Effects of Magnetic Shear on Toroidal Rotation in C-Mod Plasmas with LHCD

Application of lower hybrid current drive (LHCD) in Alcator C-Mod plasmas can induce both co- and counter-current directed changes in toroidal rotation, depending on the core q profile. For discharges with q$_{\mathrm{0}}$ \textless~1, rotation increments in the counter-current direction are observed. If the LH driven current is sufficient to suppress sawteeth and increase q$_{\mathrm{0}}$ above unity, the core toroidal rotation change is in the co-current direction. This observation unifies the results from several tokamaks. This change in sign of the rotation increment is consistent with a change in sign of the residual stress (the divergence of which constitutes an intrinsic torque that drives the flow) through its dependence on magnetic shear.