ELLIPTICITY INDUCED ALFVEN EIGENMODES

It is shown that noncircularity of tokamak flux surfaces leads to frequency gaps in the magnetohydrodynamic Alfven continuum. Within these gaps discrete modes having macroscopic structure are shown to exist and have many common features with toroidicity induced Alfven eigenmodes. The present work focuses on ellipiticity. Since kappa - 1 > epsilon in many tokamaks the ellipticity induced Alfven eigenmode may indeed be a more robust mode. The most global mode couples the m = 1, n = 1 and m = 3, n = 1 "cylindrical" eigenmodes. The region of strong coupling occurs at the q(r) = 2 surface and the width of the coupling region is finite and of order (kappa - 1)a. Furthermore, for typical limiter (r) profiles satisfying 1 less-than-or-similar-to q less-than-or-similar-to 3, the dominant mode harmonics do not intersect the continuum Alfven spectrum.