A magnetic cage keeps the hot plasma in the fusion device above 100 million degrees Celsius away from the vessel walls and prevents it from melting.
Now, researchers at the Max Planck Institute for Plasma Physics (IPP) have found a way to significantly reduce this distance. This could enable the construction of smaller and cheaper fusion reactors for energy production. This work was published in the journal “Physical Review Letters”.
The international experimental reactor ITER, currently under construction in the south of France, is the most advanced way to generate energy in a fusion power plant. This design follows the tokamak principle. In other words, fusion plasma with a temperature of 100 million degrees or more is confined in a doughnut-shaped magnetic field. This concept prevents the hot plasma from contacting and damaging the surrounding walls. The ASDEX upgrade tokamak experiment at the IPP in Garching near Munich serves as the blueprint for ITER and subsequent fusion power plants. Here the key elements of ITER were developed. And plasma operating conditions and components of later power plants are already being tested today.
Hot plasma approaches the divertor
A central element of the ASDEX upgrade and all modern magnetic fusion facilities is the divertor. This is the part of the container wall that has particularly high heat resistance and requires a precise design. “In the divertor, the heat from the plasma reaches the walls. In later power plants, the fusion product, helium-4, is also extracted there,” said IPP’s head of the plasma edge and walls division. One professor Ulrich Stroth explained. “In this region, the wall loads are particularly high.” Therefore, the ASDEX Upgrade and ITER divertor tiles are made of tungsten, a chemical element with the highest melting temperature of all (3422°C).
Without countermeasures, 20% of the plasma’s fusion power reaches the divertor surface. at approx. 200 megawatts per square meter, which is about the same as the surface of the sun. But ITER and future fusion power plant divertors can only handle up to 10 megawatts per square meter. This adds a small amount of impurity (often nitrogen) to the plasma. They extract most of the heat energy by converting it into ultraviolet light. Nevertheless, the plasma edge (separatrix) should be placed away from the divertor to protect it. In previous ASDEX upgrades this was at least 25 cm (measured from the bottom plasma tip (X point) to the edge of the diverter).
X-point radiator opens up new possibilities for fusion reactor design
IPP researchers have now managed to reduce this distance to less than five centimeters without damaging the wall. IPP Researcher Matthias Bernert, Ph.D. “When the amount of added nitrogen exceeds a certain value, in a magnetic cage with a special shape he X points he radiator.” volume is formed. “Such impurities lead to somewhat worse plasma properties, but if we change the nitrogen injection and set the X-point radiator to a fixed position, we can achieve higher output without damaging the device/divider.” We can run the experiment,” Dr. Bernert explained.
In a camera image from the vacuum vessel, the X-point radiator (XPR for short) can be seen as a blue glowing ring within the plasma. This is because they emit visible light in addition to UV radiation. IPP researchers have recently investigated XPR intensively. Nevertheless, chance also played a role in this discovery. “We were very surprised that ASDEX Upgrade coped with this without any problems.” This effect could be confirmed in further experiments, so the researchers now believe that in the presence of X-point radiators, more than previously assumed We know that much more thermal energy is converted into UV radiation. Plasmas radiate up to 90% of their energy in all directions.
Fusion power plants could be more compact and cheaper to build
This leads to very favorable conclusions for the construction of future fusion power plants.
Diverters are smaller and technically much easier to build than before (compact radial diverters).
Since the plasma is closer to the divertor, the volume of the vacuum vessel can be used more effectively. Initial calculations show that with an optimally shaped container, it is possible to almost double the plasma volume while maintaining the same dimensions. This also increases the achievable fusion power. However, researchers need to verify this in further experiments first.
Additionally, using the X-point radiator helps prevent Edge Localized Mode (ELM). This is a violent burst of energy at the plasma edge that repeats at regular intervals, releasing about a tenth of the plasma energy toward the wall. ITER and future fusion reactors would be damaged by such an eruption.
“We are working on important discoveries in fusion research,” is also the verdict of Ulrich Stroth, Head of the IPP Division. “The X-point radiator opens up completely new possibilities in the development of power plants. Tokamak will soon be ideally equipped for this purpose. A new upper diverter will be installed by summer 2024. Its special coil allows to freely deform the magnetic field near the divertor and optimize the conditions of the X point radiator.
Original: New discovery points the way to more compact fusion power plants
Than: Max Planck Institute for Plasma Physics
