Gyrokinetic modelling of the Alfvén mode and EGAM activity in ASDEX Upgrade

Abstract

Energetic particles present in tokamak machines can drive through resonant wave-particle interaction different plasma instabilities, e.g Alfvén modes and energetic particle-driven geodesic acoustic modes (EGAMs). While the former are potentially detrimental as they can enhance the energetic particle transport and damage the machine wall, the latter are axisymmetric, possibly benign modes that can act to regulate turbulence. A unique scenario, the so-called NLED-AUG case, has been developed in ASDEX Upgrade by tuning the plasma parameters so that the energetic particle kinetic energy is 100 times higher than that of the background plasma, like in ITER. An intense energetic particle-driven activity is observed, most prominently various Alfvén mode bursts triggering chirping EGAMs. The present work reports studies on the Alfvén mode and EGAM dynamics showing, for the first time, many toroidal mode gyrokinetic simulations with ORB5 where the NLED-AUG case scenario is considered. We study the mode dynamics modelling the energetic particles with different equilibrium distribution functions, such as: isotropic slowing-down, double-bump-on-tail and equivalent Maxwellian. We retain, at the beginning, the nonlinearities only in the energetic particle dynamics. Later, also the background plasma species nonlinearities are taken into account.

Nonlinear interaction of Alfvénic instabilities and turbulence via the modification of the equilibrium profiles

“…The radial structure of the mode, as given by Vannini et al (2022a), shows a mode peaked in the core, at s = ψ/ψ edge 0.25 (see Figure 6). The corresponding modification of the temperature profiles of thermal ions and electrons can be studied.…”

“…In this case, the injection energy is E = 93 keV and the EPs have a density concentration of n EP / n e = 0.0949, where • • • indicates volume average. The reader should refer to Vannini et al (2022a) for more details on this case. Note that for the physics of interest in this paper, there is no qualitative dependence of the results on the initial conditions of the energetic particles (as long as they drive the AM unstable).…”