Observations of central toroidal rotation in ICRF heated Alcator C-Mod plasmas

Impurity toroidal rotation has been observed in the centre of Alcator C-Mod ion cyclotron range of frequencies (ICRF) heated plasmas, from the Doppler shifts of argon X lav lines. Rotation velocities greater than 1.2 x 10(7) cm/s (omega = 200 krad/s) in the co-current direction have been observed in H mode discharges that had no direct momentum input. There is a correlatio n between the increase in the central impurity rotation velocity and the in crease in the plasma stored energy (confinement enhancement), induced by IC RF heating, although other factors may be at play. The toroidal rotation ve locity is highest near the magnetic axis, and decreases with increasing min or radius. A radial electric field of 300 V/cm at r/a = 0.3 has been inferr ed from the force balance equation. The direction of the rotation changes w hen the plasma current direction is reversed, remaining co-current. Impurit y toroidal rotation in ICRF heated plasmas is in the direction opposite to the rotation in ohmic L mode plasmas; co-current rotation has also been obs erved during purely ohmic H modes. When the ICRF heating is turned off, the toroidal rotation decays with a characteristic time of order 50 ms, simila r to the energy confinement time, and much shorter than the calculated neoc lassical momentum damping time.