Pressure driven high-n/high-m tearing modes in low shear regions with high pressure gradients and high resistivity

High-n modes near q = 1 observed in ASDEX Upgrade discharges with high impurity accumulation have been shown to be tearing modes. High-n tearing modes are usually stable; however, owing to high current gradient and low shear they become unstable. The coupling of different poloidal harmonics due to toroidal geometry and plasma shaping supports the mode growth. Changes in the pressure alter the relationship between the various sideband harmonics within one mode even to the extent that its tearing character may be destroyed.