Compressional Alfven eigenmode dispersion in low aspect ratio plasmas

A new theory of compressional Alfven eigenmodes (CAEs) applicable to low aspect ratio toroidal plasmas is developed. The theory is based on the ballooning representation of the poloidal dependence of the perturbed quantities. The theory provides localized CAE solution with discrete eigenfrequencies determined by the Alfven velocity of the plasma. The eigenmode localization is determined by the effective "potential" well given by the minimum of the magnetic field on a magnetic surface, so that the plasma equilibrium is essential to accurately determine the dispersion of CAEs. The mode structure is localized in both minor radius and poloidal angle directions near the low magnetic field side. Properties of recently observed fast ion beam driven instabilities in MHz frequency range in National Spherical Torus experiments (NSTX) [Kaye, , Fusion Technol. 36, 16 (1999)] are consistent with ones of CAEs. (C) 2002 American Institute of Physics.