Speaker
Description
This contribution presents first experimental observations on the effect of nitrogen (N2) seeding on the scrape-off layer (SOL) properties of type-I ELMy H-mode plasmas in the TCV device. This work is motivated by the significant uncertainties still persisting in the physical understanding of SOL transport processes in seeded regimes, which represents a crucial aspect for reliable extrapolation to reactor scenarios. Indeed, impurity seeding is among the most attractive solutions for mitigating the heat and particle loads traveling in the SOL region towards material surfaces. The addition of extrinsic impurities leads to significant divertor momentum and power dissipation, thereby detaching the divertor targets and reducing the exhaust fluxes reaching the target plates [1]. Further benefits of seeded impurities have been demostrated on improving particle confinement [2] and buffering or suppressing edge-localized modes [3,4].
Herein, two datasets have been acquired in TCV at respectively low density, high power and high density, medium power. The deuterium (D2) gas puffing and the N2 seeding rates have been varied independently to disentangle their effect on the SOL profiles and transport characteristics. N2-seeded phases are associated with an increase in average effective ion charge, core radiation level and, at high density, a more irregular ELM frequency, possibly indicating core impurity accumulation. Spectrally filtered images of the outer leg show detachment of the CIII emission front from the machine floor during N2 seeding, although limited to inter-ELM phases only. At high density, N2 injection does not correlate with modifications in either the near SOL density gradient length or far SOL shoulder amplitude, in line with previous TCV L-mode results [5]. At low density, higher D2 fueling is associated to shallower SOL density gradients, supporting prior TCV results [6]. Conversely, in the presence of N2 seeding the near SOL density profile becomes considerably steeper and the far SOL shoulder amplitude is distinctly reduced. These findings, as well as the observed differences, make interpretation of the underlying physical mechanisms challenging and call for further experimental investigation, focusing in particular on ionization sources and turbulent fluctuation properties.
[1] P.C. Stangeby (2018) Plasma Phys. Control. Fusion 60 044022
[2] C. Giroud et al (2025) 30th IAEA-FEC (Chengdu, China)
[3] M. Zurita et al (2026) This conference
[4] M. Bernert et al (2021) Nucl. Fusion 61 024001
[5] O. Février et al (2020) Plasma Phys. Control. Fusion 62 035017
[6] A. Stagni et al (2022) Nucl. Fusion 62 096031