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 $\mathit{E}\times \mathit{B}$ 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.

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• Magnetic confinement fusion
• Fusion reactors
• First-principles calculations in plasma physics
• Perturbative methods