Researchers at the Max Planck Institute for Plasma Physics (IPP) in Germany have reported a discovery that could lead to more compact fusion power plants. Within a fusion plant, a magnetic cage is used to keeps hot plasmas of more than 100m degree Celsius at some distance from the vessel wall to prevent damage. The researchers have now found a way to significantly reduce this distance, which could make it possible to build smaller and cheaper fusion reactors for energy production.
In a tokamak fusion plant, such as the International Thermonuclear Experimental Reactor (ITER) under construction at Saint-Paul-les-Durance in southern France, the hot plasma is confined in a magnetic field shaped like a donut. The ASDEX Upgrade tokamak experiment at IPP in Garching near Munich serves as a blueprint for ITER and important elements for ITER were developed there. Plasma operating conditions and components for later power plants are also tested at ASDEX.
A central element of the ASDEX Upgrade and all modern magnetic fusion facilities is the divertor. This is a part of the vessel wall that is especially heat-resistant and it requires an elaborate design. “The heat from the plasma at the wall arrives at the divertor. In later power plants, the fusion product helium-4 will also be extracted there," explained Professor Ulrich Stroth, head of the Plasma Edge & Wall Division at IPP. "In this region, the wall load is particularly high." In the ASDEX Upgrade and also in ITER, the divertor tiles of are therefore made of tungsten, which is the chemical element with the highest melting temperature (3422°C).
Without countermeasures, 20% of the fusion power of the plasma would reach the divertor surfaces. At some 200 MW per square metre, that would be roughly the same conditions as on the surface of the sun. However, the divertor in ITER and also in future fusion power plants will only be able to deal with a maximum of 10 MW per square metre. For this reason, small amounts of impurities (often nitrogen) are added to the plasma. These extract most of its thermal energy by converting it into ultraviolet light. Nevertheless, the plasma edge (the separatrix) must still be kept at a distance from the divertor to protect it. In the ASDEX Upgrade until now, this has been at least 25 centimetres (measured from the lower plasma tip – the X-point – to the edges of the divertor).
Now, researchers at IPP have succeeded in reducing this distance to less than 5 centimetres without damaging the wall. "We specifically use the X-point radiator for this, a phenomenon we discovered about a decade ago during experiments at ASDEX Upgrade," IPP researcher Dr Matthias Bernert said. "The X-point radiator occurs in specifically shaped magnetic cages when the amount of added nitrogen exceeds a certain value."
This leads to the formation of a small, dense volume that radiates particularly strongly in the UV range. "Such impurities give us somewhat poorer plasma properties, but if we set the X-point radiator to a fixed position by varying the nitrogen injection, we can run the experiments at higher power without damaging the device/divertor," Dr Bernert explained.
In camera images from the vacuum vessel, the X-point radiator (XPR) can be seen as a blue glowing ring in the plasma, as it also emits some visible light in addition to the UV radiation. Although IPP researchers have intensively investigated the XPR, chance also played a role in the current discovery: "We accidentally moved the plasma edge much closer to the divertor than we had intended," said IPP physicist Dr Tilmann Lunt.
"We were very surprised that ASDEX Upgrade coped with this without any problems." Because the effect could be confirmed in further experiments, the researchers now know: when the X-point radiator is present, significantly more thermal energy is converted into UV radiation than previously assumed. The plasma then radiates up to 90% of the energy in all directions.
Using this discovery future fusion power plants could be more compact and cheaper. Divertors can be smaller and technologically much simpler than before (Compact Radiative Divertor). Because the plasma moves closer to the divertor, the vacuum vessel volume can be better utilised. Initial calculations show that, if the vessel were optimally shaped, it would be possible to almost double the plasma volume while maintaining the same dimensions. This would also increase the achievable fusion power. However, the researchers will have to verify this in further experiments.
In addition, the use of the X-point radiator also helps against edge localised modes (ELMs). These are violent energy eruptions at the plasma edge that recur at regular intervals and expel about a tenth of the plasma energy towards the wall. ITER and future fusion reactors would be damaged by such eruptions.
IPP Division Director Ulrich Stroth believes this is “a significant discovery in fusion research”. He explained that the XPR opens up new possibilities in the development of a power plant. “We will further investigate the theory behind it and try to understand it better by new experiments at ASDEX Upgrade,” he noted.
The Garching tokamak will soon be ideally equipped for this. By summer 2024, it will be provided with a new upper divertor. Its special coils will make it possible to deform the magnetic field close to the divertor freely and thus also optimise the conditions for the XPR.
The ASDEX Upgrade divertor tokamak, the "Axially Symmetric Divertor Experiment", went into operation at Garching in 1991. Its purpose is to prepare the physics base for ITER and the subsequent DEMO facility. This requires essential plasma properties, primarily plasma density, plasma pressure and the wall load, to be matched to the conditions in a future fusion power plant.
