Core transport barriers induced by fast ions in global gyrokinetic GENE simulations

TL;DR

This paper investigates a new type of internal transport barrier induced by fast ions in tokamak plasmas, using global gyrokinetic simulations to understand the conditions and parameters that trigger this barrier.

Contribution

It systematically explores the parameter space affecting the fast ion-induced transport barrier using GENE simulations, revealing the threshold conditions for its formation.

Findings

Transport barrier width and location depend on energetic particle profiles.

A threshold in fast ion temperature gradient amplitude triggers the barrier.

Resonant wave-particle interactions are key to barrier formation.

Abstract

A novel type of internal transport barrier (ITB) called F-ATB (fast ion-induced anomalous transport barrier) has been recently observed in state-of-the-art global gyrokinetic simulations on a properly optimized ASDEX Upgrade experiment [A. Di Siena et al. Phys. Rev. Lett. {\bf 127} 025002 (2021)]. Unlike the transport barriers previously reported in literature, the trigger mechanism for the F-ATB is a basically electrostatic wave-particle resonant interaction between supra-thermal particles - generated via ion cyclotron resonance heating (ICRH) - and ion scale plasma turbulence. This resonant effect strongly depends on the particular shape of the fast ion temperature and density profiles. Therefore, to further improve our theoretical understanding of this transport barrier, we present results exploring the parameter space and physical conditions for the F-ATB generation by performing a systematic study with global GENE simulations. Particular emphasis is given to the transport barrier width and its localization by scanning over different energetic particle temperature profiles. The latter are varied in amplitude, half-width, and radial localization of an ad-hoc Gaussian-like energetic particle logarithmic temperature gradient profile. For the reference parameters at hand, a threshold in the amplitude of the fast ion logarithmic temperature gradient is identified to trigger the transport barrier effectively.