Abstract Details
Abstracts
Author: Augustus A Azelis
Requested Type: Poster
Submitted: 2024-04-12 15:59:44
Co-authors: P.W. Terry, B. Tripathi, P.-Y. Li
Contact Info:
University Of Wisconsin-Madison
1150 University Ave
Madison, 53706
United States of Ame
Abstract Text:
In toroidal ion temperature gradient (ITG) driven turbulence, it remains a challenge to understand heat flux reduction at and above the threshold of linear instability until a sufficiently large driving gradient is imposed---a phenomenon called the Dimits shift. A known but unexplained feature of this regime is the observation of appreciably non-Gaussian turbulent fluctuations and resulting transport, which manifest as burstiness in time series measurements. Preexisting theory for the Dimits shift successfully attributed heat flux reduction to resonance in mode coupling, but this analysis was based on a cumulant-discard method which assumed the fluctuations obeyed quasi-Gaussian statistics. In this work, weak turbulence closures are employed to derive the growth rate of a so-called fourth order cumulant tensor. The expression predicts the conditions under which a septet of interacting fluctuations may bring about departure from Gaussian statistics. Preliminary analysis shows strong cumulant growth near the linear threshold which can be attributed to resonances in triplet correlation times, and nonlinear coupling coefficients. This suggests possible coincidence between the mechanism responsible for heat flux suppression and the inherent non-Gaussian tendencies of the Dimits regime. A statistical mechanics-based model for predicting transport, zonal flow spectra, and intermittency as a function of driving gradient is also briefly presented. Analytical work is compared against results obtained via numerical solution of the reduced two-field fluid model for toroidal ITG driven turbulence.
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