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

Abstract

Abstract The paper presents experimental and modelling results of a comparison of negative (NT) and positive
(PT) triangularity ASDEX Upgrade (AUG) discharges using the plasma shapes presently foreseen in the
DTT tokamak, under construction in Italy. This work is part of a broader effort of investigation to understand
whether the good properties observed in NT scenarios in DIII-D and TCV may be extrapolated to
the DTT device and more in general to DEMO future operations. The experimental results have shown a
practical gain of running these AUG plasmas with only ECRH and mixed NBI+ECRH phases in negative
triangularity, even if they access the H-mode. Indeed, the NT electron kinetic profiles recover in all cases
the PT electron pressures inside mid-radius due to reduced transport in the region ρtor = 0.7 − 0.9, while
exhibit lower individual ELM (Edge Localised Mode) energy losses. The ion pressure and expected fusion
performance are comparable in the case of similar densities. Integrated modelling has been performed using
the transport solver ASTRA and the quasi-linear turbulent model TGLF, investigating the transport properties
of these discharges. The modelling reproduces the experiments qualitatively with reasonable accuracy.
Nonetheless, the heat transport in NT cases is partially overestimated. This may be because TGLF uses
the Miller equilibrium, which approximates the magnetic flux surfaces as up-down symmetric. In the case
of these asymmetric NT shapes, the simulated outer surfaces lose part of the tilt with respect to the z-axis,
reducing the upper δ < 0 effect. A numerical test to discern the impact of the geometry by symmetrically
flipping the shape has shown a beneficial effect of the negative triangularity on heat transport.