Observations of impurity toroidal rotation suppression with ITB formation in ICRF and ohmic H mode Alcator C-Mod plasmas

Co-current central impurity toroidal rotation has been observed in Alcator C-Mod plasmas with on-axis ICRF heating. The rotation velocity increases wi th plasma stored energy and decreases with plasma current. Very similar beh aviour has been seen during ohmic H modes, which suggests that the rotation , generated in the absence of an external momentum source. is not mainly an ICRF effect. A scan of ICRF resonance location across the plasma has been performed in order to investigate possible influences on the toroidal rotat ion. With a slight reduction of toroidal magnetic field from 4.7 to 4.5 T a nd a corresponding shift of the ICRF resonance from r/a = -0.36 to -0.48, t he central toroidal rotation significantly decreased together with the form ation of an internal transport barrier (ITB). During the ITB phase, electro ns and impurities peaked continuously for \r/a\ less than or equal to 0.5. Comparison of the observed rotation and neoclassical predictions indicates that the core radial electric field changes from positive to negative durin g the ITB phase. Similar rotation suppression and ITB formation have been o bserved during some ohmic H mode discharges.