Abstract Details
Abstracts
Author: Richard Nies
Requested Type: Consider for Invited
Submitted: 2024-03-29 10:47:36
Co-authors: F. Parra, M. Barnes, N. Mandell, W. Dorland
Contact Info:
Princeton University / PPPL
100 Stellarator Rd
Princeton, NJ 08540
USA
Abstract Text:
Ion-temperature-gradient (ITG) turbulence simulations exhibit stationary ZFs [1] and geodesic acoustic modes [2] at large wavelengths, and a previously unnoticed propagating ZF mode at the ion gyroradius scale. This new toroidal secondary mode (TSM) is analytically derived from a generalized secondary instability theory [3] incorporating the radial magnetic drift induced by toroidicity. This theory is in good agreement with gyrokinetic simulations of the secondary instability and the ZF spectrum in nonlinear simulations. Crucially, the turbulent ExB velocity must be larger than the magnetic drift velocity for the TSM to become unstable. Due to the strong ExB shearing produced by the TSM, the turbulence amplitude remains near this stability threshold. Alongside the conjecture of comparable parallel and nonlinear timescales (critical balance) [4], the TSM threshold condition gives an additional balance with the radial magnetic drift timescale, corresponding to the 'grand critical balance' observed in MAST turbulence measurements [5]. The eddies are anisotropic in the plane perpendicular to the magnetic field, which leads to significant differences compared to the original attempt to apply critical balance theory to ITG turbulence [4], e.g. predicting a shallower dependence of the heat flux with the temperature gradient. The new scaling laws are well satisfied in gyrokinetic simulations using the GS2 [6], stella [7] and GX [8] codes. Scalings for the saturated ZF amplitude that reproduce simulation results are also derived.
[1] Rosenbluth, MN & Hinton, FL. (1998). PRL 80
[2] Winsor, N et al. (1968). PoF 11
[3] Rogers, BN et al. (2000). PRL 85
[4] Barnes, M et al. (2011). PRL 107
[5] Ghim, Y-c. et al. (2013). PRL 110
[6] Dorland, W et al. (2000). PRL 85
[7] Barnes, M et al. (2019). JCP 391
[8] Mandell, NR et al. (2022). arXiv:2209.06731
This work was supported by US DOE Contract No. DE-AC02-09CH11466.
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