PSFC/JA-04-33 The Dependence of Core Rotation on Magnetic Configuration and the Relation to the H-mode Power Threwhold in Alcator C-Mod Plasmas with No Momentum Input

The observed toroidal rotation in Alcator C-Mod Ohmic L-mode plasmas has been found to depend strongly on magnetic configuration. For standard discharges with a lower single null and the ion B rB drift downward, and with BT = 5.4 T, IP = 0.8 MA and ne = 1.4 10/m, the core toroidal rotation is measured to be in the range of 1020 km/s (counter-current). Similar plasmas with upper single null have significantly stronger counter-current rotation, in the range of 30-50 km/s. The rotation depends very sensitively on the distance between the primary and secondary separatrices in near double null plasmas, with changes of 25 km/s occurring over a variation of a few millimeters in this distance. Application of ICRF power has been found to increase the rotation in the co-current direction. The transition to H-mode is seen to occur in these standard plasmas when the core rotation reaches a characteristic value, near 0 km/s, hence higher input power is needed to induce the transition in the upper single null configuration.