The fusion reactions in a reactor take place in a very pure plasma of hydrogen isotopes which is confined by magnetic fields and heated to a temperature of more than a hundred million degrees. But at some point this hot plasma must be in contact with the cold material wall that surrounds it. This contact is essential to exhaust the power that is produced by the reactor, and also to exhaust the particles. Because a fusion reactor does produce ash, helium in our case, and this has to be removed from the reactor continuously.

The interaction between plasma and wall takes place in a designated area in the reactor, the so-called divertor. Here, the heat flux density is extremely high – similar to that on the surface of the sun – and so is the particle flux density. Moreover, these particles are hydrogen radicals, the most reactive and erosive atom in existence. And to complicate things further, this area is close to the plasma and receives a high neutron flux as a result of the fusion reactions.

The combination of requirements leaves only very few elements. In ITER, only tungsten, beryllium and carbon are considered as wall materials, and in the divertor the choice is even limited to tungsten and carbon only. But neither of the two is ideal. Carbon reacts with the hydrogen isotopes and therefore erodes fast and binds fuel. Tungsten cannot handle pulsed heat fluxes very well.

A relatively novel concept is the use of liquid metal. In the reactor this would be liquid lithium. Liquid metals cannot melt (they are already melted), are self-healing, are neutron resistant and can be replaced continuously. Moreover, they have superior properties in the interaction with plasma. In particular, they are very effective particle pumps, which is beneficial for the removal of ash.

Preliminary studies have been carried our at the Princeton Plasma Physics Laboratory. Further tests of the concept could be done at the Magnum-PSI linear plasma generator at DIFFER.

We are very interested in this field and are starting a research activity, focusing on the controlled flow of liquid metal in a strong magnetic field. This brings together the expertises that we have in the collaborating groups: plasma surface interaction, fluid dynamics, mhd, and control (yes, including sensorics). But there is, interestingly, also a link with the turbulence in the hot plasma. And on the other end of the spectrum: this technology to control the flow of a liquid metal surface is expected to find application in industrial settings, too. For instance for the plasma light source for ASML.

PhD-student Merlijn Jakobs, with a small team of students, is setting up a pilot experiment, backed-up by numerical simulations. In time our concepts will be tested in Magnum-PSI.

Thus, this topic is a melting pot of the various research interests and expertises that we have in the fusion-related groups at the TU/e.