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Title: Modeling and Physics Design of a Lithium Vapor Box Divertor
Authors: Emdee, Eric
Advisors: Goldston, Robert J
Contributors: Astrophysical Sciences—Plasma Physics Program Department
Keywords: Divertor
Subjects: Physics
Computational physics
Issue Date: 2022
Publisher: Princeton, NJ : Princeton University
Abstract: Diverted operation, where magnetic field lines intersect with plasma facing components (PFCs) far from the main plasma in a region known as the divertor, has long been seen as necessary for any future tokamak fusion device. However, a consequence of this is that the target, the set of PFCs which intersect the field lines, faces extremely high heat flux. In fact, any solid material would be destroyed if faced with unmitigated reactor-level heat fluxes. Divertor detachment, whereby the power of the plasma is dissipated before striking a material surface, is a proposed solution to this problem. This power dissipation is typically achieved by impurity injection. Divertor detachment with medium-Z impurities can result in radiating regions within the last closed flux surface, which has the tendency to reduce plasma performance via degradation of confinement at the plasma edge. The lithium vapor box divertor seeks to detach via near-target lithium evaporation with condensation of the lithium vapor at walls further upstream. A vapor gradient would thus form, with high density lithium vapor at the target, and low density lithium beyond the condensation region. This vapor gradient results in a stable detachment front, and the low-Z lithium cannot form a radiating region within the main plasma. In this thesis, we show Stochastic PArallel Rarefied-gas Time-accurate Analyzer (SPARTA) and Scrape-Off Layer Plasma Simulator (SOLPS) predictions for the effect of a lithium vapor box divertor in a tokamak. Using SOLPS, fuel puffing is found to have a significant effect on the plasma via increases to the bulk ion friction force acting on the lithium plasma fluid. PFC geometry choices are examined and compared with lithium evaporation in the open divertor geometry on the NSTX-U tokamak. Divertor closure is found to have significant benefits in reducing upstream lithium content. In high power cases, where the unmitigated heat flux to the target is found to be 65MW/m$^2$, different closure designs are considered. Single baffling of the divertor is found to have benefits when compared to a slot divertor geometry for both heat flux and upstream lithium content reduction, as well as isolation of the divertor cooling from the outer midplane. The baffled geometry is found to be resistant to flow reversal in the far SOL where main ion flow is weak, thus the baffles eliminate a path for lithium contamination of the main plasma. The baffled system is able to reach sub-5 MW/m$^2$ heat fluxes at the cost of lithium density around 5$\%$ of the electron density at the outer midplane. The results of this study are tested for their sensitivity to choices of transport coefficients, upstream pumping rate, and puffing location. Even when transport coefficients are reduced to provide less particle flow from the core and higher heat flux at the target, sub-10MW/m$^2$ solutions are available to the lithium vapor box from an unmitigated 92 MW/m$^2$. Private Flux Region (PFR) puffing is seen to be more effective at reducing upstream lithium content while Common Flux Region (CFR) puffing is seen to be more effective at heat flux reduction. The efficacy of both puffing locations is increased by increases to the divertor recycling coefficient. Reducing pumping at walls upstream of the baffles improves the effect of the puffs, leading to cases with lower upstream lithium content for less heat flux. Ultimately, predictions for the upstream lithium content depend heavily on several assumptions made in the input to the code. With more conservative transport parameters $n_{Li}/n_e$ around 0.07 could be expected in order to reduce the target heat flux to sub-10MW/m$^2$.
Type of Material: Academic dissertations (Ph.D.)
Language: en
Appears in Collections:Plasma Physics

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