First Measurement of Drift-Alfvén Wave Polarization in magnetically confined Fusion Plasmas

Drift-Alfvén waves play crucial roles in both space and fusion plasmas, influencing turbulence, magnetic reconnection, and heat and particle transport. In theory, these waves exhibit a mixture of electrostatic and electromagnetic perturbations, their ‘polarization’. Owing to a lack of measurements on the polarization, these waves have not been unambiguously identified in fusion experiments. We report the first measurements of drift-Alfvén wave polarization in a hot, magnetically-confined plasma. The breakthrough is enabled by a novel methodology developed from gyrokinetic theory, utilizing fluctuations of electron temperature and density, quantities that are readily measured. The method is general and applicable to all waves with frequencies smaller than compressional Alfvén and ion cyclotron frequencies, and transit frequency of thermal electrons. Analysis of data from the DIII-D tokamak reveals that waves with frequencies above the geodesic acoustic mode (GAM) frequency such as toroidal and reversed-shear Alfvén eigenmodes exhibit dominant electromagnetic polarization. Waves below GAM frequency, such as fishbone and low-frequency modes show a mixture of electromagnetic and electrostatic polarization, indicating a strong coupling between shear Alfvén waves and drift-acoustic waves. For the first time, drift-Alfvén waves are clearly identified among various instabilities observed in the DIII-D tokamak [1].