Observations of Toroidal Rotation Suppression with ITB Formation in ICRF and Ohmic H-mode Alcator C-Mod Plasmas

Co-current central toroidal rotation has been observed in Alcator C-Mod plasmas with on-axis ICRF heating. The rotation velocity increases with plasma stored energy and decreases with plasma current. Very similar behavior has been seen during Ohmic H-modes, which suggests that the rotation, generated in the absence of an external momentum source, is not principally an ICRF effect. A scan of the ICRF resonance location across the plasma has been performed in order to investigate possible influences on the toroidal rotation. With a slight reduction of the toroidal magnetic field from 4.7 to 4.5 T and a corresponding shift of the ICRF resonance from r/a = −0.36 to −0.48, the central toroidal rotation significantly decreased in concert with the formation of an internal transport barrier (ITB). During the ITB period, the electrons and impurities peaked continuously for |r/a| ≤ 0.5. Comparison of the observed rotation and neo-classical predictions indicates that the core radial electric field changes from positive to negative during the ITB phase. Similar rotation suppression and ITB formation have been observed during some Ohmic H-mode discharges.