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 magnetic configuration.For standard discharges with a lower single null and the ion B¢ÖB drift downward, and with B Ì = 5.4 T, I È = 0.8 MA and n = 1.4 ¢ 10 ¾¼ /m ¿ , the core toroidal rotation is measured to be in the range of 10-20 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.