Significant effect of plasma compressibility on nonlinear mode coupling of Alfvn eigenmodes

Hirota, Makoto; Aiba, Nobuyuki

Nonlinear mode coupling of the (shear) Alfvn eigenmodes is studied both analytically and numerically to understand the effect of the magnetohydrodynamic (MHD) nonlinearity on the saturation mechanism. The energy transfer via the mode coupling can restrict the amplitude to a level that is estimated by the frequency mismatch and the coupling coefficient among the coupled modes. For the cases of cylindrical and toroidal geometries, new numerical codes are developed to calculate the coupling coefficient directly according to the Lagrangian theory of weakly nonlinear perturbations. It is shown that the shear Alfvn eigenmodes can couple with each other through the small compressional components of their eigenfunctions. By assuming low- plasma, an analytic estimation of the coupling coefficient () is derived in the cylindrical case. A coupling of reversed shear Alfvn eigenmodes in toroidal geometry indicates a more effective energy transfer at a lower amplitude level owing to the enhanced compressional perturbations in the poloidal sidebands.