Role of Shafranov shift, zonal structures on the behavior of TAEs, AAEs and microinstabilities in the presence of energetic particles

TL;DR

This study uses gyrokinetic simulations to analyze how the Shafranov shift and zonal flows influence the stability and nonlinear behavior of Alfvén eigenmodes, microturbulence, and energetic particle interactions in fusion plasmas.

Contribution

It provides the first detailed numerical investigation of the combined effects of self-consistent Shafranov shift and zonal flows on various plasma instabilities including TAEs, AAEs, and microturbulence.

Findings

Shafranov shift affects the stability of Alfvén eigenmodes.

Including zonal flows alters turbulence saturation levels.

AAEs influence the nonlinear evolution of plasma instabilities.

Abstract

In future nuclear fusion reactors, even a small fraction of fusion-born energetic particles (EP) about 100 times hotter than the thermal bulk species, contributes substantially to the kinetic pressure and therefore affect the MHD equilibrium, mainly via the Shafranov shift. In this work, we perform first-principles numerical simulations using the gyrokinetic, electromagnetic, global code ORB5 to study the effect of a self-consistent finite $\beta$ equilibrium on the arising Alfv\'en Eigenmodes (destabilized by EPs), Ion Temperature Gradient (ITG), and Kinetic Ballooning Modes (KBM) microturbulence (destabilized by thermal species). Linearly, we explore the complex interplay between EP fraction, bulk gradients and a self-consistent Shafranov shift on the plasma stability. We choose single toroidal mode numbers to represent the system's instabilities and study the characteristic nonlinear evolutions of TAEs, KBMs and ITGs separately and including the axisymmetric field response to each mode separately. This study focuses on the impact of Shafranov shift equilibrium consistency, as well as the self-generated zonal ${E \times B}$ flows, the saturation levels and resulting heat and particle fluxes. In the ITG cases including the $n=0$ perturbations reduces turbulent fluxes, as expected, however, for the TAE cases including the $n=0$ perturbations is shown to enhance the fluxes. We show for the first time that Axisymmetric Alfv\'en Eigenmodes (AAEs) play a role in this mechanism.