On the Way to the next Stellarator Generation

How can even better stellarators be built in the future? This is the key question that an international group of theoretical physicists pursued at the Simons Workshop at IPP in Greifswald. The two-week format was the culmination of a scientific collaboration that is probably unique worldwide – funded by the Simons Foundation.

For a fortnight, not all eyes were focused on the display screen in the Günter Grieger lecture theatre. Researchers walked around the room. Others sat together in groups and discussed with each other, while other participants joined in via video conference. Coffee breaks were scheduled, but many of the scientists just kept working - and decided for themselves when to go outside. The Simons Workshop at the Max Planck Institute for Plasma Physics (IPP) in Greifswald (27 June to 8 July 2022) deliberately left researchers free space so that the unexpected could emerge. "Of course, we also set a framework," explains host Prof. Dr. Per Helander, head of the stellarator theory department at IPP in Greifswald. So there were usually two to three lectures a day that specified topics. "But otherwise, we fully rely on self-organisation of the participants."

Theorists optimise magnetic fields

Technically, the Simons Workshop was about the next generation of stellarators. These devices pursue one of the two concepts (the other being tokamaks) that physicists hope to use in the future to generate energy through magnetic confinement of fusion plasmas. While the sun uses powerful gravitational forces to cause atomic nuclei to fuse, physical tricks are needed on earth to mimic this kind of energy generation. And this is where magnetic cages come into play, such as those used by tokamaks and stellarators. Whereas these extremely strong magnetic fields are shaped axially symmetrically in tokamaks, stellarators follow a completely different concept: using specifically twisted magnetic coils, they generate complex asymmetric fields, which can overcome the technical disadvantages of tokamaks.

The largest and most powerful stellarator facility in the world is Wendelstein 7-X at the IPP in Greifswald - whose design demanded high performance from theoretical physicists and required the use of elaborate computer simulations. "With stellarators we have many more possibilities than with tokamaks to achieve better results by optimising the magnetic field," says Prof. Helander. IPP scientists had demonstrated how effective optimisation strategies are for stellarators in a publication in the renowned journal Nature in 2021.

Funding from the Simons Foundation

The participants of the Simons Workshop are working on using their theories to make stellarators possible that will one day far surpass the performance of Wendelstein 7-X. To this end, the international specialists have come together to form what is probably a unique scientific collaboration worldwide: the Simons Collaboration on Hidden Symmetries and Fusion Energy. Since 2018, the international project has been funded by the US Simons Foundation with two million dollars annually.

Stellarators are a unique scientific concept of such complexity that we can only advance it together, says Prof. Dr. Amitava Bhattacharjee, a physicist at Princeton University in New Jersey, USA, and leader of the Simons Collaboration. All of our members agree that they discuss all of their interim findings with each other and hold nothing back. There is a video conference every fortnight - always in the morning between eight and nine o clock US East Coast time. We call it the Simons Hour, explains Prof Bhattacharjee. A core team of 20 researchers is almost always there, he says. Sometimes, however, 80 stellerator specialists join in. Once a year in March, they meet in person in Princeton and in New York.

The highlight of the collaboration, however, is the Simons Workshop, which has had to be cancelled so far due to the Corona pandemic, but which is to take place regularly in the coming years. The event in Greifswald was therefore also a premiere. 74 scientists took part. Because some of them fell ill with Covid-19, not all of them were able to come to the IPP in person, but were only connected via video conference. They only partially experienced the intensive character of the Simons Workshop: Scientists from different institutions around the world meeting in one place for a fortnight - to work together and also spend free time together.

Accelerated scientific progress

During two weeks of Simons Workshop, many of the most important aspects of stellarator optimisation were addressed - such as minimising turbulence in the plasma, fast ions that carry much of the energy generated during fusion, and divertors that cleanse the plasma of reaction products in fusion facilities. "A particularly promising result of the collaboration is the new computer code SIMSOPT, which achieves better results than previous methods," says Prof. Helander. This makes it possible, for example, to design stellarators that can confine alpha particles (i.e. the helium-4 nuclei produced during fusion) better than the large-scale international fusion experiment ITER - a facility based on the tokamak principle which is currently under construction. Alpha particles are supposed to heat the plasma in fusion power plants, thereby helping to sustain the fusion reaction. SIMSOPT can also design stellarator concepts in which micro-turbulence of the ITG (Ion Temperature Gradient) type is significantly reduced compared to Wendelstein 7-X.

For Prof. Bhattacharjee, the successes of the Simons Collaboration are no coincidence. He is certain: The intensive collaboration dramatically accelerates scientific progress.