Speaker
Description
The MAST Upgrade super-X divertor [1,2] creates ideal conditions for detachment with a long connection length and high neutral pressure in the divertor region. While results have shown the super-X strongly mitigates heat flux during steady state plasma conditions, the survivability of the divertor during transient events is a key requirement for future plasma devices[3]. Although large transients such as ELMs are not expected to be tolerable in future devices, certain types of transients, such as H-L back transition, are unavoidable and the ability to tolerate slightly larger transients may open up useful operational regimes.
The impact of transients in the Super-X configuration has been investigated on MAST-U using advanced diagnostics, focused experiments and improved modelling. An Ultra Fast Divertor Spectroscopy providing fast Fulcher measurements of the detachment front position has offered new insight into what is occurring during transients. This has been combined with fast Infra-Red camera data, divertor Thomson scattering at the target and camera imaging. Dedicated experiments have been performed on both ELMs and Sawteeth transients, scanning neutral deuterium gas pressure, performing nitrogen seeding in the Super-X chamber and using X-point Target Configuration to mitigate the heat flux, as well as dedicated comparisons with conventional divertor configurations.
The experimental results have quantified the dependence of the buffering on transient energy and neutral deuterium gas pressure. An approximately linear increase in peak power flux is observed with transient energy, or reducing neutral gas pressure, up to a certain point where it enters a second regime with larger increase in target heat flux. It may be that the second regime is where the neutral inventory has been depleted and after which point additional energy causes proportionately greater increase in heat flux. Comparing Super-X with the conventional divertor has shown reduced heat flux in line with the improved geometry of the Super-X divertor. Very strong mitigation of transients with Nitrogen gas have also been demonstrated albeit with some impact on core performance, offering a promising path forward using impurities to mitigate transients. The heat flux from transients greatly increase Te and ne in the divertor. These increases have been captured in Thomson scattering data and are compared with modelling in the ReMKiT1D framework[4]. This modelling has shown that recycling plays a key role in understanding the transient heat flux.
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). It has also been supported by the EPSRC Energy Programme (grant number EP/W006839/1).
References
[1] Leonard, A. W, Plasma Phys. Control. Fusion 60 (2018) 044001
[2] J. Harrison et al., “Overview of the mast upgrade super-x divertor”, Nuclear Materials and Energy 12, 1072–1076 (2017).
[3] G. Federici et al., “Plasma-material interactions in current tokamaks and their implications for next step fusion reactors”, Nuclear Fusion 41, 1967–2137 (2001)
[4] S. Mijin et al, “ReMKiT1d: a framework for building reactive multi-fluid models of the tokamak scrape-off layer with coupled electron kinetics in 1d”, Computer Physics Communications 300, 109195 (2024).