Description
Negative triangularity (NT) has emerged as a promising alternative to the conventional positive triangularity (PT) shape of tokamak plasmas. Owing to the suppression of large ELMs, combined with outstanding L-mode confinement, NT appears as a suitable candidate for core-edge integration [1,2]. While the fundamental reason for the NT L-mode performance improvement over PT remains unclear, experiments emphasize the role of the NT edge region [3,4]. This contribution investigates the edge radial electric field (Er) and its shear as possibly relevant distinguishing features between the NT and PT edge. The study is based on recent experiments on the TCV tokamak that, for the first time, compare the edge Er profile in matched NT/PT discharges [5]. Measurements using a Doppler backscattering diagnostic [6] reveal an Er well that is deeper in NT than in PT, along with improved L-mode confinement. The stronger edge Er × B shear in NT is observed systematically under Ohmic, NBI, or ECRH heating. It coincides—at equal heating power—with higher edge density and temperature. Notably, a deeper Er well is still observed in NT, even when the auxiliary power in PT is adjusted to match the edge density and temperature profiles of NT. Implications of this decoupling of Er from kinetic profile differences are dicussed. Overall, the results are compatible with edge Er × B shear stabilization as a possible cause for the beneficial L-mode confinement properties of the NT edge. Turbulent transport modelling based on the presented data is underway to elucidate the origin of the differing Er behavior between NT and PT.
[1] K. E. Thome et al. Plasma Physics and Controlled Fus. 2024
[2] O Février et al. Submitted to Nuclear Fusion 2026
[3] A. Balestri et al. Plasma Physics and Controlled Fus. 2024
[4] A. O. Nelson et al. Plasma Physics and Controlled Fus. 2024
[5] S. Rienäcker et al. Nuclear Fusion 2026
[6] S. Rienäcker et al. Plasma Physics and Controlled Fus. 2025