of fast ions, depending on their radial density or temperature variation, have different stabilizing properties. Secondly, that zonal flows are an important ingredient in this effect precisely because the fast ion zonal response is negligible. Finally, that in the limit of highly-energetic fast ions, their response approaches that of the "dilution" model; in particular, alpha particles are expected to have little, if any, stabilizing effect on plasma turbulence. We support these conclusions through detailed linear and nonlinear gyrokinetic simulations.