First-principle based predictions of the effects of negative triangularity on DTT scenarios

Abstract

Plasmas with negative triangularity (NT) shape have been recently shown to be able to achieve H-mode levels of confinement in L-mode, avoiding detrimental edge localised modes. Therefore, this plasma geometry is now studied as a possible viable option for a future fusion reactor. Within this framework, a NT option is under investigation for the full power scenario of the Divertor Tokamak Test (DTT) facility, under construction in Italy, with δtop = −0.32/δbottom ≃ 0.02 top/bottom triangularity values at the separatrix. The transport properties of this scenario are studied in this work. Gyrokinetic GENE simulations and integrated modelling using ASTRA
with the quasi-linear model TGLF have been performed. The emerging picture from the ASTRA-TGLF runs with boundary conditions at ρtor = 0.94 is that, in the L-mode NT option, the larger peaking of the kinetic profiles in the edge region is not sufficient to recover the loss of the PT H-mode pedestal, and reach similar central temperature values. Two additional shapes are also considered, obtained by flipping the triangularity of the scenarios, to single out the effect of the triangularity sign. A negligible ‘direct’ effect of the triangularity is
found for the L-mode, while a small beneficial effect is observed for the H-mode. The ASTRA-TGLF results are validated by GENE and TGLF stand-alone at two selected radii. GENE shows ITG dominant micro-instability and explains the small beneficial effect of the NT for the H-mode as due to a strong reduction of the heat fluxes, when reversing the triangularity, with a relatively high Ti stiffness. An improvement of the predicted performances of the NT DTT scenario could come from ρtor ≳ 0.9, as indicated by some recent experiments at TCV and AUG.

Experiments and modelling of negative triangularity ASDEX Upgrade plasmas in view of DTT scenarios

“…TGLF predicts ITG modes as the dominant unstable drift for most of the radial points, as predicted for the DTT full power scenario in both δ configurations [30,35]. A numerical test by changing only the shape is done to discern the impact of the triangularity.…”

“…The turbulent transport is calculated by trapped-gyro-Landaufluid (TGLF), a quasi-linear (QL) electromagnetic gyro-fluid model with shaped flux surfaces, using the SAT2 saturation rule. Predictions of the QL model TGLF in NT plasmas were already compared with the more precise gyrokinetic simulations in previous studies [30], finding a good agreement between the two approaches, making TGLF suitable for the study of NT geometry.…”

Reducing transport via extreme flux-surface triangularity

“…Validation of gyrofluid-based quasilinear fluxes against experimental transport [19] confirms the applicability of mixing-length models for moderate magnitudes of δ, whereas for extreme δ, it remains to be ascertained whether such quasilinear models can successfully predict fluxes.…”