Cryopump and fuelling location impacts on upstream density and detachment on MAST-Upgrade

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Abstract
A cryopump was installed in the MAST-U lower divertor at the start of its fourth scientific campaign (MU04) to enhance density control, detachment front management, and impurity removal, while expanding the operational space to lower-density regimes. The tightly baffled divertor chamber quasi-isolates the divertor neutral environment from the main chamber, enabling localised neutral pressure (P n ) tuning via divertor fuelling and cryopump operation, with minimal impact on upstream plasma density. MU04 experiments and SOLPS-ITER simulations both confirm that activating the cryopump reduces the outer lower divertor P n by 50-65%, while maintaining line-averaged density (n̄ e ) and midplane P n . The cryopump facilitates lower divertor P n , aiding plasma attachment and drawing the detachment front closer to the target. Modelling indicates a 25-40% increase in upstream density required for rollover onset in both CD and SXD configurations, enabling a broader operational scan from attached conditions through detached to radiative collapse. Divertor fuelling exhibits a low fuelling efficiency (~5% of injected neutrals reach the separatrix), resulting in a weak scaling between upstream separatrix density and divertor neutral pressure (n e,sep ∝P n 0.3 ), consistent with observations on other devices. In contrast, main-chamber fuelling achieves substantially higher efficiency (~15% for LFS and up to ~40% for HFS fuelling) and yields a stronger density scaling (exponent >0.6), reflecting the different plasma–neutral coupling pathways associated with the fuelling location. These findings demonstrate that the combined use of different fuelling locations and cryopumping provides complementary control of upstream density and divertor detachment, with direct relevance for exhaust optimisation in future fusion devices.
Funding Information
  • Engineering and Physical Sciences Research Council (EP/T022159/1)
  • EPSRC Centre for Doctoral Training in Science and Engineering in Arts, Heritage and Archaeology (EP/W006839/1)
  • Euratom Research and Training Programme (101052200 — EUROfusion)

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