The use of low atomic number liquid metals has been shown to have the potential to solve many of the prevalent problems like erosion and radiation losses associated with the interaction of fusion plasma with the plasma facing component (PFC) structures in tokamaks. Since the first evidence of lithium increasing plasma performance in TFTR [1], the benefits of using lithium in fusion environments have been seen in many devices, including CDX-U [2], NSTX [3], LTX [4], and DIII-D [5]. While both fast flow and slow flow concepts have been studied with regards to liquid lithium first wall alternatives, this report will focus on efforts placed on fast flow research and will mainly focus on advancements in the LiMIT device that help to eliminate concerns over the broad use of liquid lithium. Due to the promising TFTR results along with results obtained at the University of Illinois at Urbana-Champaign [6], suitably designed trench structures holding liquid lithium could be an appropriate fast flow candidate for PFC modules in future fusion devices. There are four potential shortcomings of this approach: (1) Droplet ejection, (2) Wetting control, (3) Tritium retention, and (4) Limited heat flux handling. Droplet ejection is discussed in a companion publication [7], while this paper addresses the topics of wetting control and heat flux handling. Limitations in wetting and prevention of lithium creep (i.e. getting and keeping the lithium only where it should be) have been solved by laser-texturing the base material with extreme short laser pulses (pico -femto second) of high power (several 10 s of W). Micro-and nano-structuring results indicate that the textured substrates displayed significant change in their wetting properties, increasing the temperature needed to wet from 310 degrees C to 390 degrees C. Lastly, initial designs for the Lithium Metal Infused Trenches (LiMIT) [6] showed dryout above 3 MW/m(2), but new designs of the trench shaping show potential to be able to handle up to 10 MW/m(2). Dryout is accompanied by lithium evaporation which is shown to mitigate the incident heat flux, which may be viewed as beneficial [8]. The advances shown here will increase the viability of the LiMIT system in large-scale testing, and allow for extensive design iteration to begin tackling the large powers and heat fluxes present in reactor-relevant systems. (C) 2017 The Authors. Published by Elsevier Ltd.

Liquid metal plasma facing components (LMPFC) have shown a potential to supplant solid plasma facing components materials in the high heat flux regions of magnetic confinement fusion reactors due to the reduction or elimination of concerns over melting, wall damage, and erosion. To design a workable LMPFC, one must understand how liquid metal interacts with solid underlying structures. Wetting is an important factor in such interaction, several designs of LMPFC require liquid metal to wet the underlying solid structures. The wetting of lithium compounds (lithium nitride, oxide, and carbonate) by 200 degrees C liquid lithium at various surface temperature from 230 to 330 degrees C was studied by means of contact angle measurements. Wetting temperatures, defined as the temperature above which the contact angle is less than 90 degrees, were measured. The wetting temperature was 257 degrees C for nitride, 259 degrees C for oxide, and 323 degrees C for carbonate. Surface tensions of solid lithium compounds were calculated from the contact angle measurements. (C) 2016 Elsevier B.V. All rights reserved.

An outstanding concern raised over the implementation of liquid metal plasma facing components in fusion reactors is the potential for ejection of liquid metal into the fusion plasma. The influences of Rayleigh-Taylor-like and Kelvin-Helmholtz-like instabilities were experimentally observed and quantified on the thermoelectric-driven liquid-metal plasma-facing structures (TELS) chamber at the University of Illinois at Urbana-Champaign. To probe the stability boundary, plasma currents and velocities were first characterized with a flush probe array. Subsequent observations of lithium ejection under exposure in the TELS chamber exhibited a departure from previous theory based on linear perturbation analysis. The stability boundary is mapped experimentally over the range of plasma impulses of which TELS is capable to deliver, and a new theory based on a modified set of the shallow water equations is presented which accurately predicts the stability of the lithium surface under plasma exposure.

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. (C) 2016 Elsevier B.V. All rights reserved.

The application of liquid metal, especially liquid lithium, as a plasma facing component (PFC) has the capacity to offer a strong alternative to solid PFCs by reducing damage concerns and enhancing plasma performance. The liquid-metal infused trenches (LiMIT) concept is a liquid metal divertor alternative which employs thermoelectric current from either plasma or external heating in tandem with the toroidal field to self-propel liquid lithium through a series of trenches. LiMIT was tested in the linear plasma simulator, Magnum PSI, at heat fluxes of up to 3 MW m−2. Results of these experiments, including velocity and temperature measurements, as well as power handling considerations are discussed, focusing on the 80 shots performed at Magnum scanning magnetic fields and heat fluxes up to ~0.3 T and 3 MW m−2. Comparisons to predictions, both analytical and modelled, are made and show good agreement. Concerns over MHD droplet ejection are additionally addressed.

