Modeling of divertor heat flux limits and lithium vapor shielding in NSTX-U using UEDGE code

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Abstract
Managing extreme heat and particle fluxes to divertor targets remains a critical challenge for high-power spherical tokamaks such as NSTX-U. In this work, UEDGE simulations coupled self-consistently with a two-dimensional wall transport solver (Wall-Li) are used to investigate two closely related issues important for NSTX-U operation: (1) plasma-surface interactions of graphite plasma-facing components (PFCs) under increasing input power for standard and snowflake (SF) divertors and (2) lithium PFC vapor shielding and its dependence on upstream plasma conditions for a standard lower single null divertor with a 5 mm layer of lithium on the graphite tiles. Simulations with graphite target PFCs show a monotonic increase in surface temperature with increasing heat flux incident on the target plates. For a broad range of operating conditions, surface temperatures exceed the graphite sublimation temperature ( > 1200 ∘ C , when graphite starts massively sublimating) once peak heat fluxes exceed ∼ 7 MW m − 2 . This occurs despite enhanced carbon radiation (up to ∼ 2.5 MW for a 10 MW input-power case), indicating that carbon radiation alone is insufficient to dissipate NSTX-U divertor heat loads with graphite PFCs. A SF-minus divertor configuration is analyzed to assess divertor heat handling on the secondary strike point (SP#2) under conditions where both magnetic field-line incidence and cross-field transport are uncertain. A poloidally non-uniform transport model is employed to approximate the enhanced cross-field transport associated with churning-mode (CM) plasma convection in the SF zone. Simulation results indicate that the heat flux to SP#2 is dominated by enhanced CM transport rather than parallel conduction, leading to increased deposited heat flux with increasing CM strength and incidence angle. Lithium vapor shielding is investigated using a self-consistent model in which lithium sourcing depends on local plasma and surface conditions. The model demonstrates that lithium vapor shielding provides passive thermal cooling, maintaining surface temperatures below ∼ 700   ∘ C even as the core power increases. However, lithium accumulation upstream increases once surface temperatures exceed ∼ 600   ∘ C. Increasing the core density augments the drag force on impurity ions due to the parallel plasma flow in the scrape-off layer, confining lithium closer to the target and thereby extending the operational window for effective vapor shielding. Overall, these results highlight the necessity of (1) thick lithium layers to prevent strike-point depletion, (2) active cooling mechanisms (such as fast-flowing liquid lithium) to suppress excessive surface temperatures and evaporation, as lithium operation is constrained by surface temperature, and/or (3) deuterium gas puffing near the divertor to further increase SOL flow and density, leading to higher frictional drag on lithium impurities.
Funding Information
  • Lawrence Livermore National Laboratory (DEAC52- 07NA27344)

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