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

Linear and nonlinear magnetohydrodynamic (MHD) calculations have been perfo rmed to study the stability of tokamak discharges with reversed or negative central magnetic shear using the finite aspect ratio (FAR) suite of MHD co des [Charlton , J. Comput. Phys. 86, 270 (1990)]. The linear calculations c onfirm anew that radially localized resistive interchange modes can be unst able in the reversed or negative central magnetic shear region. The calcula tions 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 nonline ar couplings of the linearly more unstable higher n toroidal harmonics, dom inates the nonlinearly saturated steady state. While the resistive intercha nge modes may account for the precursors detected experimentally, the satur ated levels of magnetic fluctuations obtained in the nonlinear calculations do not appear large enough to cause the experimentally observed global col lapse of such discharges. (C) 2001 American Institute of Physics.