The use of liquid walls for fusion reactors could help solve problems associated with material erosion from high plasma heat-loads and neutronic activation of structures. A key issue analyzed here is the influx of impurity ions to the core plasma from the vapor of liquid side-walls. Numerical 2D transport simulations are performed for a slab geometry which approximates the edge region of a reactor-size tokamak. Both lithium vapor (from Li or SnLi walls) and fluorine vapor (from Flibe walls) are considered for hydrogen edge-plasmas in the high- and low-recycling regimes. It is found that the minimum influx is from lithium with a low-recycling hydrogen plasma, and the maximum influx occurs for fluorine with a high-recycling hydrogen plasma.

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Interactions between Liquid-Wall Vapor and Edge Plasmas

Abstract The liquid lithium divertor (LLD) to be installed in NSTX has four toroidal panels, each a conical section inclined at 22° like the previous graphite divertor tiles. Each LLD panel is a copper plate clad with ∼0.25 mm of stainless steel (SS) and a surface layer of flame sprayed molybdenum (Mo) that will host lithium deposited from an evaporator. LITER (evaporators) already used in NSTX will be upgraded for the LLD. Each has twelve 500 W cartridge heaters with thermocouples, 16 other thermocouples, and a channel for helium cooling. During LLD experiments, the LLD will be heated so that the lithium is just above its melting temperature. The length of each shot will be preset to prevent excessive evaporation of lithium from the LLD. This duration depends on the heat load and is likely to be in the range of less than a second to several seconds. Careful thermal control of the LLD is important to maximize the shot times and to guide operation of the LLD. This paper describes the layout of the LLD, its expected thermal performance, the control system, and supporting experiments and analysis. A companion paper in this conference, “Physics design requirements for the national spherical torus experiment liquid lithium divertor,” provides other information.

Thermal Control of the Liquid Lithium Divertor for NSTX (poster).

Several liquid metals are being considered as plasma-facing materials in tokamak fusion reactors. Liquid metal plasma-facing materials have several advantages over conventional solid materials, such as the ability to self-replenish during reactor operation. Liquid lithium, tin, and lithium-tin alloys are some of the most promising candidates. Before these materials can be utilized, a sound understanding of basic properties relevant to plasma-facing applications needs to be established. While many experimental studies have been performed with this goal in mind, there is still a noticeable lack of important data. Classical molecular simulation techniques present an alternative method for studying relevant properties and can complement existing experimental studies. However, before simulation studies can take place, careful development and validation of liquid metal force fields needs to be done. In this thesis, embedded-atom method force fields for lithium and tin were compared to experimental and first-principles data in order to assess their ability to predict properties of liquid metal plasma-facing materials. The wetting properties of liquid lithium on solid molybdenum were also studied using molecular simulations. Six lithium force fields found in the literature were tested by examining their ability to predict a variety of liquid and coexistence properties. It was found that the force field developed by Cui et al. [Modell. Simul. Mater. Sci. Eng. 20, 015014 (2012)] is the most robust due to its accurate prediction of the melting temperature and liquid-phase data. Further simulation studies of liquid lithium utilized this force field. The initial conclusion is supported by the fact that this force field accurately predicts the self-diffusivity and viscosity of liquid lithium. A new liquid tin force field was also developed in this work. It was found that the Ravelo and Baskes force field [Phys. Rev. Lett. 79, 2482 (1997)] is suitable for the solid phases of tin, but not the liquid phase. Therefore, a simulated annealing procedure was used to construct the new force field by primarily fitting to experimental iii liquid data. The new force field accurately reproduces a majority of the experimental data used in the fitting procedure and also accurately predicts liquid data not used in the optimization. Finally, the wetting properties of liquid lithium on solid molybdenum were examined. A lithium-molybdenum force field was developed by fitting to first-principles data. It was found that liquid lithium perfectly wets the (110) surface of molybdenum, in contrast with experimental data. This suggests the presence of oxygen and surface structure can significantly effect the ability of lithium to wet molybdenum. It was also found that the lithium atoms close to the molybdenum surface exhibit solid-like behavior as evidenced by their low mobility and ordered structure, even at temperatures well above the bulk melting temperature of lithium. This shows that lithium strongly adheres to the molybdenum surface. These results suggest that if the solid walls of the reactor are primarily composed of molybdenum, liquid lithium would be a strong candidate for liquid plasma-facing material due to its ability to wet and adhere to the solid surface. This thesis highlights the importance of careful force field development and validation when performing molecular simulation studies, especially for systems with limited experimental data. The work done here establishes tools, mostly in the form of force fields, for the further study of plasma-facing materials. It also illustrates the constant interplay between classical simulations with experiments and first-principles calculations. These tools can be used to understand new materials and applications.

Force Field Development and Validation for Liquid Metal Plasma-Facing Materials

The US Enabling Technology Program in fusion is investigating the use of free flowing liquid surfaces facing the plasma. We have been studying the issues in integrating a liquid surface divertor into a configuration based upon an advanced tokamak, specifically the ARIES-RS configuration. The simplest form of such a divertor is to extend the flow of the liquid first wall into the divertor and thereby avoid introducing additional fluid streams. In this case, one can modify the flow above the divertor to enhance thermal mixing. For divertors with flowing liquid metals (or other electrically conductive fluids) MHD (magneto-hydrodynamics) effects are a major concern and can produce forces that redirect flow and suppress turbulence. An evaluation of Flibe (a molten salt) as a working fluid was done to assess a case in which the MHD forces could be largely neglected. Initial studies indicate that, for a tokamak with high power density, an integrated Flibe first wall and divertor does not seem workable. We have continued work with molten salts and replaced Flibe with Flinabe, a mixture of lithium, sodium and beryllium fluorides, that has some potential because of its lower melting temperature. Sn and Sn-Li have also been considered, and themore » initial evaluations on heat removal with minimal plasma contamination show promise, although the complicated 3D MHD flows cannot yet be fully modeled. Particle pumping in these design concepts is accomplished by conventional means (ports and pumps). However, trapping of hydrogen in these flowing liquids seems plausible and novel concepts for entrapping helium are also being studied.« less

Donald F. Cowgill

Papers

Design integration of liquid surface divertors.