Mitigation of Sawtooth transient heat flux in the MAST-U Super-X Divertor with Deuterium and Nitrogen
The MAST Upgrade Super-X divertor protects plasma-facing components from heat fluxes during steady-state operation and transient events. This paper examines heat loads from sawtooth events with energies of ΔWsawtooth ≈ 2–9 kJ in lower single null plasmas. The impact of deuterium and nitrogen gas pressures on mitigating these transients is investigated. During the deuterium gas scan up to ΔWsawtooth = 6kJ, although the transient energy is the dominant factor in determining the peak heat flux, the neutral pressure is also shown to play a role. Together, the variation in transient energy and deuterium gas pressure accounts for 70% of the variation in peak heat fluxes at the target as measured by infrared thermography and 80% of the “Burn Through Factor“ as inferred by D2 Fulcher band spectroscopy. Larger sawtooth transients (ΔWsawtooth ≈ 6–9 kJ) fall outside this trend and shows much higher heat fluxes even at the highest deuterium pressures. A comparison between Super-X and conventional divertor configurations across a range of deuterium pressures shows that the Super-X configuration experiences significantly lower peak heat fluxes for similar transient energies, consistent with, but not significantly exceeding, expectations based on geometry. Nitrogen gas seeding was applied to transients with energies below ΔWsawtooth ≈ 6kJ. Increasing the nitrogen pressure yields a very large reduction in peak heat flux, about 4.5MW/m2/Pa, leading to complete buffering of sawtooth transients at higher nitrogen pressures. Direct divertor electron temperature and density profiles were obtained during transients along a chord running from the target to 0.4m poloidally from the target, aligned along the separatrix. These indicate quiescent inter-transient temperatures of 1eV, corresponding to strong detachment. The temperatures rise to 6–8eV during sawtooth transients, with profiles showing decreasing Te and increasing ne toward the target. In cases where divertor heat loads exceed q⊥ > 2MW/m², we observe Te along this chord exceeding 10eV. The experimental data on transient heat flux mitigation and profiles obtained are compared to modelling results using a 1D exhaust code in the ReMKiT1D framework. The modelling showed an increase in heat load for transients with energy > 4−6kJ, which is qualitatively similar to the rapid increase in heatload observed for experimental transients with energies > 6kJ. Similar temperature and density profiles to those obtained in experiment were predicted. Themodelling has shown the importance of target recycling. Delaying recycling to 1ms after the transient event, that is a 1ms wall neutral retention time, is found to match well with experimental data.