Linear and nonlinear resistive magnetohydrodynamic stability of tokamak discharges with negative central shear

Abstract

Linear and nonlinear magnetohydrodynamic (MHD) calculations have been performed to study the stability of tokamak discharges with reversed or negative central magnetic shear using the finite aspect ratio (FAR) suite of MHD codes [Charlton et al., J. Comput. Phys. 86, 270 (1990)]. The linear calculations confirm anew that radially localized resistive interchange modes can be unstable in the reversed or negative central magnetic shear region. The calculations further show that these resistive interchange modes are more unstable for toroidal mode numbers n higher than n=1. The nonlinear calculations do however demonstrate that toroidal mode number n=1, enhanced by the nonlinear couplings of the linearly more unstable higher n toroidal harmonics, dominates the nonlinearly saturated steady state. While the resistive interchange modes may account for the precursors detected experimentally, the saturated levels of magnetic fluctuations obtained in the nonlinear calculations do not appear large enough to cause the experimentally observed global collapse of such discharges.