Speaker
Description
Spherical tokamaks (STs) represent one of the most promising research pathways towards fusion energy. Owing to their low aspect ratio, devices such as the Mega Amp Spherical Tokamak Upgrade (MAST-U) allow access to high-β plasma regimes, potentially offering a route towards more compact and efficient fusion reactors.
In spherical tokamaks such as MAST-U, Alfvén Eigenmodes (AEs) can play an important role, as the ST configuration strongly influences the Alfvén continuum, favouring the excitation by fast ions of global coherent modes, such as Toroidal Alfvén Eigenmodes (TAEs), for which the toroidicity-induced frequency gap is enhanced.
Despite AEs being widely recognised as an important factor in the confinement of both energetic particles and bulk plasma, in spherical tokamaks their role remains only partially explored compared to large-aspect-ratio devices. In particular, in spherical tokamaks such as MAST-U, the combination of low aspect ratio, strong natural plasma shaping, and high-β values introduces a regime in which AEs–energetic-particles interactions are expected to be strongly dependent on plasma shaping parameters, such as elongation and triangularity, which modify the Alfvén continuum, poloidal mode coupling, and the spatial structure of the eigenmodes.
The quantitative impact of plasma shaping on Alfvén Eigenmode stability around MAST-U like configurations has been investigated in both the linear and nonlinear regimes using the ORB5 gyrokinetic code, with particular emphasis on dynamical phenomena such as frequency chirping and convective losses of energetic ions. These aspects are essential for the development of reliable predictive models of wave–particle interaction and for assessing the robustness of the spherical tokamak concept in reactor-relevant burning plasma regimes.