Local threshold conditions and fast transition dynamics of the L-H transition in Alcator C-Mod

Edge profiles during the L-H transition and pedestal evolution in the Alcator C-Mod tokamak have been measured with high spatial and time resolution. For input power near the threshold, periodic 'dithering' cycles are seen, and the sustained transition occurs in a series of steps that appear related to this oscillatory behaviour. Even at a higher power, there is evidence of non-smooth T_e evolution, and the pedestal T_e shows a double break-in-slope at the transition. Calculations with a fluctuation-shear flow model, for parameters typical of this experiment, reproduce much of the observed behaviour. Profiles just before the L-H transition, averaged over steady or dithering periods, are compared with an analytic criterion based on shear suppression by zonal flows (Guzdar P N et al 2002 Phys. Rev. Lett. 89 265004). The experimental values of T_e/√L_n are about 50% below the theoretical threshold, for a range of B_T.