Stability of infernal and ballooning modes in advanced tokamak scenarios

A numerical parameter study has been performed in order to find MHD stable operating regimes for advanced tokamak experiments. In this study we have concentrated on internal modes. Ballooning stability and stability with respect to infernal modes are considered. The calculations confirm that pressure gradients are the main driving force for infernal modes. It is possible to stabilize infernal modes by elimination of pressure gradients in the region of small shear. In the type of equilibrium considered a flat or slightly non-monotonic q profile is most unstable against infernal modes. Higher shear around the minimum q surface stabilizes these modes. Resistivity does not lead to a significantly more unstable situation. In the region of positive shear pressure, gradients are limited by the destabilization of ballooning modes. The results of the study have been used to construct an equilibrium that is stable up to beta p=3.59, beta N=5.44