Helium ion bombardment of tungsten at temperatures between approximately one third and one half of its melting point has shown growth of nanostructures colloquially referred to as “fuzz”. The ...nanostructures take the form of thin tendrils of diameter about 30 nm and grow out of the bulk material. Tungsten will and does compose one of the key materials for plasma facing components (PFCs) in fusion reactors. The formation of nanostructured fuzz layers on PFCs would be detrimental to the performance of the reactor, and must therefore be avoided. Previous experiments have shown evidence that tungsten fuzz is initially grown by loop punching of helium bubbles created in the bulk. However, once the tendrils grow to sufficient length, the tendrils should intercept the entire helium flux, halting the production of fuzz. Fuzz continues to grow though. To increase the understanding of the mechanisms of tungsten fuzz formation, and thereby aid the avoidance of its production, a series of tests were performed to examine the validity of several theories regarding later stage tungsten fuzz growth. Tests showed that the fuzz formation was dependent solely on the bombardment of helium ions, and not on electric fields, or adatom diffusion. Experiments employing a tungsten coated molybdenum sample indicate the presence of a strong mixing layer and strongly suggest that tungsten fuzz growth continues to occur from the bottom up even as the tendrils grow in size. Tests also show a similarity between different metals exposed to helium ion fluxes where the ratio of bubble diameter to tendril diameter is constant.
•Tritium recovery will be critical in an energy-producing fusion utility.•Recovering hydrogen from liquid lithium is the focus of this article.•A distillation column was designed and constructed at ...the University of Illinois.•The COMSOL Multiphysics software was used to model induction heating.•Proof-of-concept tests were performed on Li-rich and LiH-rich samples.
Recovery of tritium from plasma-facing components in fusion devices will be vital to future full-scale operation. Liquid, low-Z materials have demonstrated many inherent advantages over solid first wall materials. To this end, a thermal treatment method in the form of a distillation column for extraction of hydrogen isotopes from liquid lithium has been designed, developed, and constructed at the Center for Plasma-Material Interactions at the University of Illinois at Urbana-Champaign. Use of induction heating and lithium condensation stages are the two qualities that set this design apart from other thermal treatment systems. Induction heating capabilities were modeled using the COMSOL Multiphysics software, which were validated when commissioning the physical heater module. Proof-of-concept tests were performed in the prototype column, which were undertaken as batch processes to investigate the efficacy with which the column could remove hydrogen gas from lithium-rich and lithium hydride-rich samples. All of the tests reported used lithium hydride as a surrogate for lithium deuteride and lithium tritide. The design process and results from the initial tests will be discussed, along with the envisioned placement of this treatment scheme in a fully-functional lithium loop.
•Open surface liquid metal flows can exhibit dryout under high heat flux.•Dryout phenomenon is modeled in 2-D using COMSOL Multiphysics.•Coupled moving mesh and laminar flow modules capture behavior ...of the liquid lithium free surface.•Shaping the trench bottom can mitigate the effect to keep solid PFC materials protected.
Liquid lithium displays increasing promise as a replacement for solid plasma facing components (PFCs) in fusion device applications. The Liquid Metal Infused Trench (LiMIT) system, developed at the University of Illinois (UIUC), has demonstrated how thermoelectric magnetohydrodynamics (TEMHD) can be harnessed to drive liquid lithium flow in an open surface PFC. However, in the highest heat flux applications, large local acceleration is created via TEMHD, and the sudden increase in velocity can cause the liquid level to expose the underlying solid, eliminating the protective benefits of the lithium. In order to study potential mitigation strategies, a 2-D COMSOL Multiphysics model was developed using the moving mesh module to capture free surface flow. The model depicts the development of the dryout phenomenon for 2 test cases – slow (1 cm/s) and medium (10 cm/s) flow in 5 mm deep trenches – including the liquid level reduction under the high heat flux and the pileup of slower flow both upstream and downstream of the heat stripe. The effectiveness of trench shaping dryout mitigation strategies is examined. For the slow flow case, it is shown that a 1.8 mm ledge placed under the heat stripe will stop dryout, and for the medium flow case, a 2.7 mm ledge is required to mitigate the effect. This model can be used to identify strategies for increasing the viable heat load for open surface liquid lithium PFCs.
•Liquid lithium wets W, Mo, 316 SS, Ta, and TZM at sufficiently high temperatures.•Wetting temperatures between 284°C (TZM) and 353°C (Ta) for untreated materials.•Argon GDC and lithium evaporation ...treatments reduce wetting temperature.
Research into lithium as a plasma facing component material has illustrated its ability to engender low recycling operation at the plasma edge leading to higher energy confinement times. Introducing lithium into a practical fusion device would almost certainly require the lithium to be flowing to maintain a clean lithium surface for gettering. Several conceptual designs have been proposed, like the LiMIT concept of UIUC (Ruzic, 2011). Critical to the implementation of these devices is understanding the interactions of liquid lithium with various surfaces. For a device that relies on thermoelectric magnetohydrodynamic drive, such as the LiMIT concept, two of the critical interactions are the wetting of materials by lithium, which may be characterized by the contact angle between the lithium and the surface, and the relative thermopower between lithium and potential substrate materials.
