Direction reversals of intrinsic toroidal rotation have been observed in Alcator CMod Ohmic L-mode plasmas following modest electron density or toroidal magnetic field ramps. The reversal process occurs in the plasma interior, inside of the q = 3/2 surface. For low density plasmas, the rotation is in the co-current direction, and can reverse to the counter-current direction following an increase in the electron density above a certain threshold. Reversals from the co-to counter-current direction are correlated with a sharp decrease in density fluctuations with k R ≥2 cm −1 and with frequencies above 70 kHz. The density at which the rotation reverses increases linearly with plasma current, and decreases with increasing magnetic field. There is a strong correlation between the reversal density and the density at which the global Ohmic L-mode energy confinement changes from the linear to the saturated regime.

Several seemingly unrelated effects in Alcator C-Mod Ohmic L-mode plasmas are shown to be closely connected: non-local heat transport, core toroidal rotation reversals, energy confinement saturation and up/down impurity density asymmetries. These phenomena all abruptly transform at a critical value of the collisionality. At low densities in the linear Ohmic confinement regime, with collisionality ν * ≤ 0.35 (evaluated inside of the q=3/2 surface), heat transport exhibits non-local behavior, core toroidal rotation is directed co-current, edge impurity density profiles are up/down symmetric and a turbulent feature in core density fluctuations with k θ up to 15 cm −1 (k θ ρ s ∼ 1) is present. At high density/collisionality with saturated Ohmic confinement, electron thermal transport is diffusive, core rotation is in the counter-current direction, edge impurity density profiles are up/down asymmetric and the high k θ turbulent feature is absent. The rotation reversal stagnation point (just inside of the q=3/2 surface) coincides with the non-local electron temperature profile inversion radius. All of these observations can be unified in a model with trapped electron mode prevalence at low collisionality and ion temperature gradient mode domination at high collisionality.

Ohmic energy confinement saturation is found to be closely related to core toroidal rotation reversals in Alcator C-Mod tokamak plasmas. Rotation reversals occur at a critical density, depending on the plasma current and toroidal magnetic field, which coincides with the density separating the linear Ohmic confinement regime from the saturated Ohmic confinement regime. The rotation is directed co-current at low density and abruptly changes direction to counter-current when the energy confinement saturates as the density is increased. Since there is a bifurcation in the direction of the rotation at this critical density, toroidal rotation reversal is a very sensitive indicator in the determination of the regime change. The reversal and confinement saturation results can be unified since these processes occur at a particular value of the collisionality.