Theory of Alfven eigenmodes in shear reversed plasmas

Plasma configurations with shear reversal are prone to the excitation of unusual Alfven eigenmodes by energetic particles. These modes exhibit a quasiperiodic pattern of predominantly upward frequency sweeping (Alfven cascades) as the safety factor q changes in time. This work presents a theory that employs two complementary mechanisms for establishing Alfven cascades: (1) a nonstandard adiabatic response of energetic particles with large orbits and (2) toroidal magnetohydrodynamic effects that are second-order in inverse aspect ratio. The developed theory explains the transition from Alfven cascades to the toroidicity induced Alfven eigenmodes (TAEs), including modifications of the TAEs themselves near the shear reversal point. (C) 2003 American Institute of Physics.