PhD defense Jos Scholte: The viability of liquid metals as a plasma-facing material in nuclear fusion devices

Jos Scholte will defend his PhD thesis titled: 'The viability of liquid metals as a plasma-facing material in nuclear fusion devices'.

• Date: 3 June 2025, 11:00h
• Location: Campus TU/e
• Promotors: T.W. Morgan; N.J. Lopes Cardozo

Summary

There is a process that yields over 20 million times the energy per reaction as the combustion of coal, without emitting any greenhouse gasses. This is the process that fuels the Sun and is known as nuclear fusion. Here two atoms are fused to form a new, more stable atom, releasing energy. However, because the atoms both have the same charge, they repulse each other. To achieve fusion, they need to be fast or hot enough to overcome this. In a fusion reactor, the necessary temperature exceeds 150 million degrees Celsius, which is about ten times hotter than the core of the Sun. At this temperature, the hydrogen fuel becomes a plasma: an ionized gas where the electrons are no longer bound to the nucleus. To keep the fuel hot it is kept in place using magnets in a device called a tokamak.

Every engine needs an exhaust where the “ash” is removed. In a tokamak, this exhaust is called the divertor. Fusion produces a lot of energy in a small volume. This is great for a power plant, but makes designing the divertor heat shield challenging. The heat flux which the divertor has to withstand is similar to a rocket re-entering the Earth’s atmosphere. However, it must endure this for years instead of several minutes. 

A solution explored in this thesis is the use of a liquid metal heat shield rather than a solid one. This may sound counter-intuitive, however unlike a solid, a liquid wall by definition cannot fail through melting or breaking. If it evaporates, it can cool the component underneath it like sweat on your skin. This evaporated metal can easily be replaced by pumping fresh material towards it. This liquid metal is kept in place using a sponge, referred to in this thesis as a capillary porous structure or CPS for short. Tin is a suitable candidate to be used as a liquid metal: it is not reactive and has a low melting point and low vapour pressure and thus it is easy to keep liquid while not strongly polluting the plasma.

This thesis experimentally explores the potential of tin as a plasma-facing heat shield for a fusion reactor divertor. We successfully tested our tin-filled CPS design in the Magnum-PSI and GLADIS facilities. We then replaced a tile of the divertor of ASDEX Upgrade, a tokamak in Germany, with this CPS. While the design worked without damage, we observed radiation losses which were far too high. We showed that this is because tin can behave similarly to a glass of champagne when it is exposed to a hydrogen plasma. Part of the hydrogen can dissolve in tin, where it forms gas bubbles. Once these bubbles collapse, they eject a droplet. These droplets can radiatively cool the plasma, preventing the conditions for fusion to occur.

This effect can be reduced by reducing the pores of the sponge, which forms a physical barrier preventing bubble growth. However, current production methods do not allow us to make pores with a sufficiently small size. 

A path forwards for liquid metals is to shield the plasma core from these droplets or to consider other metals like gallium or lithium. More hydrogen can dissolve in gallium, delaying the formation and growth of bubbles, giving more time to de-gas the liquid elsewhere. Lithium, on the other hand, reacts with hydrogen, preventing bubbles from forming all together. Both have other disadvantages, gallium is corrosive and for lithium forming hydrides has downsides as well. However, if these problems can be overcome, liquid metals still show great promise to extend the lifetime of the divertor.

Read more on the TU/e website: PhD defense Jos Scholte