Beta-induced Alfvén-acoustic eigenmodes in stellarator plasmas with low shear

Eremin, D. Yu.; Könies, A.
January 2010
Physics of Plasmas;Jan2010, Vol. 17 Issue 1, p012108
Academic Journal
The coupling of low-frequency Alfvénic modes with acoustic oscillations due to curvature of the background magnetic field is considered for stellarator plasmas with low shear. Magnetohydrodynamic (MHD) analysis demonstrates that the interaction between these branches can generate gaps in the continua with a width proportional to beta as well as the magnitude of the Fourier harmonics of the magnetic field strength which cause the coupling. The gaps can provide a habitat for beta-induced Alfvén-acoustic eigenmodes (BAAEs). Using the causality principle, a technique is developed to resolve the singular behavior of the MHD BAAE eigenmode equation at the points of resonance with the acoustic continuum. Alternatively, the singularities arising in the reduced MHD description can be resolved by accounting for the finite parallel electrical field. Both approaches yield consistent continuum damping rate, which proves to be small. Numerical calculations for analytically fitted experimental profiles of electron-dominated plasma in Helically Symmetric eXperiment (HSX) facility yield two weakly damped BAAE modes with different frequencies: one is close to the maximum of the lower-frequency Alfvén-acoustic continuum, and the other is located well within the BAAE gap. The numerically found BAAEs have frequencies in the same range as the experimentally observed electromagnetic modes in HSX, even when the finite diamagnetic frequency effects are considered.


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