Gyrokinetic Simulation of Alfvén Eigenmodes and Energetic Particle Confinement in Stellarators

The success of a fusion pilot plant (FPP) relies critically on confining energetic fusion products (EPs) for self-heating. While optimized stellarator designs show promising neoclassical EP confinement [1], they are susceptible to significant EP losses driven by Alfvén eigenmodes (AEs), which degrade plasma heating. Given the lack of experimental data for future FPP burning plasmas, gyrokinetic simulations are vital for understanding plasma behavior and assessing designs for stellarators.

In this work, Gyrokinetic Toroidal Code (GTC) [2] is utilized to investigate the interplay between AEs and EPs in stellarators. We are conducting simulations using equilibria from W7-X and LHD to understand the driven of EPs on Beta-induced Alfvén Eigenmodes (BAEs) and Toroidal Alfvén Eigenmodes (TAEs) in these geometries. For the W7-X case, we employ an experimental equilibrium with artificially enhanced EP drive (due to W7-X's relatively weak beam) to study the AEs. In these W7-X simulations, GTC is benchmarked against Far3d. A BAE is initially identified in the equilibrium using GTC. When energetic particles are included, GTC naturally observes a TAE, while Far3d continues to find the BAE; therefore, driven differences between these codes' observations are studied. For the LHD case, which has experimentally observed AE phenomena, a TAE is identified using antenna in GTC. By carefully adjusting the energetic particle slowing-down distribution, this expected mode is successfully inducted by EPs in the simulations and its electromagnetic phenomenon is further studied. This research validates GTC's capability to accurately represent fast ion-driven AEs in stellarators, laying a crucial foundation for predictive modeling for future FPPs.