Gyrokinetic simulation of Alfvén eigenmodes in Stellarator W7-X

The ignition in the fusion pilot plant (FPP) heavily relies on self-heating by energetic fusion products, alpha particles. Consequently, the confinement of these energetic particles (EPs) becomes a critical issue for maintaining high-temperature plasmas. Recent designs of optimized stellarators with quasi-symmetry (QS) or quasi-isodynamic (QI) configurations have demonstrated good neoclassical confinement of alpha particles [1]. However, with optimized neoclassical transport, EP instabilities such as Alfvén eigenmodes (AEs) can induce significant EP losses and degrade plasma heating efficiency. Due to the absence of experimental data, plasma confinement properties in the FPP burning plasmas remain uncertain. Therefore, gyrokinetic simulations are expected to play a crucial role in providing important insights into plasma behavior and enabling more comprehensive assessments of stellarator designs.

 

In this work, Gyrokinetic Toroidal Code (GTC) [2] is utilized to investigate the interplay between Alfvén eigenmodes and energetic particles in stellarators. We conduct the  GTC AE simulations using the equilibrium of existing devices like W7-X to enhance our understanding of plasma behavior in stellarators. This demonstrates the capability of GTC to realistically represent the mechanisms and behaviors of fast ion-driven AEs in stellarators. For the analysis of AEs, the Alfvén continuum plays a very important role thanks to its capability to display the dispersion relation with different gaps. The 3D geometry of stellarators brings additional couplings between different toroidal harmonics. We have upgraded ALCON, the Alfvén continuum solver for GTC, for 3D geometry to verify the AEs observed in the GTC simulations.