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Description
Recent fusion experiments have observed significant improvement in plasma confinement in the presence of fusion-born alpha particles [1,2], with Alfven eigenmodes playing a central role in regulating drift-wave microturbulence. First-principles gyrokinetic theory has proposed two mechanisms: direct three-wave interaction [3] and indirect interaction mediated via zonal fields or phase-space zonal structures (PSZS) [4], with the latter attracting increasing attention in both experimental and simulation studies.
In this work, we employ the global gyrokinetic code GTC to investigate the beat-driven excitation of zonal fields by toroidal Alfven eigenmodes (TAEs). In particular, the zonal current (ZC), characterized by radially fine-scale structures, shows good agreement between simulation and theoretical predictions [5]. This ZC further induces local modification of the q-profile within the TAE saturation amplitude (delta_B/B_0~10^-3), and may potentially break the radial correlation of microturbulence and reduce bulk plasma transport through oscillating magnetic shear. The zonal flow (ZF) exhibits both meso-scale and fine-scale structures, attributed to contributions from energetic particles (EPs) and thermal ions. The ZF plays a dominant role in TAE saturation by influencing bulk plasmas, while also playing a subdominant role that weakly increases the TAE linear growth rate via its effect on EPs [5]. Progress in the verification and validation of TAE-driven zonal fields through gyrokinetic theory and simulation will be reported.
Reference
[1] Mazzi S et al. 2022 Enhanced performance in fusion plasmas through turbulence suppression by megaelectronvolt ions Nature Physics 18 776–782
[2] Ruiz Ruiz J et al. 2025 Measurement of zero-frequency fluctuations generated by coupling between alfven modes in the jet tokamak Phys. Rev. Lett. 134 095103
[3] Chen L et al. 2023 On nonlinear scattering of drift wave by toroidal Alfven eigenmode in tokamak plasmas Nucl. Fusion 63 106016
[4] Fang Q et al. 2025 Indirect nonlinear interaction between toroidal Alfven eigenmode and ion temperature gradient mode mediated by zonal structures Nucl. Fusion 65 066004
[5] Chen L et al. 2025 The effects of zonal fields on energetic-particle excitations of reversed-shear Alfven eigenmode: simulation and theory Nucl. Fusion 65 016018