Speaker
Description
In future fusion reactors, the transient heat loads associated with large edge-localised modes (ELMs) are not tolerable, motivating strong interest in regimes with small or suppressed ELM activity. Among these, the Quasi-Continuous Exhaust (QCE) regime has emerged as particularly promising. It is characterised by the absence of large ELMs with a small confinement degradation. The QCE regime is accompanied by a broadening of the scrape-off layer (SOL), which can reduce heat fluxes to the divertor but may increase particle and heat loads on poloidal limiters [1,2].
Most research has focused on the compatibility of QCE with divertor detachment and divertor integrity, while its impact on other plasma-facing components (PFCs), such as limiters, has received less attention. Although several studies have addressed fluxes in the far SOL [5,6], a quantitative estimation of peak limiter heat fluxes in the QCE regime is still lacking. This work aims to address this gap.
Three ASDEX Upgrade (AUG) discharges, ranging from an ELMy H-mode to a QCE discharge achieved through increased gas puffing, have been simulated using the three-dimensional code EMC3-EIRENE [4]. The simulations are validated against experimental data, yielding two-dimensional maps of heat and particle fluxes on limiter surfaces. A peak parallel heat flux of approximately 5.5 MW m⁻² is found on one limiter, corresponding to a peak perpendicular flux of about 1.8 MW m⁻² when accounting for the limiter geometry. This value corresponds to roughly 40% of the peak perpendicular heat load on the divertor target, indicating that first-wall heat loads in the QCE regime are significant.
An estimate of the ITER first-wall heat load has also been performed using a simple analytic model, suggesting that the limiter–separatrix distance may influence the heat and particle fluxes impacting ITER PFCs. To experimentally test this model, scans of the separatrix–wall clearance have been carried out in QCE discharges at AUG, and corresponding modelling activities are ongoing. The goal is to assess the influence of the wall gap on limiter loads and to investigate the role of recycled particles in the formation of the density shoulder and the toroidal asymmetries observed in recent AUG experiments [3].
[1] M.Faitsch et al., Nucl. Mater. Energy (2021)
[2] A.Redl et al., Nucl. Fusion (2024)
[3] B.Tal et al., Nucl. Fusion (2024)
[4] Y.Feng et al., J. Nucl. Mater. (1999)
[5] T.Lunt et al., Plasma Phys. Control. Fusion (2020)
[6] T.Lunt et al., J. Nucl. Mater. (2015)