Low frequency Alfvén waves induced by toroidal flows

Abstract

The magnetohydrodynamic (MHD) spectra of toroidally rotating, axisymmetric tokamaks are analyzed. The continuous spectrum equations for the coupled localized Alfvén and slow modes are derived. The effect of flow on this spectrum is threefold: Doppler shift, centrifugal, and Coriolis effects enter. By exploiting a low-β, large aspect ratio expansion, the newly found low frequency Δm=0 Alfvén gap [van der Holst et al., Phys. Rev. Lett. 84, 2865 (2000)] located on the rational mode surfaces is analyzed. This gap is created by the geodesic curvature of the field lines, together with finite pressure, and by centrifugal and Coriolis effects. It is determined by a three mode interaction involving a central Alfvén mode and two sideband slow modes. From the same scheme, another new purely flow-induced gap inside the Δm=0 gap is found. Also, toroidal flow-induced Alfvén eigenmodes (TFAE) are found in the Δm=0 gap. All of these waves may be useful for MHD spectroscopy. Since the gap structures as well as the global wave are in the low frequency regime, they will have important implications for stability.