Speaker
Description
Axisymmetric (n=0) perturbations in magnetically confined plasmas include distinct global oscillatory modes occurring in the Alfvénic frequency range, such as Global Alfvén Eigenmodes and Vertical Displacement Oscillatory Modes, both of which can couple resonantly to energetic particles and undergo nonlinear saturation in the presence of fast-ion drive. This is particularly relevant in NBI or ICRH-heated plasmas, where fast ion populations provide a source of resonant drive for low-n Alfvénic activity, motivating a quantitative test of whether the observed amplitudes are compatible with energetic-particle-driven nonlinear saturation.
In this work, we develop an analytical framework for the nonlinear saturation of generic n=0 modes by energetic particles. Starting from reduced and hybrid MHD descriptions, we derive the mode structure and the corresponding wave-particle coupling coefficients in a form suitable for the weakly nonlinear theory of marginally driven instabilities. This allows us to extract explicit scaling relations for the nonlinear saturation level, the dependence on equilibrium parameters, and the role of the resonant particle population.
The resulting framework provides predictive expressions for the coupling strength, the saturation amplitude scalings, and the expected order of magnitude of the relative magnetic field perturbation amplitude in the weakly nonlinear regime. These scalings are designed to connect analytic theory with experimentally accessible quantities, enabling direct comparison with observations of axisymmetric fluctuations in present-day tokamaks and offering a route to assess whether their measured saturation levels are consistent with energetic-particle-driven nonlinear dynamics.