Advancements in Fusion Reactor Design for Efficient Energy Production
Nuclear fusion devices use a magnetic cage to keep the extremely hot plasma, which is over 100 million degrees Celsius, away from the vessel wall to prevent melting. Researchers at the Max Planck Institute for Plasma Physics (IPP) have discovered a method to significantly decrease this distance. This finding may lead to the development of smaller and more affordable fusion reactors for energy generation. The study was published in the journal Physical Review Letters.
The international experimental reactor ITER, currently under construction in southern France, is at the forefront of fusion energy production. Its design is based on the tokamak principle, which involves confining fusion plasma at temperatures above 100 million degrees within a donut-shaped magnetic field. This technique keeps the hot plasma from touching and damaging the surrounding wall. The ASDEX Upgrade tokamak experiment at IPP in Garching near Munich is a model for ITER and future fusion power plants. Key components for ITER were developed in this experiment, and it allows for testing plasma operating conditions and components for upcoming power plants.
Divertor Technology and Challenges in Fusion Reactors
The divertor is a crucial component of ASDEX Upgrade and all modern magnetic fusion facilities. This part of the vessel wall is designed to be highly heat-resistant and has a complex structure. Prof. Ulrich Stroth, head of the Plasma Edge and Wall Division at IPP, explains that the heat from the plasma reaches the wall at the divertor, and in future power plants, helium-4, a fusion product, will also be extracted there. The wall load is particularly high in this area, and as a result, the divertor tiles of both ASDEX Upgrade and ITER are made from tungsten, the element with the highest melting temperature of all (3422°C).
Without any intervention, 20% of the fusion power from the plasma would impact the divertor surfaces, creating conditions similar to those on the sun’s surface at around 200 megawatts per square meter. However, the divertor in ITER and future fusion power plants can only handle a maximum of 10 megawatts per square meter. To address this issue, small amounts of impurities, often nitrogen, are introduced into the plasma, which absorbs most of the thermal energy by converting it into ultraviolet light. Despite this, the plasma edge (the separatrix) must be maintained at a safe distance from the divertor to protect it. In ASDEX Upgrade, this distance has been at least 25 centimeters, measured from the lower plasma tip (the X-point) to the edges of the divertor.
Reducing Plasma Distance and Enhancing Fusion Reactor Efficiency
Scientists at IPP have successfully decreased the distance between the plasma and the wall to less than 5 centimeters without causing damage. Dr. Matthias Bernert, an IPP researcher, explains that they used the X-point radiator, a phenomenon discovered about ten years ago during experiments at ASDEX Upgrade. The X-point radiator forms in specifically shaped magnetic cages when the amount of nitrogen added surpasses a certain threshold.
The result is a small, dense volume that emits strong UV radiation. Dr. Bernert states that while the impurities can lead to slightly worse plasma properties, controlling the X-point radiator’s position by adjusting nitrogen injection allows them to conduct experiments at higher power without damaging the device or divertor.
Camera images from the vacuum vessel show the X-point radiator (XPR) as a blue glowing ring in the plasma, as it emits some visible light in addition to UV radiation. IPP researchers have been extensively studying the XPR recently. However, the current discovery also involved some luck, as Dr. Tilmann Lunt, an IPP physicist, mentions that they accidentally moved the plasma edge much closer to the divertor than intended.
The team was surprised that ASDEX Upgrade handled this situation without any issues. After confirming the effect in further experiments, researchers now understand that when the X-point radiator is present, a significantly higher amount of thermal energy is converted into UV radiation than previously thought. The plasma then radiates up to 90% of the energy in all directions.
Implications of X-Point Radiator Discovery for Future Fusion Power Plants
The findings suggest that divertors can be constructed smaller and much more technologically straightforward than previously thought, leading to the development of Compact Radiative Divertors.
As the plasma moves closer to the divertor, the vacuum vessel’s volume can be used more effectively. Preliminary calculations indicate that with optimal vessel shaping, it might be possible to nearly double the plasma volume while maintaining the same dimensions, which would also increase the achievable fusion power. However, researchers must first confirm this through additional experiments.
Additionally, the X-point radiator helps mitigate edge localized modes (ELMs), which are violent energy bursts at the plasma edge that occur regularly and release about a tenth of the plasma energy towards the wall. Such eruptions could damage ITER and future fusion reactors.
IPP Division Director Ulrich Stroth asserts that the X-point radiator discovery is significant for fusion research, as it offers entirely new opportunities for power plant development. Researchers will continue to investigate the underlying theory and seek to better understand it through new experiments at ASDEX Upgrade.
The Garching tokamak will soon be ideally suited for these investigations, as it will be equipped with a new upper divertor by summer 2024. Special coils in the divertor will enable the magnetic field near the divertor to be freely deformed, optimizing conditions for the X-point radiator.
What Does All This Mean?
The benefits of this research for the scientific community and humanity lie in the potential for significant advancements in fusion reactor design. By utilizing the X-point radiator phenomenon and optimizing the plasma confinement and divertor technology, researchers can potentially create more efficient, compact, and cost-effective fusion reactors. These advancements would pave the way for harnessing fusion energy, which is a clean, safe, and nearly limitless energy source.
Fusion energy can help address global energy demands while reducing dependence on fossil fuels and minimizing greenhouse gas emissions. This would contribute to combating climate change and promoting a sustainable future for humanity. Additionally, the technological innovations resulting from this research could stimulate further advancements in related scientific fields, fostering interdisciplinary collaboration and driving further innovation.
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Original Article: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.145102