Experiments have been performed into the contact angle of liquid lithium droplets with various surfaces, as well as methods to decrease the contact angle of lithium with a given surface. The contact angle, as well as its dependence on temperature was measured. For example, at 200°C, tungsten registers a contact angle of 130°, whereas above its wetting temperature of 350°C, the contact angle is less than 80°. Glow discharge cleaning of the target surface as well as evaporation of a thin layer of liquid lithium onto the surface prior to performing wetting measurements were both found to decrease the wetting temperature.
The thermopower of W, Mo, Ta, Li and Sn has been measured relative to stainless steel, and the Seebeck coefficient of each of these materials has then been calculated. These are materials that are ...currently relevant to fusion research and form the backbone for different possible liquid limiter concepts including TEMHD concepts such as LiMIT. For molybdenum the Seebeck coefficient has a linear rise with temperature from SMo=3.9μVK−1 at 30°C to 7.5μVK−1 at 275°C, while tungsten has a linear rise from SW=1.0μVK−1 at 30°C to 6.4μVK−1 at 275°C, and tantalum has the lowest Seebeck coefficient of the solid metals studied with STa=−2.4μVK−1 at 30°C to −3.3μVK−1 at 275°C. The two liquid metals, Li and Sn have also been measured. The Seebeck coefficient for Li has been re-measured and agrees with past measurements. As seen with Li there are two distinct phases in Sn also corresponding to the solid and liquid phases of the metal. In its solid phase the SSn-solid=−1.5μVK−1 at 30°C and −2.5μVK−1 near the melting temperature of 231°C. There is a distinct increase in the Seebeck coefficient around the melting temperature as the Sn melts and stays relatively constant over the rest of the measured temperatures, SSn-melt=−1.4μVK−1 from 235°C to 275°C.
As the use of liquid metals in plasma facing components becomes more widespread, it is important to investigate how these liquid metals interact with the surfaces onto which they are deposited. An ...important example of these interactions is the ability to control liquid metal wettability on fusion relevant substrates. In this work, we explore the influence of femtosecond laser induced nanostructured surfaces on the wetting degree of liquid lithium versus temperature. Three material candidates as a lithium wall in magnetic fusion devices have been investigated: molybdenum, tungsten and 304 L stainless steel. Laser parameters were tuned to induce periodical self-organized nanostructures (ripples or LIPSS) formation on each material. Wettability of laser treated materials was changed from lithium-philic to lithium-phobic for temperatures beyond 320 °C - 360 °C compared to untreated material. The effect of both laser induced topography and chemistry are quantified to explain the observed liquid lithium contact angles on each material. Finally, it was shown that topography in the form of self-organized periodical nanostructures as well as the surface chemistry in the form of oxides enrichment, both induced by a single step laser process, strongly influence the wetting degree of liquid lithium and enhance lithium-phobicity at high temperatures.
The Center for Plasma-Material Interaction (CPMI) has developed innovative coating method of evaporative coating at atmospheric pressure (ECAP). This new idea is an atmospheric-pressure-based ...process. Following the similar concept as the laser-assisted plasma coating at atmospheric pressure (LAPCAP) 1, the material captured by the plasma plume is atomic in nature (the evaporated metal atom) and should therefore end up deposited molecule-by-molecule as in a PVD fashion. By using the thermal energy from the microwave plasma, solid 99.99%+purity aluminum were evaporated and then produce a PVD-like alumina coating on a work piece. The aluminum target was inserted in the center of the microwave torch feeding a melt pool and evaporated into the surrounding plasma plume. A bottle neck was made in the antenna and could reduce the heat loss by 84%, thus allowing higher temperatures to exist in the sample-holder antenna tip. Gas shielding was used to keep the work gas pure. The film was deposited as Al2O3 using oxygen from the environment. Deposition rate was around 2μm/min. Gas flow rate around the antenna tip was about 0.9m/s, and the temperature of the plasma was about 1400°C at 1350W input power from simulations. Alpha and other metastable phases of aluminum oxide were found on the deposited films.
•Aluminum oxide film deposited with metal aluminum target in atmospheric pressure.•Film characterization techniques were performed on the film deposited.•Film deposition rate was found to be around 2μm/min.•Phase of alumina nanocrystal could be identified by measuring the d-spacing on the TEM images and comparing to known data.
In 2012, lithium coating with an upgraded system on EAST, the first application of lithium granules injection for ELMs pacing on EAST, and the first flowing lithium limiter experiments on HT-7 have ...successfully been carried out and several new results were obtained. On EAST, it was found that both the Mo first walls and the C divertors were well coated by lithium and the lithium film coverage was increased up to 85%, which greatly contributed to the new achievements of EAST, especially stationary H-mode plasma over 30s and long pulse plasma over 400s. And at the same time, ELMs suppression by active lithium conditioning and ELMs pacing using lithium granules injection were demonstrated and reported for the first time on EAST. On HT-7, flowing liquid lithium limiters using the TEMHD concept and using a thin flowing film concept were also initially tested and some references were obtained for the future development. Those experiments show that lithium should be an important material for fusion devices. It could be used for wall conditioning, ELMs mitigation and also provide a self-recovery plasma facing components in future fusion devices.
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