Liquid Metal Walls, Lithium, And Low Recycling Boundary Conditions In Tokamaks

Abstract

Abstract. At present, the only solid material believed to be a viable option for plasma-facing components (PFCs) in a fusion reactor is tungsten. Operated at the lower temperatures typical of present-day fusion experiments, tungsten is known to suffer from surface degradation during long-term exposure to helium-containing plasmas, leading to reduced thermal conduction to the bulk, and enhanced erosion. Existing alloys are also quite brittle at temperatures under 700°C. However, at a sufficiently high operating temperature (700 -1000 °C), tungsten is selfannealing and it is expected that surface damage will be reduced to the point where tungsten PFCs will have an acceptable lifetime in a reactor environment.The existence of only one potentially viable option for solid PFCs, though, constitutes one of the most significant restrictions on design space for DEMO and follow-on fusion reactors. In contrast, there are several candidates for liquid metal-based PFCs, including gallium, tin, lithium, and tin-lithium eutectics. We will discuss options for liquid metal walls in tokamaks, looking at both high and low recycling materials. We will then focus in particular on one of the candidate liquids, lithium.Lithium is known to have a high chemical affinity for hydrogen, and has been shown in test stands 1 and fusion experiments 2,3 to produce a low recycling surface, especially when liquid. Because it is also low-Z and is usable in a tokamak over a reasonable temperature range (200 -400 °C), it has been now been used as a PFC in several confinement experiments (TFTR, T11-M, CDX-U, NSTX, FTU, and TJ-II), with favorable results. The consequences of substituting low recycling walls for the traditional high recycling variety on tokamak equilibria are very extensive. We will discuss some of the expected modifications, briefly reviewing experimental results, and comparing the results to expectations.

Liquid li structure and dynamics: A comparison between OFDFT and second nearest‐neighbor embedded‐atom method

“…Liquid metals have attracted considerable attention recently, due to their potential use as plasma‐facing components in tokamak reactors . Liquid lithium (Li) and its alloys, such as lithium‐tin, show the greatest promise as plasma‐facing components . Simulation studies are a useful complement to experimental ones, particularly so for liquid metal systems that are difficult to study experimentally.…”

Initial Exploration of High-Field Pulsed Stellarator Approach to Ignition Experiments

“…This, however, would largely reduce the attractiveness of the approach, unless a solution is adopted-probably based on liquid lithium, which may withstand high P d . As an added benefit, low recycling Li walls enhanced confinement in TFTR [52], TJ-II [53], NSTX [54] and other devices [55,56] by various amounts, ranging between 25% and 100%. Liquid lithium does not erode or blister.…”

“…Liquid lithium does not erode or blister. Low Li impurity in the core plasma was obtained in NSTX and TFTR [55], which allowed low Z eff (* 1.3), e.g. in TFTR [37].…”

Particle control and plasma performance in the Lithium Tokamak eXperiment

“…Shell coatings are applied with a simple system of evaporators, which operate with a helium gas fill of the vacuum vessel to [1][2][3][4][5] of one of the evaporator systems, after an evaporation cycle and prior to cleaning, is shown in Figure 2. The lithium coatings evident on the structure surrounding the evaporation crucible, and in the crucible itself, are removed by reacting with common white vinegar (8% acetic acid solution), which reduces the residual metallic lithium to water-soluble lithium acetate.…”

“…The liquid metals generally considered as candidates for PFCs are gallium, tin, and lithium. 3 Of these candidates, virtually no experimental tests of gallium or tin have been conducted in confinement devices, whereas lithium wall coatings and wall conditioning have been tested in a number of devices, and observed to strongly affect tokamak performance, since the experiments on the Tokamak Fusion Test Reactor (TFTR). 4 Several, more recent, experiments [5][6][7][8] have employed localized limiters of liquid lithium, but the liquid lithium surface area in these systems has never exceeded a few percent of the total plasma surface.…”