Speaker
Description
Transient events such as ELMs, sawteeth and H-L back-transitions are inevitable in some form in next step reactor scale tokamaks. Such transients lead to heat fluxes on divertor target tiles which risk cracking and further damage [1, 2]. Mitigation of these fast transient heat fluxes has been shown to be possible in part by increasing neutral gas pressure in the detachment cloud [3, 4] and by impurity seeding in the scrape off layer [5]. Demonstration and advanced understanding of buffering capability for small transients would facilitate the consideration of small ELM regimes for future plasma scenarios.
In this work progress on analysis of transient buffering by detachment and impurities at MAST-U is shown. Ultrafast divertor spectroscopy has been utilised to measure how the speed of burn-through of the detachment front changes with detachment pressure, transient energy and burn-through state. These measurements are compared to SOLPS-ITER simulations of transient burn-through speeds. Further attempts are made to measure the atomic and molecular neutral density in the detachment cloud which is expected to relate to its buffering capacity.
Recent studies on MAST-U are presented which show the effectiveness of nitrogen seeding in buffering transients in a tightly baffled Super-X divertor, with attempts to quantify the reduction in heat flux achievable without significant degradation of core performance. The buffering of ELM and sawteeth instabilities is compared, as well as the buffering performance of the Super-X compared to the conventional divertor.
[1] Eich et al., Nucl. Mater. Energy, 12 84-90, (2017)
[2] Linke et al., Nucl. Fusion, 51 073017, (2011)
[3] Federici et al., Nucl. Fusion, 64 126068, (2024)
[4] Flanagan et al., Nucl. Fusion, 65 116031, (2025)
[5] Komm et al., Nucl. Fusion, 63 126018, (2023)
Acknowledgements
This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) (Grant Number EP/S022430/1) and has been part-funded by the EPSRC Energy Programme (Grant Number EP/W006839/1).
This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No. 101052200—EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.