Theory and Observations of Low Frequency Eigenmodes due to Alfvén Acoustic Coupling in Toroidal Fusion Plasmas

New theory developments and experimental observations of two classes of energetic particle driven instabilities are reported. First is a new class of global MHD solutions resulting from coupling of the Alfvénic and acoustic fundamental MHD oscillations due to geodesic curvature. These modes, predicted theoretically and numerically and called Beta-induced Alfvén-Acoustic Eigenmodes (BAAEs), have been recently observed in low beta JET, DIII-D and high beta NSTX plasmas. The second class is the instability of Reversed Shear Alfvénic eigenmodes (RSAEs), which are shown to be suppressed due to strong magnetic pressure in NSTX plasma. A kinetic theory is required if the modes strongly interact with the Alfvénic continuum. Both RSAEs and BAAEs can potentially deteriorate the fast ion confinement in next step fusion plasmas, especially if excited together with multiple global TAE instabilities [3]. At the same time they can be used as a diagnostic tool for fast ion and safety factor profiles, a technique known as MHD spectroscopy.
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