Speaker
Description
Managing extreme heat and particle fluxes to divertor targets remains a major challenge in spherical tokamaks. In this work, we use UEDGE simulations to investigate three interconnected topics relevant to NSTX-U operation: (1) graphite plasma-facing components (PFCs) performance, (2) lithium PFCs vapor shielding and its dependence on upstream plasma conditions, and (3) the impact of snowflake (SF)-type magnetic configurations on divertor heat handling.
The 2D heat-transport model (Wall-Li) is coupled self-consistently with UEDGE to monitor target surface-temperature evolution over discharge times up to 5 s, representative of NSTX-U pulses. Graphite target simulations show a monotonic rise in surface temperature with increasing deposited heat flux; for a wide range of input-power conditions, the surface temperature exceeds the graphite sublimation threshold (≈1400 °C) once peak heat fluxes surpass ≈7 MW/m². This occurs despite enhanced impurity radiation (≈2 MW radiated for a 10 MW input power), indicating that graphite radiation alone is insufficient to accommodate projected NSTX-U heat loads.
This work also explores lithium vapor shielding using a self-consistent coupling between UEDGE and Wall-Li, which models lithium sourcing based on local plasma and surface conditions. A broad parameter scan over input power and core density shows that lithium can form a dense vapor layer near the divertor target that provides strong passive thermal regulation: the surface temperature remains below ≈700 °C even as core power is increased. However, core lithium accumulation increases with surface temperature, with core lithium concentration rising sharply once the surface temperature exceeds ≈600 °C. Increasing core density enhances main ion–impurity friction, which suppresses upstream lithium transport, confines lithium closer to the target, and thereby extends the temperature window over which effective shielding occurs.
The modeled equilibrium for the SF configuration is analyzed using an “umbrella-type” transport model, which approximates the enhanced cross-field transport associated with the churning mode (CM) of plasma convection near the X-points. Power-scan simulations (4–10 MW) indicate that CM-driven transport substantially broadens the divertor heat-flux profiles and increases power deposition to the outer wall, while significantly reducing peak heat loads compared to a uniform radial-transport model applied to the same SF geometry. Analytical variation of the field-line incidence angle up to 90° further shows that the peak divertor heat flux remains below 7 MW/m² even at the highest power-loading conditions.
*This work was performed under the auspices of the U.S. Department of Energy (DOE) Office of Fusion Energy Sciences under contract No. DE-AC52-07NA27344.