Magnetohydrodynamic calculations with a nonmonotonic q profile and equilibrium, sheared toroidal flow

The linear and nonlinear stability of a nonmonotonic q profile is examined using a reduced set of magnetohydrodynamic (MHD) equations with an equilibr ium, sheared toroidal flow. The reversed shear profile is shown to be unsta ble to a rich variety of resistive MHD modes including pressure-driven inst abilities and tearing instabilities possessing a tearing/interchange charac ter at low Lundquist number, S, and taking on a double/triple tearing struc ture at high S. Linear calculations show that the destabilizing effect of t oroidal velocity shear on tearing modes is enhanced at finite pressure. In addition, this velocity shear decreases the stabilizing effect of finite pr essure seen previously for tearing modes at high S. Nonlinear calculations show the generation of a large, m=1, n=0, Reynolds-stress-driven poloidal f low in the absence of significant flow damping. Calculations in which the p oloidal flow was heavily damped show that sub-Alfvenic, sheared toroidal fl ows have a minimal effect on weakly coupled, localized instabilities. (C) 1 999 American Institute of Physics. [S1070-664X(99)00603-5].