Physicist helps confirm a major advance in stellarator performance for fusion energy

Andreas Langenberg (IPP physicist) and Novimir Pablant (PPPL physicist), before the installation of the XICS Diagnostic on the W7–X. Credit: Scott Massida
The twisty magnetic devices known as Stellarators that harness the fusion energy of the sun and stars on Earth have been second to the more common doughnut-shaped tokamaks. Complex twisted stellarator magnets are difficult to design. They have also allowed for greater heat leakage from fusion reactions.

Researchers at the Max Planck Institute for Plasma Physics, IPP, and the U.S. Department of Energy (DOE), Princeton Plasma Physics Laboratory, (PPPL) have now shown that the Wendelstein 7X (W7X) device in Greifswald (Germany), which is the most advanced stellarator in existence, can confine heat to temperatures that are twice that of the sun's core.

Key indicator

The XICS is a diagnostic instrument that PPPL physicist Novimir Pablant, in collaboration with Andreas Langenberg, has been designed, built, and operated by Novimir Pablant. It's a key indicator for a sharp decrease in a type heat loss known as "neoclassical transportation" which was historically greater in tokamaks than in classical stellarators. Frequent collisions can cause troublesome transport by knocking heated particles out their orbits and causing them to swirl around the magnetic fields that contain them. Transport is also caused by drifts in particle orbits.

Recent Nature magazine report confirms that designers have succeeded in shaping the stellarator magnets into a more efficient form to reduce neoclassical transportation. Craig Beidler, physicist at the IPP Theory Division, was the first author of this paper. Pablant, who is coauthor with Langenberg, said that it was a great news for fusion that the design was successful. It clearly shows that optimization is possible.

David Gates, the head of the Advanced Projects Department, PPPL, which oversees the laboratory’s stellarator research, was also very excited. Gates stated, "It's been very thrilling for us at PPPL, and all other U.S. collaborating institutes, to be part in this really exciting experiment." "Novi's research has been at the heart of this incredible experimental team's efforts. Our German colleagues were very kind to allow us to participate.

Power that is carbon-free

Scientists are trying to create fusion by combining light elements in plasma, a hot, charged state composed of electrons and nuclei. This form of fusion can generate large amounts of energy. Controlled fusion would allow for an almost unlimited supply of clean, safe, and carbon-free electricity to be generated on Earth. It also contributes to the transition away fossil fuels.

The first Stellarators were built under Lyman Spitzer's PPPL founder in 1950s. They can operate in a stable state without the risk of plasma disruptions like tokamaks. Their complexity and poor heat confinement history have kept them from being successful. The W7-X optimized design, which produced its first plasma in 2015 was designed to show the suitability of an optimized stellarator for a future fusion power station.

The XICS results show that hot ion temperatures cannot be achieved without a drastic reduction in neoclassical transportation. These measurements were also taken by the CXRS diagnostic, built and operated IPP. Although they were a bit more precise, they could not be made under all conditions. The Nature report's final temperature profiles were taken from CXRS, and supported by measurements made with XICS in plasmas similar to the one used.

"Extremely valuable"

Robert Wolf, co-author of this paper and head of W7-X heating operation division, said that without the XICS "we probably wouldn't have discovered this [good confinement] regime." We needed an easily accessible ion temperature measurement, and this was very valuable."

Researchers did a thought experiment to see if optimization was a factor in the confinement results. It was found that large neoclassical transport in a non-optimized stargazer would have rendered the high temperatures measured on W7X for the given heating force impossible. Pablant stated that this experiment showed that W7X's optimized shape reduced neoclassical transportation and was essential for the W7X experiments. It was a way to show how important optimization was.

He said that the results are a step towards allowing stellarators based upon the W7X design to be used to build a practical fusion reactor. "But, reducing neoclassical transportation is not the only thing that you need to do. You must also show stability and reduce turbulent transport. The second source of heat loss is turbulent transport, which is caused by ripples and eddies in the plasma.

After a three year upgrade, the W7-X will reopen its doors in 2022. The upgrades included a water-cooling system to extend fusion experiments and an improved divertor to exhaust high-performance heat. These upgrades will allow the next stage in the W7-X research into the value of optimized stellarators to be blueprints for power plant designs.

Further exploration The Wendelstein 7X concept proves its efficacy

C. D. Beidler et. al., Demonstration reduced neoclassical energie transport in Wendelstein 7X, Nature (2021). Information from Nature C. D. Beidler et. al., Demonstration reduced neoclassical energie transport in Wendelstein (7-X),(2021). DOI: 10.1038/s41586-021-03687-w