Global resistance MHD calculations for high-beta DIII-D divertor tokamak

Resistive MHD phenomena at high pressure or high current may cause magnetic fluctuations and determine various discharge limitations for tokamaks. The linear and nonlinear resistive MHD phenomena for finite-β (Β is the ratio of plasma pressure to toroidal magnetic field energy) noncircular tokamak equilibria are studied using a 3-D initial value code CART. This code, employing Cartesian coordinate mesh, is particularly suited for treating noncirvular shapes, updown asymmetry, open field lines and a magnetic separatirix, all of which characterize realistic DIII-D single-null divertor discharges with high pressure or high current. More than two dozen magnetic signals calculated from inferred profiles show a good agreement with the measurements both in relative amplitudes and phases when the vacuum vessel is treated as conducting and its exact noncircular shape is taken into account. With a simple modeling of finite Larmor radius effect, these calculations suggest that the prevailing mode structure of the pressure-driven resistive mode might change from short to long wavelength (namely, from toroidal mode number n = 3 to n = 1) as the plasma pressure is increased, in accordance with experimental observation. The line-typing effect in terms of reduced resistivity in open field lines and the localized change in the interior resistivity due to local heating are found to have some weak effect on these global resistive MHD modes for the DIII-D plasmas. The effects of a magnetic separatrix and its x-point location are also examined