Within the Plasma Material Interactions group we concentrate on three main research areas by exploiting our globally unique linear plasma devices and other in-house equipment, in combination with our modelling capabilities.
1. Solving plasma material interaction questions for ITER
How much fuel will be trapped in the divertor wall over long timescales? Is there a synergistic effect of transient plasma loading with the steady-state plasma flux? What sort of shaping should be applied to the ITER monoblocks in order to avoid edge melting during transients? These are all questions we try to tackle.
Synergistic effects of transient plasma loading
One question for the performance of the ITER wall materials is how they will behave under transient plasma loading from ELMs. Especially if the surface temperature gets too high the wall components (W-monoblocks) can recrystallize, leading to decrease in the yield and tensile strength of the tungsten. This can make them more susceptible to thermal-shock damage from the ELMs, which could lead to cracking and degradation of performance. To test this, as part of a contract for ITER we exposed a monoblock mock-up to combined steady-state plasma loads and 17,600 transient plasma pulses to simulate this effect. We found that even at a base temperature of 1500 °C the material did not recrystallize or crack. Because similar tests with an electron beam showed clear cracking it indicates that a dynamic process may be taking place due to the plasma loading into the material which can suppress recrystallization and thus cracking. We are also studying what happens in cases where there is a misalignment of the monoblocks, giving a leading edge which could lead to melting, and how such damaged or melted blocks can perform.
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Understanding divertor detachment via advanced diagnostics
Using a combination of recycling neutrals and injected impurities, the plasma which enters the divertor area can be cooled and slowed so that much of the heat is spread via radiation and momentum loss away from the hot strikepoints. This phenomenon, called detachment, is essential to limit the power and particle loading to the divertor materials to acceptable limits. Detachment is hard to study in tokamaks due to geometry and difficult diagnostic access. However, Magnum-PSI is an excellent simulator of the detaching region close to the strikepoints, and the linear geometry, flexibility and good diagnostics make it easy to study these sorts of plasmas. We have recently developed a unique Collective Thomson Scattering (CTS) system able to measure the ion temperature and plasma velocity, giving extremely detailed information about the plasma for the first time. We are also developing tunnel probes and Langmuir probes which can be combined with modelling to give a better understanding of the physical processes of detachment.
2. Developing divertor material solutions for DEMO
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 reactor DEMO and its successors. Higher neutron loading and longer duty cycles mean we may need more advanced solutions. We investigate advanced tungsten grades, as well as using liquid metals as a the plasma facing material instead.
Using liquid metals as a replenishing plasma facing material
Liquid metals have several advantages over solid plasma facing components (PFCs): any eroded material can be replaced by new material flowing in, they are capable of cooling other than by conduction, and that the liquid is not so susceptible to damage from neutrons. For these reasons we study the performance of liquid metals as a PFC for DEMO. Recently we found that when the surface temperature increases under intense plasma loading, a regime is reached where the high vapour pressure balances the incoming plasma pressure. This results in a reduced heat load to the surface as the vapour cools the plasma and increases radiation. We also study the erosion properties and have developed a Cavity Ring-Down Spectroscopy (CRDS) diagnostic to measure the tin content in the plasma via absorption of laser light with high accuracy. Other topics include fuel retention, surface stability, as well as material wetting and compatibility, with the overall aim of demonstrating its feasibility ready for use in the next generation fusion reactors.
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3. Exploiting extreme conditions for novel materials processing
Our linear devices create conditions where the rate of particles hitting the material is extremely high, and the path lengths between collisions extremely small, so that the material can be pushed out of equilibrium. We explore the strange morphology changes these create and see how we can use them in catalysts, batteries or solar cells.
Nanostructuring photocatalysts with helium bombardment
To convert solar energy to hydrogen requires catalytic surfaces with large surface area. Using dense helium plasma bubbles form in metal surfaces which can then form foamy nanostructured surfaces. Following oxidation these can act as photocatalysts for water splitting. We have demonstrated that we can precisely control and tune the nanostructure properties as a function of plasma exposure time and temperature, and that the metal oxides work as photocatalysts for these applications.
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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.