Self-acceleration of a tokamak plasma during ohmic H mode

Core plasma rotation is observed to change from counter direction to co-cur rent direction during the transition from low (L) to high (H) confinement m ode, in Alcator C-Mod plasmas that are heated purely Ohmically and, hence, have no momentum input. The changes of the toroidal velocities, deduced ind ependently from impurity Doppler measurements and from magnetic perturbatio ns associated with sawteeth, agree. The magnitude of the change is consiste nt with the previously documented scaling for rotation in ion cyclotron rf- heated H modes. The rotation in this Ohmic experiment is obviously nor an r f effect but demonstrates unequivocally a transport effect accelerating the plasma.