Low-frequency magnetohydrodynamic spectra of plasmas with anisotropic pressure in axisymmetric toroidal systems

Abstract

The low-frequency ideal magnetohydrodynamic (MHD) spectra of anisotropic pressure plasmas in axisymmetric toroidal systems are analyzed. In the framework of the Chew-Goldberger-Low model, the continuum spectrum equations for the coupled localized Alfvén and slow magnetosonic modes are derived. The equations are applied to study the continuous spectra for low-pressure plasmas in large aspect ratio tokamaks. It is shown that for some classes of plasma equilibria the pressure anisotropy results in instability of continuous modes. Such an instability is due to anisotropic plasma stratification over poloidal angle on the magnetic surfaces. It takes place for both the zonal flow modes with (m, n) = 0 and the general electromagnetic modes with (m, n) ≠ 0. The latter are unstable only if they are localized near the rational magnetic surface. A stabilization of this instability due to the Alfvén effect is shown for the modes localized away from the rational magnetic surface.