How does negative triangularity mitigate ITG turbulence and transport?

Journal Article · 30 December 2024 · Nuclear Fusion

DOI: https://doi.org/10.1088/1741-4326/ad9ea0 · OSTI ID:2484213

Singh, Rameswar  ^[1]; Diamond, P. H.  ^[1]; Marinoni, A.  ^[1]

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1. University of California San Diego, La Jolla, CA (United States)

Improved confinement in negative triangularity (NT) experiments is attributed to reduced fluxes driven by micro-turbulence. The physical mechanism of why thermal confinement improves in NT relative to PT is unknown. This study employs gyrokinetic flux tube simulations using the GENE code with local Miller equilibrium to elucidate the physical mechanisms behind the beneficial effects of NT flux surface shapes. The focus is on collisionless ion temperature gradient (ITG) driven turbulence with adiabatic electrons. The kinetic profiles are held fixed across a scan of triangularity values, thus enabling comparisons on a level playing field. The reduced linear growth rates for NT is shown to be due to a reduced eigenmode averaged magnetic drift frequency and a wider, stronger negative local magnetic shear region about the outboard mid-plane. The nonlinear heat flux is lower for NT than that for PT, due to reduced radial correlation length and increased correlation time (τ_c) of fluctuations. These, in turn, are due to a comparatively higher level of self-generated zero-frequency E × B zonal shearing rate ω_E in NT as compared to PT. Though the linear zonal potential residual is lower for NT, the nonlinearly generated E × B zonal shearing rate is higher for NT than for PT. This outcome is linked to the distinctive features of the radial wavenumber spectra of the zonal potential and the zonal shearing rate. The dimensionless parameter ω_Eτ_c is suggested as a figure of merit. This is higher for NT than for PT. Thus, the reduced heat diffusivity for NT is linked to increased ω_Eτ_c. Self-generated temperature corrugations (i.e. zonal temperature gradients) are much weaker than the background mean temperature gradient. Nevertheless, temperature corrugations are more pronounced in NT than in PT.

• Research Organization: University of California San Diego, La Jolla, CA (United States)
• Sponsoring Organization: USDOE; USDOE Office of Science (SC), Fusion Energy Sciences (FES)
• Grant/Contract Number: FG02-04ER54738
• OSTI ID: 2484213
• Journal Information: Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 2 Vol. 65; ISSN 0029-5515; ISSN 1741-4326
• Publisher: IOP PublishingCopyright Statement
• Country of Publication: United States
• Language: English