The role of rotation in the H-mode transition in different magnetic configurations and anomalous momentum transport in Alcator C-Mod plasmas with no momentum in

Full text: Anomalous momentum transport has been observed in Alcator C-Mod tokamak plasmas. Following the L-mode to EDA H-mode transition in both Ohmic and ICRF heated discharges, the ensuing co-current toroidal rotation velocity, which is generated in the absence of any external momentum source, is observed to propagate in from the edge plasma to the core with a time scale of order of the observed energy confinement time, but much less than the neo-classical momentum confinement time. With the aim of understanding the higher H-mode power threshold observed in plasmas with an upper single null, the role of rotation in the L-H transition has been examined in discharges with different magnetic configurations. In L-mode plasmas, the ambient rotation in upper single null plasmas is found to be substantially more counter-current than in lower single null. With application of ICRF power, the rotation velocity increases in the co-current direction, in proportion to the stored energy increase. The H-mode transition occurs when the velocity reaches a characteristic value, which for the core is near 0 km/s. L-mode plasmas with stronger counter-current rotation consequently have a higher H-mode power threshold. (author)