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 Te evolution, and the pedestal Te 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 are about 50% below the theoretical threshold, for a range of BT.
Zonal flow production in the L–H transition in Alcator C-Mod