April 4-6

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Author: Taweesak Jitsuk
Requested Type: Poster
Submitted: 2022-03-04 16:37:20

Co-authors: P. W. Terry

Contact Info:
University of Wisconsin-Madison
1150 University Ave.
Madison, WI   53706

Abstract Text:
Simulations of ITG turbulence in various plasma configurations show that saturation occurs by transfer of energy from an unstable mode to a stable mode with different intermediary modes: i.e., the zonal flow in the toroidal ITG regime and a marginally stable mode in the slab regime. This is consistent with a study from a three-field fluid model where the assumption of parallel-flow decoupling under strong ballooning reduces the equations to two fields for the toroidal limit, eliminates the marginal mode, and leaves the zonal flow as the only dominant transfer channel. However, if decoupling is not assumed, both the marginal mode and the zonal flow coexist, allowing investigation of the selection rules that govern the preferred channel for each regime. Here, we evaluate the saturation-governing factors in the nonlinearities of the three-field fluid equations in the ballooning representation. These include the eigenmode overlap, the triplet correlation time, and the mode coupling coefficients. The eigenmode overlap assesses the degree of spatial overlap of coupled eigenmodes in ballooning angle. Because eigenmode structures are sensitive to magnetic curvature and shear, and the wavenumber constraint on kx in ballooning space imposed by mode coupling k''=k-k', these parameters are varied to expose selection rules. Non-zero kx-values in interacting triads fix the offset of modes from the outboard midplane, making mode overlap a significant factor on the selection rules. The triplet correlation time determines the duration for modes to interact nonlinearly. Calculations with physical parameters from plasma devices ranging from slab to toroidal limits show that the triplet correlation times are comparable across the devices. The mode coupling coefficients are prefactors of the nonlinearities, determined by k-values in the triads. They also play crucial roles in selection of channel for saturation. These factors help to understand the prior simulation results.