Nonlocal effects in negative triangularity TCV plasmas

Abstract

Global gradient driven gyrokinetic simulations performed with the Gyrokinetic Electromagnetic Numerical Experiment (GENE) code are used to investigate Tokamak à configuration variable (TCV) plasmas with negative triangularity. Considering limited L-mode plasmas, the numerical results are able to reproduce the actual transport level over a major fraction of the plasma minor radius for a plasma with δ LCFS = − 0.3 and its equivalent with standard positive triangularity δ. For the same heat flux, a larger electron temperature gradient is sustained by δ < 0 , in turn resulting in an improved electron energy confinement. In agreement with the experiments, a reduction of the electron density fluctuations is also seen. Local flux-tube simulations are used to gauge the magnitude of nonlocal effects. Surprisingly, very little differences are found between local and global approaches for δ > 0 , while local results yield a strong overestimation of the heat fluxes when δ < 0 . Despite the high sensitivity of the turbulence level with respect to the input parameters, global effects appear to play a crucial role in the negative triangularity plasma and must be retained to reconcile simulations and experiments. Finally, a general stabilizing effect of negative triangularity, reducing fluxes and fluctuations by a factor dependent on the actual profiles, is recovered.

Effect of triangularity on ion-temperature-gradient-driven turbulence

“…The DIII-D and TCV experiments have explored negative triangularity configurations, achieving improved confinement and lower fluctuation levels relative to their positive triangularity counterparts [9][10][11][12][13][14] . These observations have been reproduced in gyrokinetic and gyrofluid simulations [14][15][16][17][18][19] .…”

A brief history of negative triangularity tokamak plasmas

“…Indeed, while the experimental results reported in [157] reconstructed a uniform reduction of the electron heat diffusivity across the plasma cross section, non-linear local gyro-kinetic modeling work [163,171] recovered the experimental results only in the outer part of the cross section, consistent with the local value of triangularity quickly decreasing towards the magnetic axis. While it was immediately realized that such discrepancy hinted at global effects, such impact has only been very recently demonstrated using gradient driven global non-linear gyro-kinetic simulations, which found quantitative agreement across a large fraction of the minor radius between predicted non-linear fluxes and the corresponding experimental values derived from a power balance analysis [177]. In that work it was also concluded that global effects appeared important for the δ < 0 case but not for the δ > 0 case, for which flux-tube simulations yielded adequate results.…”

Interplay between magnetic shear and triangularity in ion temperature gradient and trapped electron mode dominated plasmas

“…2009; Merlo et al. 2015, 2019, 2021; Duff et al. 2022) and the edge-Scrape Off Layer physics (Riva et al.…”

“…Although there is an increasing amount of experimental evidence from several machines corroborating the beneficial effects of negative triangularity on confinement (Camenen et al 2007;Fontana et al 2017;Huang & Coda 2018;Marinoni et al 2019;Coda et al 2021), as well a large number of numerical studies based on those plasmas, analysing both the core (Marinoni et al 2009;Merlo et al 2015Merlo et al , 2019Merlo et al , 2021Duff et al 2022) and the edge-Scrape Off Layer physics (Riva et al 2017;Laribi et al 2021), an investigation of the interplay of negative triangularity with other plasma parameters is missing, or at least not complete. The goal of this paper is thus to attempt to partly fill this gap via gyrokinetic modelling.…”