The effect of the ion bounce resonance on ideal and resistive MHD instabilities

The effect of parallel compressibility on the stability of resistive ballooning mode and of the m=1 internal kink mode in collisionality regimes appropriate to present day tokamak experiments has been recently investigated. As resistive instabilities are described by a two spatial scale structure in toroidal geometry, the effect of compressibility can be defined either with reference to the short scale, of the order of the connection length qR, with q being the safety factor and R being the major radius, or with reference to the long scale associated to the radial localization of the mode. The purpose of the present work is twofold: first we want to analyze the effects of ion bounce resonance on resistive ballooning modes in tokamak plasmas; second we want to discuss the effect of the transit resonances on the m=1 internal kink mode with particular regard to the effect of the external region. (author) 8 refs