Kinetic analysis of MHD ballooning modes in tokamaks

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Abstract
A comprehensive analysis of the stability properties of the appropriate kinetically generalized form of MHD ballooning modes, together with the usual trapped-particle drift modes, is presented. The calculations are fully electromagnetic and include the complete dynamics associated with compressional ion acoustic waves. Trapped-particle effects, together with all forms of collisionless dissipation, are taken into account without approximations. The influence of collisions is estimated with a model Krook operator. Results from the application of this analysis to realistic tokamak operating conditions indicate that unstable short-wavelength modes with significant growth rates can extend from β = 0 to values above the upper ideal-MHD critical beta associated with the 'second stability regime'. Since the maximum growth rates of the relevant modes appear to vary gradually with beta, these results support a 'soft' beta-limit picture involving a continuous (rather than abrupt or 'hard') modification of anomalous transport already present in low-beta tokamaks. However, at higher beta the increasing dominance of the electromagnetic component of the perturbations indicated by these calculations could also imply significantly different transport scaling properties.