Within the Plasma Material Interactions group we concentrate on two main research areas by exploiting our globally unique linear plasma devices Magnum-PSI and UPP as well as other in-house equipment. We evaluate the behaviour of materials under the conditions expected in future fusion reactors, and how they can themselves influence the plasma. And we investigate and develop the technology for implementing liquid metals as a robust and resilient divertor solution.
1. Plasma-Material Interactions in fusion devices
Pure tungsten blocks will be used as the armour material for the ITER divertor, but they may not be suitable for the first generation fusion reactors and its successors. Higher neutron loading and longer duty cycles mean we may need more advanced solutions. Neutrons will significantly affect the thermal performance and trapping of hydrogen isotopes for example. Reactors also need to consider wall conditioning like adding boron to the wall (boronization), but the properties of these coatings, how they form and how stable they are, are not fully understood.
The fundamentals of Boronization in ITER
The next-generation fusion experiment ITER will be the biggest and most advanced fusion experiment when it is complete. It aims to generate ten times more fusion power than the power used to heat the plasma. Designing walls that can withstand the large heat and power loads from the plasma, while also maintaining a clean core plasma without radiating the heat away, is a big challenge. ITER has recently changed its wall composition, and to ensure that the plasma remains clean, it will apply a thin boron layer around 50 nm thick on top of the tungsten wall tiles. The boron captures oxygen which would otherwise pollute the plasma, but this new approach raises new questions. The PMI group evaluates the erosion and fuel-trapping of such boron layers, as well as investigating how the eroded boron would migrate and deposit into new thick layers. These deposits could be a significant sink of fusion fuel (tritium), as well as potentially forming dust by delamination. We have developed an in-situ method to produce thick boron layers using Upgraded Pilot-PSI which enables relevant layers to be grown in relevant plasma conditions as can be expected in ITER.
Tungsten aging during slow-transient loading
ITER operates normally in a mode called detachment by deliberately adding impurities to help cool the edge plasma and radiate power away before it reaches the exhaust region (divertor). This protects the surfaces of the divertor from erosion and damage from the plasma, and limits the loading to only 10 MW m-2. However, with imperfect control, it can be anticipated that the detachment can be temporarily lost or reduced. This can result in surface temperatures for the divertor surfaces reaching above 2000 °C while at the same time being transiently loaded by Edge Localized Modes (ELMs). ELMs are periodic bursts of energy that erupt from the edge of the confined plasma in high-performance discharges in tokamaks. When they travel down the magnetic field lines into the divertor their impact on the tungsten wall surface stresses the material and can lead to cracking. Magnum-PSI is one of the few devices in the world where such conditions can be replicated, and we investigate how these combined loadings modify and degrade the tungsten armour material.
Synergistic loading of tungsten by plasma and IRRADIATION
We investigate the effect of irradiation damage in tungsten (W) on deuterium retention. In fusion reactors, neutron irradiation creates defects in tungsten divertor materials, which act as trapping sites for hydrogen isotopes. As a result, irradiated tungsten can retain up to two orders of magnitude more fuel than pristine material under repeated plasma exposure.
To study this, we use proton irradiation as a proxy for neutron damage. At DIFFER, we can combine simultaneous proton and plasma exposure to replicate reactor-relevant conditions using the UPP device.
2. Liquid Metals: a resilient heat exhaust solution
Liquid metals have long been considered attractive as a plasma facing material for the divertor of future fusion reactors. They are self-repairing, generally immune to neutron bombardment effects, enable lower thermal stresses in the surrounding materials and enable additional heat loss channels beyond thermal conduction via evaporative cooling and vapour shielding. This last process can be considered a form of impurity-induced detachment, where strong evaporation from the surface of the liquid metal interacts with the incoming plasma flux resulting in additional volumetric power dissipation. Typically relatively abundant and inexpensive metals with low melting points and without strong activation such as lithium and tin are considered the most promising. Often the liquids are held in place by capillary forces in a metal sponge known as a Capillary Porous Structure (CPS). The PMI group evaluates and develops liquid metal divertor solutions.
The Liquid Metal Shield Laboratory (LiMeS-lab)
A wide variety of concepts for applying liquid metals in fusion reactors exist, from static CPS designs to fast-flowing divertors. To advance the technological development of liquid metal we are developing a new dedicated laboratory: LiMeS-lab. This facility combines manufacturing, preparation, testing and analysis in a single chain: bespoke structures can be additively manufactured and integrated into components to confine the liquid metal, which is wetted to the substrate via a dedicated device. A dedicated linear plasma device incorporating flowing liquid metals, active high temperature cooling, protected diagnostics and high flux plasma provides an unparallelled test-bed. A liquid-metal capable thermal desorption spectroscopy system enables high quality data on hydrogen isotope trapping. Together this forms a coherent laboratory that can push the concept of liquid metal divertors to the next level.
A first experimental validation of the vapour box divertor
A promising approach for implementing liquid metals is the vapour box divertor, where a closed divertor structure enables a high temperature evaporation region where liquid metal vapours strongly interact with the plasma, cooling and slowing it. A lower temperature condensation region re-condenses the vapour and returns it for re-evaporation, while preventing upstream transport to the core. A vapour box test module has been developed and tested at DIFFER, representing in linear geometry the same principles. The experiments show that the heat load to the target is reduced by more than 50%, while the plasma pushes the lithium back towards the target direction, preventing it from being transported in the upstream direction. In a reactor this would help to keep the lithium in the divertor and not dilute the core plasma. This is a first experimental demonstration of how a vapour box would work in a fusion reactor, and represents a large advance in developing this as a technology.
Trapping and Recovery of hydrogen isotopes in lithium
Fusion fuel consists of deuterium (D) and tritium (T). While deuterium is abundant, tritium is scarce due to its 12.3-year half-life, making its retention in plasma-facing components a critical issue. While lithium (Li) is known to form hydrides, the trapping in co-deposits is not well understood. We used the UPP device to create co-deposited layers and then study their properties in-situ and operando using an ion beam. Using deuterium as a proxy, we observe Li:D ratios close to 1:1 in co-deposited layers up to 450 °C, indicating that lithium can act as a strong sink for fuel. The effect of oxygen present in the machines has also been studied, showing that it will lead to D desorption at higher temperatures if the layers is thin. Importantly, we demonstrate that this retained fuel can be recovered. Through isotope exchange, exposure of LiD layers to hydrogen plasma removes deuterium while leaving lithium intact. This finding supports the viability of lithium as a plasma-facing material.
Liquid metal performance in tokamaks
As well as experiments using Magnum-PSI and UPP, we have carried out experiments to evaluate the performance of liquid metals in tokamaks. We carried out experiments in two leading fusion facilities: ASDEX-Upgrade (Germany) and DIII-D (USA) to evaluate the performance of Sn-filled and Li-filled CPSs respectively. Both used our in-house designed additively manufactured concepts. As an example, in the DIII-D experiments the Li was found to remain stable and did not modify the core plasma, demonstrating its benign properties.
Collaboration makes us stronger
As well as our own in-house research we work closely with dozens of researchers and research groups around the world on many additional topics. We work together to plan and execute experiments in our linear devices and collaborate in interpreting the results, maximising the effectiveness of our research. If you have an experimental proposal please fill in the web form.