Amanda E. Hubbard
Published: 2003
Total Pages: 36
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Edge profiles during the L-H transition and pedestal evolution on 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 which appear related to this oscillatory behaviour. Even at higher power, there is evidence of non-smooth Te evolution and 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, Phys. Rev. Lett. 89, 265004 (2002).]. Experimental values of ... are about 50% below the theoretical threshold, for a range of BT.