Turbulence nonlinearities and the L-to-H transition threshold

Istvan Cziegler, UC San Diego, Center for Energy Research

Thursday March 5, 1:00pm, SERF 329

Abstract: Nonlinear transfer processes between large-scale edge flows and the ambient broadband fluctuations have been shown to be significant in the dynamics of turbulence in the edge region of tokamak plasmas. Their effects include spreading power spectrally away from coherent modes and driving linearly stable modes. Recent studies have strongly suggested that zonal flows play a key role in mediating transitions of confinement regimes. In order to predict the thresholds of these transitions, both their direct trigger mechanism and the parametric dependence of nonlinear transfer processes must be studied. 

Transitions from low- to high-confinement on the Alcator C-Mod tokamak are examined in detail with a focus on the interaction between turbulence and the local shear flow. Results clearly show that the drive of the edge zonal flow and the initial reduction of turbulence fluctuation power are consistent with a lossless kinetic energy conversion mechanism, which consequently mediates the transition into H-mode. The edge pressure gradient is then observed to build on a slower (~ 1 ms) timescale, locking in the H-mode state. These results unambiguously establish the time sequence of the transition and thus its trigger as flow organization. 

Since the expected flow damping terms depend on ion collision rates and local safety factor, recent experiments were performed at various values of the plasma current, density and amount of auxiliary heating. Nonlinear interactions between zonal flows and turbulence in L-mode are estimated using bispectral as well as time-resolved methods. Results show that energy transfer from broadband turbulence to slowly evolving flows increases monotonically with cross-field heat flux while showing a clear dependence on plasma current or local safety factor. Taken together, these results delineate the physics preceding the L-H transition. 

Work supported by the U.S. Department of Energy under Award Numbers DE-SC-0008689 and DE-FC02-99ER54512.