It is shown that neoclassical theory explains quite well the origin of the co-current toroidal rotation velocity measured in the core of stationary Alcator C-Mod edge localized mode (ELM)-free Ohmic high confinement (H)-mode discharges. Both edge and core toroidal rotation velocity profiles are determined to a good approximation by the edge ion temperature and density pedestals, where the gradients are large and the plasma is in the high collisionality regime. Under these conditions, the predicted radial electric field profile is similar to those measured in the DIII-D tokamak whereas the usual expression for the poloidal velocity is modified by finite Larmor radius (FLR) effects. Over the entire plasma cross section, the expression of the toroidal velocity can approximately be cast as the product of a dimensionless non-local functional of the pedestal normalized profiles T i (r)/T i (r inf) and N i (r)/N i (r inf) with powers of the plasma density, temperature, safety factor and magnetic field at the pedestal inflexion point r inf provided the FLR related corrections are independent of the latter parameters. The collapse of the core toroidal rotation velocity when either an internal transport barrier forms (that leads to impurity accumulation), or the plasma experiences a transition from the H-to the low confinement (L)-mode, or ELMs appear, and the spin up at the L-H transition are also explained. In the edge region, power balance is consistent with the prediction from subneoclassical ion energy transport theory at high collisionality. The role of charge exchange neutrals is discussed and the critical density above which they are expected to noticeably slow down the rotation is estimated. The toroidal velocity gradient predicted by theory at the edge of the ELM-free Ohmic H-mode discharge mainly under study (q s = 3.4) is near the onset value for the Kelvin-Helmholtz (K-H) parallel velocity shear (PVS) instability; this result is very interesting since a transition from ELM-free to enhanced D α (EDA) H-modes occurs at q ∼ = 3.5-4; the PVS K-H instability appears to have the characteristics of the 'quasi-coherent' mode that is present in all EDA plasmas, but not in ELM-free H-modes.

Alcator C-Mod [I. H. Hutchinson, et al., Phys. Plasmas 1 , 1511(1994] has operated a High-confinement-mode (H-mode) plasma together with a dissipative divertor and low core Z eff . The initially attached plasma is characterized by steady-state enhancement factor, H ITER89P [P. N. Yushmanov, et al., Nucl. Fusion 30, 1999(1990], of 1.9, central Z eff of 1.1, and a radiative fraction of ~50%. Feedback control of a nitrogen gas puff is used to increase radiative losses in both the core/edge and divertor plasmas in almost equal amounts. Simultaneously, the core plasma maintains H ITER89P of 1.6 and Z eff of 1.4 in this nearly 100% radiative state. The power and particle flux to the divertor plates have been reduced to very low levels while the core plasma is relatively unchanged by the dissipative nature of the divertor.