Gyrokinetic simulation of Alfvén Wave-Energetic Particle Interplay in Spherical Tokamaks MAST-U and ST40

The Gyrokinetic Toroidal Code (GTC) is utilized to investigate the interplay between Alfvén waves and Energetic Particles (EPs) in spherical tokamaks (STs). In MAST-U discharge #26887, Toroidal Alfvén Eigenmode (TAE) is simulated using antenna and EPs excitation, matching the ideal MHD simulation results and qualitatively agree with experimental measurements. In the ST40 discharge #09781, electromagnetic modes near the core region in the sub-TAE frequency range are observed. These modes, which are accompanied by the transitions between the low-confinement mode (L-mode) and the high-confinement mode (H-mode), are identified as beta-induced Alfvén–acoustic eigenmodes (BAAE) by MHD simulations using the NOVA code. The MHD simulation using GTC successfully reproduces this low-frequency mode with similar frequency and mode structure as predicted by the NOVA. However, in GTC gyrokinetic simulations, higher EP density is needed to drive such modes. The agreement of these low-frequency instabilities has also been observed in two additional shots, namely #09831 and #09894. Ongoing efforts focus on detailed verifications of frequency, growth rate, and mode structure, aiming for more realistic and comprehensive results.

These simulation results represent the potential of GTC for studying the EPs effect and various phenomena in STs. Validating gyrokinetic simulations with experimental measurements represent the first step toward a better understanding and interpretation of AE phenomena in STs.