Compressional Alfvén eigenmode structure in spherical tokamaks

Abstract. The two-dimensional structure of compressional Alfvén eigenmodes below the ion cyclotron frequency is studied numerically by solving the cold plasma Hall-MHD equations for a realistic spherical tokamak equilibrium. The simplest of the computed eigenmodes have a standing wave-like structure and the higher frequency solutions show a more travelling wave-like behaviour. The effects of the equilibrium current and the Hall terms in the eigenmode equation are investigated, and these terms are found to shift the frequencies of eigenmodes with different sign of the toroidal mode number away from each other. A classification scheme is proposed which relates each spherical tokamak eigenmode to the three mode numbers of the corresponding large aspect ratio circular cross section eigenmode obtained by gradually decreasing the elongation and increasing the aspect ratio