Magnetic shear due to localized toroidal flow shear in tokamaks

Abstract

We investigate the impact of toroidal rotation on axisymmetric MHD equilibria analytically and numerically, with a focus on the change of the safety factor and magnetic shear induced by the flow. When the toroidal rotation is radially localized by an external momentum source, the pressure due to the flow can change the magnetic configuration and the safety factor significantly, even for subsonic flows. Specifically, when the radial profile of the safety factor is not conserved throughout a discharge, toroidal flow can lead to magnetic shear reversal for the equilibrium magnetic field, and we find that the magnitude of the negative shear is amplified for cross sections with large negative triangularity. This has implications for both magnetohydrodynamic stability and anomalous transport. Furthermore, even when the magnetic flux is conserved and the radial magnetic shear profile does not change, the toroidal flow can modify the local magnetic shear. The poloidal variations of the local magnetic shear due to the flow can stabilize magnetohydrodynamic ballooning modes.