The dependence of core rotation on magnetic configuration and the relation to the H-mode power threshold 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 the magnetic configuration. For standard discharges with a lower single null and the ion B ? ?B drift downward, and with BT = 5.4?T, IP = 0.8?MA and ne = 1.4 ? 1020?m?3, the core toroidal rotation is measured to be in the range of 10?20?km?s?1 (counter-current). Similar plasmas with upper single null (USN) have significantly stronger counter-current rotation, in the range 30?50?km?s?1. 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?1 occurring over a variation of a few millimetres 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?1; hence higher input power is needed to induce the transition in the USN configuration.