Description
Good plasma confinement is essential for harnessing fusion energy.
Negative triangularity (NT) has been shown to reduce turbulent transport, thereby improving confinement.
While the impact of NT on ion temperature gradient (ITG) instabilities remains unclear, both experiments and simulations consistently indicate its beneficial effect on trapped electron mode (TEM) instabilities [1,2].
A full understanding of the physical mechanism underlying TEM stabilization by NT is crucial for extrapolating towards reactor-relevant regimes.\
In a previous analysis, we have provided the first modeling-based explanation for the linear stabilization of TEMs by NT, obtained using a reduced semi-analytical model [3,4]. It identifies the ballooning character of the instability—i.e. its poloidal localisation on the low-field side—as the dominant stabilization mechanism, since it puts strong weight on the stabilizing contribution from deeply trapped electrons in NT configurations.\
While this analysis provides deep physical insight into the stabilization mechanism, the resulting linear stabilization remains weak. Therefore, in this contribution, we explore nonlinear effects via flux-driven global simulations with the GYSELA code [5] in TEM-dominated turbulence. These simulations capture turbulence self-organization from eddy size to large-scale flows. In TCV-like highly shaped configurations, turbulent heat fluxes are found to be lower in NT than in PT in the outer half of the tokamak ($0.5<r/a<1$), in qualitative agreement with experiments. Zonal flows (ZF) are likely responsible for the reduced heat fluxes, with higher drive and shear in NT than in PT. Contributions from electric and diamagnetic Reynolds stresses are of similar magnitude and in phase, similarly to ITG turbulence [6]. A detailed analysis shows that the phase-shift is responsible for the observed difference in ZF-drive for NT versus PT configurations.\
In order to critically evaluate the impact of boundary conditions , which are particularly important in this type of study since geometrical effects are most sensitive at the edge, simulations with a poloidally homogeneous and a more realistic limiter-like configuration in the scrape-off layer are compared. This highlights the key role played by the well of the radial electric field in the vicinity of the separatrix, consistently with recent experimental observations [7].
[1] Y. Camenen et al. In: Nuclear fusion 47.7 (2007), p. 510.
[2] A. Marinoni et al. In: Plasma Physics and Controlled Fusion 51.5 (2009), p. 055016.
[3] X. Garbet et al. In: Nuclear Fusion 64.10 (2024), p. 106055.
[4] L. De Gianni et al. https://hal.science/view/index/docid/5477865. submitted. 2025.
[5] V. Grandgirard et al. In: Computer physics communications 207 (2016), pp. 35–68.
[6] Y. Sarazin et al. In: Plasma Physics and Controlled Fusion 63.6 (2021), p. 064007.
[7] S. Rienaecker et al. In: Nuclear Fusion 66 (2026), p. 014002.