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FIREFLY: heat load and particle exhaust approximations for rapid evaluation of divertor designs
IOP PublishingAbstract The divertor in a magnetic confinement fusion reactor is an essential
component for power dissipation and particle removal. This article introduces the
FIREFLY package for rapid evaluation of divertor designs based on an extension of
the FLARE code for field line reconstruction from an unstructured flux tube mesh. First, divertor heat
loads are approximated with a simplified heat transport model. Neutralized particles
are then sampled from the resulting distribution, and the EIRENE code is used to
track molecules and atoms in a plasma background while accounting for dissociation,
charge exchange and ionization. Particles are removed on pumping surfaces in order
to estimate the exhaust efficiency for a given divertor geometry. Optimization of the
divertor geometry for more efficient particle exhaust is explored by using W7-X as an
example, and the sensitivity to model parameters for the plasma background in the
proxy calculations is evaluated.
Investigation of Drift Effects in UEDGE Simulations of NSTX-U Edge Plasma With Lithium Divertors
WileyLithium is a low-Z material, and lithium-based plasma-facing components (PFCs) are planned for the National Spherical Torus Experiment Upgrade (NSTX-U) to explore potential benefits for divertor power exhaust and core plasma management. NSTX-U is a medium-sized spherical tokamak with up to 12 MW of auxiliary heating, capable of generating reactor-relevant plasma conditions. This work presents boundary plasma simulations for NSTX-U with lithium PFCs using the UEDGE code, incorporating full magnetic and E ×B drift physics. The simulations show that drifts strongly influence heat and particle transport: they enhance convective transport, broaden the scrape-off layer heat-flux width λ<sub>q</sub> , and reduce the anomalous heat diffusivity χ required to reproduce predicted SOL heat-flux width. E ×B drifts provide poloidal transport, while ∇B (which includes both gradB and curvature) drifts provide radial heat and particle transport. Lithium transport is also affected by drifts, with lithium ions migrating from the outer divertor to the inner divertor through the private flux region (PFR) following the E ×B drifts flow, lowering upstream impurity lithium densities. UEDGE is self-consistently coupled with the Wall-Li model to study plasma lithium PFC interactions depending on the local lithium sourcing based on local plasma conditions and lithium surface temperature. In these simulations, lithium evaporation shows a vapor-shielding effect that reduces divertor heat flux and increases radiative losses once surface temperatures exceed 450°C. This research work provides a first step toward self-consistent modeling of lithium PFCs in NSTX-U, demonstrating the impact of drift-driven plasma transport in SOL and divertor regions.
Assessing plasma face component thermal response to rotating 3D magnetic fields for SPARC tokamak
American Institute of Physics (AIP)Thermal response simulations of plasma-facing components (PFCs) in the SPARC tokamak, performed with the HEAT code, show that three-dimensional (3D) heat loads resulting from stationary n=1 perturbations require highly radiative scenarios, with up to 95% of the power crossing the separatrix (P<sub>SOL</sub>) being radiated, to maintain PFC temperatures within acceptable operational limits, whereas the application of slowly rotating 3D fields substantially reduces the thermal loads. The HEAT module, developed to predict heat loads from non-axisymmetric plasmas, is extended to model time-dependent heat flux patterns generated by rotating 3D fields, and a comprehensive thermal analysis is performed on PFCs subjected to both the maximum and minimum power loads, as well as to rotating heat flux distributions, to evaluate the temperature evolution for varying perturbation amplitudes and rotation frequencies. The extension of this analysis to 3D fields with toroidal mode number n=2 shows that this configuration leads to weaker localized heat flux peaks relative to the n=1 case, enabling safe operation with less than 80% of the power radiated when static 3D fields of low amplitude are applied, while using slowly rotating fields at higher amplitudes. These results indicate that n=2 perturbations are generally less detrimental to divertor power exhaust, emphasizing the strong dependence of divertor power exhaust on the characteristics of the applied 3D fields.
Effects of transient heating events on tungsten plasma-facing materials in a steady-state divertor-plasma environment*
IOP PublishingNearly all heat-pulse tests of plasma-facing materials (PFMs) have been completed in vacuum environments without the presence of a background plasma. Thus, combined effects of thermal transients on materials undergoing plasma exposure need to be explored. Heat-pulse experiments have been conducted in the PISCES-A device using a pulsed laser in a divertor-like plasma background. The results indicate that the erosion of PFMs is enhanced as compared with transient-only or plasma-only experiments, and the threshold energy for material removal by a transient heat pulse in a steady-state plasma background is reduced. There appears to be a minimum ion fluence to the surface needed to cause these effects. Initial experiments with pre-loaded material samples exposed to plasmas and heat pulsed in the lower divertor of the DIII-D tokamak using the Divertor Material Evaluation System indicate that similar effects occur in confinement devices.
A snowflake divertor: a possible solution to the power exhaust problem for tokamaks
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