EFFECTS OF A POLOIDAL DIVERTOR SEPARATRIX ON RESISTIVE MAGNETOHYDRODYNAMIC INSTABILITIES IN A TOKAMAK

A computer code package has been developed to simulate the linear and nonlinear evolution of long-wavelength resistive magnetohydrodynamic ( MHD) instabilities in a four-node poloidal divertor tokamak (e.g., Wis consin Tokapole II-[Nucl. Fusion 19,1509 (1979)]). Distinguishing feat ures of this package include the use of a full set of three-dimensiona l (3-D) nonlinear resistive MHD equations and the inclusion of the div ertor separatrix and the plasma outside the divertor separatrix in the computational domain. The present numerical results suggest that the plasma current outside the divertor separatrix tends to linearly stabi lize the resistive MHD instability dominated by the m = 2, n = 1 mode, and, to a lesser extent, that dominated by the m = 1, n = 1 mode. (He re, m and n are poloidal and toroidal mode numbers, respectively.) How ever, the nonlinear evolution of the m = 1, n = 1 dominant instability is not significantly affected by the divertor configuration; the m = 1, n = 1 island is shown to reconnect totally by developing a large re gion of magnetic stochasticity. Hence, the cause of the partial reconn ection observed in Tokapole II seems to lie beyond the scope of the cl assical resistive MHD model.