Plasma Rotation during Ohmic H-modes in the Alcator C-Mod Tokamak

Core plasma rotation is observed to change from the counterto co-current direction during the transition from low (L) to high (H) confinement mode, in Alcator C-Mod plasmas that are heated purely ohmically.The changes of the toroidal velocities deduced from impurity Doppler measurements and magnetic perturbations associated with "sawteeth" are quantitatively similar.The magnitude of the change is observed to be typically of order 30 km/s.This velocity change, which corresponds to roughly 10% of the sound speed, is consistent with the previously documented scaling for rotation in ICRFheated H-modes.The rotation in this ohmic experiment is obviously not an RF effect but demonstrates unequivocally a transport effect accelerating the plasma.