Stability of n = 1 internal modes in tokamaks

An extensive numerical study has been carried out for internal modes with toroidal mode number unity. These are internal kink modes, when the q = 1 surface falls within the plasma, and have a ballooning characteristic when qaxis > 1 Both modes show a dependence on the pressure and have a second region of stability at high β. A parameter survey has been conducted, varying the geometry, i.e. aspect ratio, ellipticity, triangularity, etc. and the current profiles, through the pressure and safety factor. The principal results show that the modes are dependent on the geometry and are strongly stabilized by high-order, noncircular effects. Broader pressure profiles and reduced shear are favourable for limiting the instability.