New stellarator design points the way for future fusion power plants

Using a new approach, researchers at the Max Planck Institute for Plasma Physics in Greifswald have developed a stellarator design that fulfils all the basic physics requirements for a viable fusion power plant. What is behind the concept?

Nuclear fusion is considered a promising option for a clean and safe energy source for the future. The most advanced form of fusion research is that of magnetic confinement of a plasma at several million degrees Celsius – known as “magnetic fusion”. Researchers around the world are focussing on two main concepts: the tokamak and the stellarator. Tokamaks generate a donut-shaped plasma, and have already achieved many important milestones in experiments on the way to a fusion power plant. The international experimental reactor ITER in southern France is also currently being built according to this concept.

Despite all the progress made, tokamaks have one main disadvantage compared to stellarators: in order to operate they require a powerful electric current to run through plasma. However, this current can destabilise the plasma, which could cause damage to the reactor. Furthermore, because this current must be switched off at regular intervals, the fusion power output would pause for a short time. This is known as pulsed operation (every pulse can last for several hours).

Stellarators, on the other hand can be operated completely without a current drive and therefore continuously, because the plasma is kept in an inherently steady-state equilibrium by a complex deformation of its donut structure. However, they still have to prove in experiments that the plasma confinement is as good as in tokamaks.

Wendelstein 7-X: Validating stellarator design

The world's largest and most powerful stellarator, Wendelstein 7-X (W7-X) — which is operated by the Max Planck Institute for Plasma Physics (IPP) in Greifswald — is working to provide this proof. The facility has already reached several important milestones and more are planned for the coming years. At the same time, however, it is clear that the design of W7-X, which was started at the end of the 1980s, needs to be further improved to make it suitable for a viable reactor. Using the lessons learned from W7-X, researchers at the IPP in Greifswald have done exactly this. The team from IPP's Stellarator Theory division recently published the design for new stellarators with key properties suitable for use in power plants. The new concept is so promising that several private fusion companies — inside Germany and abroad — have already expressed interest in continuing their own work on this basis.

“W7-X is a so-called quasi-isodynamic (QI) stellarator, which provides several enticing advantages over other stellarator types. Because W7-X has shown in experiments that these advantages are real, we decided to focus our efforts on QI stellarators specifically," explains IPP scientist Alan Goodman, who led the project as part of his doctoral thesis.

The new SQuID design: a breakthrough for future fusion power plants

These new stellarators have been given the moniker “SQuIDs”: Stable Quasi-Isodynamic Designs. The new designs exhibit highly desirable properties in computer simulations:

• They limit the net toroidal current in the plasma to very low values, which is necessary for extrapolating the plasma exhaust concept of W7-X to a reactor.
• In the simulations, SQuIDs exhibit encouraging properties concerning the plasma turbulence. This means that the energy confinement should be relatively good, which is one of the principal goals of a fusion reactor.
• High-energy particles produced by fusion reactions in the plasma do not drift out and hit the reactor wall, which would otherwise damage the plasma vessel.

SQuIDs represent the state of the art in stellarator design, in part made possible by recent developments in computational tools built for stellarator design. The success of this work is also largely a result of the collective wealth of knowledge and experience of the researchers at the Max Planck Institute for Plasma physics, and from the valuable lessons learned from experimental physicists working on W7-X. The synthesis of this fundamental scientific understanding, with modern supercomputer capabilities, allow the design of stellarators that would have been impossible to design even five years ago.

“The philosophy of our design approach was to systematically rule out design options that we realised could not be built in practice. Then, we could just focus entirely on getting the physics right,” says physicist Alan Goodman. After all, this is one of the challenges of stellarators: due to their very complex magnetic field, stellarators need to be carefully tailored to suit practical needs, and the difference between a very good design, and a very bad design, may be very small. Further, these magnetic fields can only be the generated through the use of complex-shaped magnetic coils, which have to be specially developed for this purpose.

„These designs are theoretically sound, as computationally verified by extensive simulations, but until they have been constructed, operated, and studied in the lab, one cannot know their true potential,“ explains Prof Per Helander, head of the Stellarator Theory division at IPP. „For engineers, both at IPP and in fusion startups, these new SQuIDs can be the basis for developing new magnet concepts and other technology needed for the realisation of a SQuID as an experiment or a power plant.“

Original publication: DOI: 10.1103/PRXEnergy.3.023010