Speaker
Description
Achieving good confinement in tokamak advanced scenarios requires understanding and controlling Alfvénic instabilities driven by energetic particles (EPs). Reversed-shear Alfvén eigenmodes (RSAEs) [1, 2] are commonly observed in plasmas with nonmonotonic safety-factor profiles and can cause substantial EP transport and losses, making their nonlinear saturation mechanisms critical for predicting burning-plasma performance [3]. Zonal electromagnetic fields (ZFs)—toroidally symmetric, low-frequency perturbations of the equilibrium—are a natural candidate to mediate saturation because they arise from self-consistent nonlinearities and can couple scales across the plasma [4-6]. Previous theoretical work emphasized zonal-flow effects and continuum damping as key saturation channels [7, 8], while recent large-scale simulations and analyses have shown both thermal-plasma-driven ZFs and phase-space zonal structures (PSZS) can be important, and in some cases ZFs may even enhance EP drive [9-12]. This motivates a first-principles clarification of which nonlinear channel dominates and how it leads to saturation.
In this work we isolate the thermal-plasma nonlinearity route by keeping EP dynamics linear. Global gyrokinetic simulations (GTC) [13] of a single-n RSAE show clear downward frequency chirping, excitation of radially propagating kinetic Alfvén waves (KAWs), and eventual saturation at finite amplitude, even with linear EP drive. To interpret these results we derive two compact eigenmode models. The “RSAE-ZF-MHD” model (based on the ideal-MHD continuum) demonstrates that beat-driven zonal current produces a dominant downward frequency shift (zonal flow yields a smaller opposite effect) and that continuum resonant damping is negligible. The extended “RSAE-ZF-KAW” model, which includes finite-ion-Larmor-radius terms and electron Landau kinetics, captures the resulting mode conversion to KAWs and the associated strong radiative damping responsible for mode suppression and saturation. Numerical solutions of these models quantitatively reproduce the gyrokinetic observations. These results establish zonal-current-induced frequency chirping plus KAW radiative damping as an efficient saturation mechanism for RSAEs.
References
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