Abstract Details
Abstracts
Author: Taweesak Jitsuk
Requested Type: Poster
Submitted: 2023-03-31 16:39:48
Co-authors: P. W. Terry
Contact Info:
University of Wisconsin-Madison
1150 University Ave
Madison, WI 53706
USA
Abstract Text:
The saturation of ion temperature gradient (ITG) turbulence predominantly occurs by transferring energy from unstable modes to stable modes through an intermediary mode that depends on the instability branch. For toroidal ITG, the intermediary mode is the zonal flow, while for slab ITG, it is a marginally stable mode. This distinction is also observed in the quasi-axially-symmetric and quasi-helically-symmetric configurations of stellarator plasmas, with zonal-flow-mediated saturation in the former and marginal-mode-mediated saturation in the latter. However, the factors determining intermediary mode selection remain unclear. This study aims to identify such factors by assessing nonlinear-coupling quantities, primarily the triplet correlation time and spatial mode overlap of fluctuations. A three-field fluid model is employed, providing access to unstable, stable, marginal, and zonal modes both in toroidal and slab ITG. The mode frequencies are crucial for the triplet correlation time calculation. Numerical results show that triplet correlation time involving zonal flows dominates other couplings in both limits. Zonal flows allow frequencies in correlation time to better resonate compared to marginal-modes coupling, suggesting that they are less likely a criterion for intermediary mode selection. Mode overlap involves eigenmode structures under Hermite expansion. Calculation shows that the marginal-coupling overlap in the slab limit is larger than that of zonal coupling, whereas, in the toroidal limit, they are roughly equal. This suggests that marginal modes are more important in the slab limit, but not in the toroidal limit, and vice versa. A systematic analysis of plasma parameters in both limits shows a consistent pattern. The coupling coefficients may also be a significant factor in the selection of the intermediary mode and will be explored in the future. Work supported by DOE grant DE-FG02-85ER53212.
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