May 8-10

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Author: Augustus A Azelis
Requested Type: Poster
Submitted: 2023-03-31 16:45:47

Co-authors: A. A. Azelis, P.W. Terry, P.-Y. Li

Contact Info:
University of Wisconsin - Madison
1150 University Ave
Madison,   53706
United States of Ame

Abstract Text:
Intermittency in the turbulent fluctuations and transport associated with the Dimits regime of toroidal ion temperature gradient driven turbulence are analyzed through probabilistic, combinatoric, and statistical closure based methods. The Dimits regime is a common feature of gyrokinetic simulations that describes a range of driving-temperature-gradient values for which transport is suppressed at and above the threshold of linear instability. The region of heat flux suppression is further characterized by the presence of a significant non-Gaussian tail in the turbulent fluctuation probability distribution function (PDF). While the increase in the critical gradient above the threshold predicted by linear theory has recently been attributed to resonance in mode coupling, the intermittent behavior of fluctuations was not included in that analysis. In this work, a series of statistical techniques is employed to connect the variation in both linear and nonlinear fluctuation evolution as a function of temperature gradient to the kurtosis associated with the non-Gaussian PDF tail. In a probabilistic and combinatorial sense, consideration of all possible manners in which the system may receive energy from the gradient and redistribute it across the Fourier domain should yield illumination into the underlying statistics of the energetics and ultimately transport. In addition, the tendency of the system to prefer certain channels over which to non-linearly transfer energy can be directly inferred via spectral and cumulant growth rates obtained from weak turbulence closures. An explanation of the physics driving intermittency would provide further insight into the nature of transport suppression in the Dimits regime, which allows reactors to operate at lower ratios of transport to temperature gradient than if the heat flux more closely tracked the instability growth rate.

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