THEORY OF CONTINUUM DAMPING OF TOROIDAL ALFVEN EIGENMODES IN FINITE-BETA TOKAMAKS

A general theoretical approach has been formulated for analyzing two-dimensional structures of high-n toroidal Alfven eigenmodes (TAE) in large aspect-ratio, finite-beta tokamaks. Here, n is the toroidal wave number and beta is the ratio between plasma and magnetic pressures. The present approach generalizes the standard ballooning-mode formalism and is capable of treating eigenmodes with extended global radial structures, as well as finite coupling between discrete and continuous spectra. Employing the well-known (s,alpha) model equilibrium and assuming a linear equilibrium profile, the present approach has been applied to the calculation of the resonant continuum damping rate of TAE modes. Here, s and alpha denote, respectively, the strengths of magnetic shear and pressure gradients. In particular, it is found that there exists a critical alpha value, alpha(c)(s), such that, as alpha --> alpha(c), the continuum damping rate is significantly enhanced and, thus, could suppress the potential TAE instability.