X-ray observations of central toroidal rotation in ohmic Alcator C-Mod plasmas

Impurity toroidal rotation has been observed in the centre of ohmic plasmas in the Alcator C-Mod tokamak from the Doppler shifts of argon and molybdenum X ray lines. The rotation is highest (similar to 6 x 10(6) cm/s) in the early portion of the discharges, when the loop voltage is highest and the electron density is lowest, and then typically settles to values less than or equal to 2 x 10(6) cm/s during the steady state period. The impurity rotation is in the same direction as the electron toroidal drift, opposite to the plasma current, and reverses direction when the plasma current direction is reversed. Molybdenum and argon ions rotate with the same velocity. These observations are in qualitative agreement with neoclassical theory.