ITG-turbulence-driven Alfvénic modes in the Wendelstein 7-X stellarator

Throughout several operational phases (2018–2025) of Wendelstein 7-X (W7-X), an optimized stellarator in Greifswald, Germany, magnetic and density fluctuation measurements show that shear Alfvén wave (SAW) excitation correlates with the broadband turbulence spectrum. The typical role of SAWs in magnetic confinement fusion (MCF) research relates to their excitation by energetic particles. In recent years, the interaction between SAWs and turbulence has revealed important numerical results in the presence of energetic particles. In W7-X, despite the absence of conventional energetic particle sources, SAWs persist in most experiments. The measured Alfvénic fluctuations resemble observations in ohmic tokamak scenarios, such as those at TFTR, ASDEX Upgrade, and TCV. Across various machines, common characteristics are evident for Alfvénic fluctuations correlated with the turbulence spectrum, in contrast to those driven by energetic particles. Alfvén eigenmodes (AEs) detected through Mirnov coil measurements are consistent with ellipticity, toroidicity, and non-circularity induced AEs. Phase Contrast Imaging (PCI) measurements indicate dominant ion-temperature-gradient (ITG) driven turbulence in these plasmas. We observe a correlation between the amplitudes of AEs and density fluctuations across different magnetic field configurations. Nonlinear gyrokinetic simulations using the EUTERPE code show that ITG turbulence can simultaneously drive zonal flows and generate AEs.