Long wavelength resistive MHD modes are studied using a three-dimensional stability code and experimentally determined equilibrium profiles of DIII-D H-mode discharges. Calculated magnetic signals at more than two dozen poloidal locations show good agreement with the experimental measurements, both in relative amplitudes and phases, when the vacuum vessel is treated as conducting and its exact non-circular shape is taken into account. Simulations with a simple finite Larmor radius model suggest that the prevailing mode structure of the pressure driven resistive mode might change from short wavelength to long wavelength (from toroidal mode number n = 3 to n = 1) as the plasma pressure is increased, consistent with experimental observations.
Characteristics of MHD instabilities for high beta plasmas in inward shifted LHD configurations