Description
Energetic particles (EPs) provide plasma heating and current drive in fusion devices, but their confinement can be strongly affected by Alfvén eigenmodes (AEs) and other instabilities, with implications for performance and plasma–wall interactions in ITER and DEMO. Recent experiments on the Tokamak à Configuration Variable (TCV) have advanced the physics basis for AE excitation and control in plasmas with supra-Alfvénic fast ions. Scenarios were established using both co- and counter-injected neutral beams, enabling controlled variation of fast-ion drive and systematic characterization of AE activity.
Dedicated control experiments demonstrate reproducible AE suppression using electron-cyclotron heating and current drive (ECRH/ECCD). The observations indicate that multiple mechanisms contribute, including (i) temperature and density profile modifications affecting damping and resonance conditions, (ii) magnetic-shear changes driven by localized ECCD, and (iii) direct fast-ion redistribution associated with AE dynamics and EC actuation. Comparative studies in positive and negative triangularity further show that fast-ion confinement is similar during MHD-quiescent phases, while AE activity during flat-top conditions differs between shapes, highlighting sensitivity of EP-driven stability to equilibrium and profile details.
Finally, first observations of ion cyclotron emission (ICE) on TCV open a new diagnostic window on EP dynamics, complementing established fast-ion measurements and enabling access to high-frequency signatures linked to EP populations and wave–particle coupling. The results are discussed in the context of recent findings on other tokamaks, emphasizing common trends and constraints for robust AE control and improved EP confinement in reactor-relevant regimes.