New high-confinement regime with fast ions in the core of fusion plasmas

TL;DR

This paper predicts and observes a new transport barrier in fusion plasmas, called F-ATB, caused by resonant interactions between fast ions and turbulence, leading to improved plasma confinement.

Contribution

It introduces the F-ATB as a novel transport barrier mechanism driven by fast ion interactions, supported by advanced simulations and experimental data.

Findings

F-ATB fully suppresses turbulent transport

Resonant wave-particle interactions trigger the barrier

Enhanced confinement observed in ASDEX Upgrade discharge

Abstract

The key result of the present work is the theoretical prediction and observation of the formation of a new type of transport barrier in fusion plasmas, called F-ATB (fast ion-induced anomalous transport barrier). As demonstrated through state-of-the-art global electrostatic and electromagnetic simulations, the F-ATB is characterized by a full suppression of the turbulent transport - caused by strongly sheared, axisymmetric $E \times B$ flows - and an increase of the neoclassical counterpart, albeit keeping the overall fluxes at significantly reduced levels. The trigger mechanism is shown to be a mainly electrostatic resonant interaction between supra-thermal particles, generated via ion-cyclotron-resonance heating, and plasma micro-turbulence. These findings are obtained by realistic simulations of the ASDEX Upgrade discharge $\#36637$ - properly designed to maximized the beneficial role of the wave-particle resonance interaction - which exhibits the expected properties of improved confinement produced by energetic particles.