Observations of Anomalous Momentum Transport in Tokamak Plasmas with No Momentum Input

Anomalous momentum transport has been observed in Alcator C-Mod tokamak plasmas. The time evolution of core impurity toroidal rotation velocity profiles has been measured with a crystal x-ray spectrometer array. Following the L-mode to EDA (enhanced Dα) H-mode transition in both Ohmic and ICRF heated discharges, the ensuing co-current toroidal rotation velocity, which is generated in the absence of any external momentum source, is observed to propagate in from the edge plasma to the core with a time scale of order of the observed energy confinement time, but much less than the neo-classical momentum confinement time. The steady state toroidal rotation velocity profiles in EDA H-mode plasmas are relatively flat and the momentum transport can be simulated with a simple diffusion model. Assuming the L-H transition produces an instantaneous edge velocity source (which disappears at the Hto L-mode transition), the momentum transport may be characterized by a diffusivity, with values of ∼ 0.07 m/s during EDA H-mode and ∼ 0.2 m/s in L-mode. These values are large compared to the calculated neo-classical momentum diffusivities. Velocity profiles during ELM-free H-modes are centrally peaked, which suggests inward momentum convection; the observed profiles are matched with simulations including an inward convection velocity of ∼ 10 m/s. In EDA H-mode discharges which develop internal transport barriers, the velocity profiles become hollow in the center indicating the presence of a negative radial electric field well in the vicinity of the barrier foot.