Effects of Geometric Curvature and Weak Magnetic Shear on the Ion-Temperature-Gradient Instability Near the Magnetic Axis in a Tokamak

Global gyrokinetic simulation of the ion temperature gradient mode shows that the radial electric field, E_{r} well, upshifts the critical temperature gradient near the magnetic axis, in the weak but not in the strong magnetic shear (s) configuration. The geometric curvature effect almost cancels/doubles the E×B shear in the inner/outer half of the near-axis E_{r} well, so that the E_{r} well suppresses the modes in the outer region but does not in the inner region, where the weak-s effect is necessary to suppress the modes. This new finding unravels the formation mechanism of the internal transport barrier in the weak central s discharges.