RSAE Dynamics in Tokamak Plasmas

• Reversed Shear Alfvén Eigenmodes are discrete, core-localized shear-Alfvén waves that form in regions with a reversed safety-factor profile (qmin) in tokamak plasmas.
• They exhibit unique eigenmode structures and frequency sweeping behavior driven by the local minimum in the magnetic shear profile.
• RSAEs play a critical role in energetic particle transport, nonlinear wave coupling, and reactor-grade alpha-channeling through complex wave–particle interactions.

Reversed Shear Alfvén Eigenmodes (RSAEs) are discrete, core-localized shear-Alfvén waves that arise in axisymmetric toroidal plasmas—most prototypically tokamaks—when the safety-factor profile $q(r)$ develops a pronounced local minimum in the plasma core, resulting in a reversed magnetic shear region. The resulting extremum in the Alfvén continuum creates a frequency gap in which RSAEs are discretely trapped, featuring unique eigenmode structure, frequency sweeping with $q_\mathrm{min}$, and sensitivity to energetic particle (EP) drive. RSAEs play a central role in core energetic particle transport, nonlinear wave–wave and wave–particle interactions, zonal structure generation, and reactor-grade “alpha-channeling” via three-wave decay. They have been extensively analyzed with gyrokinetic-MHD theory, sophisticated numerical modeling, and supported by experiments across major devices.

1. Formation, Mode Structure, and Dispersion

RSAEs exist when the safety-factor profile $q(r)$ exhibits a local minimum $q_\mathrm{min}$ in the core region, generating a “reversed-shear” zone. The local shear-Alfvén continuum is given by $$k_\parallel (r