Description
To design a divertor that is robust against inevitable transient changes in upstream conditions, it is important to understand the physics that governs detachment burn-through. Specifically, to understand what sets the timescales and degree to which the neutral buffer is both “burned through” (by increased plasma energy) and “re-detached” (by a subsequent reduction in plasma energy). To this effort, we present the first transient simulations of the MAST-U tokamak using the SOLPS-ITER model, operated in time-dependent mode and including time-dependent kinetic neutrals.
Here, simulations are quantitatively compared with observations of transients carried out during the third to fifth experimental campaigns [1,2]. Using a synthetic diagnostic of the Ultra-fast divertor spectroscopy system, the simulations are shown to quantitatively reproduce the transient energy threshold for ELM burn through of ~2-3 kJ on MAST-U [1]. Importantly, we find that the pre-transient neutral inventory alone is too small to explain the threshold; target recycling during the transient acts to significantly boost the neutral inventory and is required within the model to reproduce the experimentally observed threshold. According to preliminary analysis, we are also able to quantitatively reproduce the detachment front speed towards the target during the transient. Further analysis will investigate the impact of the pre-ELM neutral pressure, the divertor configuration (Conventional vs. Super-X) and the molecular reaction rates on the burn-through physics.
[1] J Flanagan et al. 2025, Nucl. Fusion 65 116031 (https://doi.org/10.1088/1741-4326/ae11c5)
[2] R Scannell et al. 2026, submitted to Nucl. Fusion