C-MOD REVIEW: Spontaneous Toroidal Rotation in Alcator C-Mod Plasmas with No Momentum Input

Spontaneous toroidal rotation of impurity ions has been observed in the core of Alcator C-Mod plasmas with no external momentum input.The magnitude of the rotation ranges from -60 km/s (counter-current) in limiter L-mode discharges to +140 km/s (co-current) in ICRF heated H-mode plasmas.The core rotation in L-mode plasmas is generally counter-current and is found to depend strongly on the magnetic topology; in near double null discharges, the core rotation changes by 25 km/s with a variation of a few millimeters in the distance between the primary and secondary separatrices.In H-mode plasmas, the rotation increments in the co-current direction with the toroidal rotation velocity increase proportional to the corresponding stored energy increase, normalized to the plasma current.These discharges exhibit a positive E r in the core.Immediately following the transition from L-mode into enhanced D α (EDA) H-mode, the co-current rotation appears near the plasma edge and propagates to the center on a time scale similar to the energy confinement time, but much less than the neo-classical momentum diffusion time, indicating both the role of the plasma boundary in the dynamics of the H-mode transition and the anomalous nature of momentum transport.Rotation velocity profiles are flat in EDA H-mode plasmas and centrally peaked for ELM-free Hmodes, demonstrating the effects of an inward momentum pinch.In EDA H-mode discharges that develop internal transport barriers (ITBs), the core toroidal rotation inside of the barrier foot is observed to drop on a time scale similar to the core pressure profile peaking (100s of ms), indicating a negative E r well in the core region.