Liquid metals are garnering increased attention as an alternative divertor solution to tungsten divertors. While tungsten suffers from a myriad of potentially critical issues, such as bulk erosion, melting under significant transient heat loads, and nanostructuring colloquially referred to as "fuzz", liquid metals avoid many of these entirely. In order to implement liquid metal concepts, the interactions between the liquid metal and the substrate it is deposited upon must be characterized. One such critical interaction is the wetting of a liquid metal on the surface of the PFC structure. The wetting ability of a substance determines many significant properties, including the thickness of the liquid film and the propensity of a flowing liquid to break into rivulets. A previous study conducted at the University of Illinois [1] characterized wetting as a measurement of the contact angle of lithium when deposited as liquid droplets onto a surface. The dependence of the contact angle on temperature was measured, finding a transition between non-wetting and wetting at a critical temperature. For example, at 215 °C, stainless steel registers a contact angle of 137°, whereas above its wetting temperature of 315 °C, the contact angle is less than 80°. The impact of nanostructuring of the surface is detailed herein. A novel method of rapid laser nanostructuring was developed to create the samples. To further the knowledge of liquid metal PFC surface interactions, results of experiments on the relationships between material and temperature and the contact angle of lithium are presented for a variety of nanostructured surfaces.

Flowing liquid metal PFCs may offer a solution to the issues faced by solid divertor materials in tokamak plasmas. The Liquid–Metal Infused Trenches (LiMIT) concept of Illinois Ruzic et al. (2011) is a liquid metal plasma facing structure which employs thermoelectric magnetohydrodynamic (TEMHD) effects to self-propel lithium through a series of trenches. The combination of an incident heat flux and a magnetic field provide the driving mechanism. Tests have yielded experimental lithium velocities under different magnetic fields, which agree well with theoretical predictions Xu et al. (2013). The thermoelectric force is expected to overcome gravity and be able to drive lithium flow along an arbitrary direction and the strong surface tension of liquid lithium is believed to maintain the surface when Li flows in open trenches. This paper discusses the behavior of the LiMIT structure when inclined to an arbitrary angle with respect to the horizontal.

Impingement of high fluxes of helium ions upon metals at elevated temperatures has given rise to the growth of nanostructured layers on the surface of several metals, such as tungsten and molybdenum. These nanostructured layers grow from the bulk material and have greatly increased surface area over that of a not nanostructured surface. They are also superior to deposited nanostructures due to a lack of worries over adhesion and differences in material properties. Several palladium samples of varying thickness were biased and exposed to a helium helicon plasma. The nanostructures were characterized as a function of the thickness of the palladium layer and of temperature. Bubbles of ~100 nm in diameter appear to be integral to the nanostructuring process. Nanostructured palladium is also shown to have better catalytic activity than not nanostructured palladium.

Determination of the mechanisms underlying the growth of tungsten fuzz is an important step towards mitigation of fuzz formation. Nanostructured tungsten was produced on resistively heated tungsten wires in a helicon plasma source (maximum flux of 2.5 × 1021 m−2 s−1). Asymmetry in the setup allows for investigation of temperature and flux effects in a single sample. An effort at elucidating the mechanism of formation was made by inspecting SEM micrographs of the nanostructured tungsten at successive fluence steps of helium ions up to a fluence of 1 × 1027 m−2. To create these micrographs a single tungsten sample was exposed to the plasma, removed and inspected with an SEM, and replaced into the plasma. The tungsten surface was marked in several locations so that each micrograph is centred within 200 nm of each previous micrograph. Pitting of the surface (diameter 9.5 ± 2.3 nm, fluence (5 ± 2) × 1025 m−2) followed by surface roughening (fluence (9 ± 2) × 1025 m−2) and tendril formation (diameter 30 ± 10 nm, fluence (2 ± 1) × 1026 m−2) is observed, providing evidence of bubble bursting as the mechanism for seeding the growth of the tungsten fuzz.

A device has been developed that produces spherical lithium pellets for the controlled excitation, or pacing, of ELMs and replenishing lithium coating on PFCs during a plasma shot. The device is based on a dripper design, where the lithium is forced through a small orifice with gas pressure. It is envisaged to use the dripper in two ways: first it is used in conjunction with the lithium granular injector developed at PPPL for ELM pacing and the second for replenishing lithium PFC coatings during a plasma discharge by “dripping” liquid lithium drops into the plasma edge and vaporization will redistribute the lithium to recoat the PFC during the shot. A theory has been developed for the drop formation and frequency using high-pressure gas. Experiments have been performed initially with Wood's metal and subsequently lithium. Using Wood's metal at a backing pressure of ΔP > 600 torr, frequencies up to f = 1200 Hz have been achieved with droplet diameters d > 600 μm. These agree well with theory. Measurements using lithium also show that the frequency does not quite match the theory with ΔP = 450 torr, a frequency of f = 2 kHz and but the diameters are in good agreement with d = 0.8 mm.

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. (C) 2014 Elsevier B.V. All rights reserved.

With lithium as a fusion material gaining popularity, a method for producing lithium pellets relatively quickly has been developed for NSTX. The Lithium Pellet Production device is based on an injector with a sub-millimeter diameter orifice and relies on a jet of liquid lithium breaking apart into small spheres via the Plateau-Rayleigh instability. A prototype device is presented in this paper and for a pressure difference of ΔP = 5 Torr, spheres with diameters between 0.91 < D < 1.37 mm have been produced with an average diameter of D = 1.14 mm, which agrees with the developed theory. Successive tests performed at Princeton Plasma Physics Laboratory with Wood's metal have confirmed the dependence of sphere diameter on pressure difference as predicted.