Access to the second stability region in a high-shear, low-aspect-ratio tokamak

Abstract

It is known that tokamaks display a second region of stability to ideal magnetohydrodynamic (MHD) internal modes. An important determining factor for MHD properties is the radial profile of toroidal current. Here it is shown that in a low-aspect-ratio tokamak with high on-axis safety factor (q0≂2) and high shear, a path to high beta can be obtained that remains completely stable against ideal MHD modes. By maintaining high shear this scenario avoids fixed boundary instabilities for both high and low toroidal mode numbers for beta values well above the Troyon limit (stability was tested up to εβp=1.4, β=10.8%). For a close fitting wall (awall/aplasma≂1.2) this configuration is also stable to low toroidal mode number balloon-kink modes.