New family of optimized omnigenous magnetic fields could display enhanced fusion plasma confinement

Physicists have been trying to design fusion reactors, technologies that can generate energy via nuclear fusion processes, for decades. The successful realization of fusion reactors relies on the ability to effectively confine charged particles with magnetic fields, as this in turn enables the control of high-energy plasma.

Researchers at Laboratorio Nacional de Fusión–CIEMAT in Madrid have introduced a new family of magnetic fields that could be better suited for confining particles in these devices without the need for complex equipment configurations. Their paper, published in Physical Review Letters, could be an important step toward the successful realization of fusion reactors.

“In the last years, there have been many initiatives proposing the design and construction of new experimental fusion devices and reactor prototypes,” José Luis Velasco, first author of the paper, told Phys.org.

“When these projects design the magnetic field that will confine the fusion plasma, practically all of them try to make the field ‘omnigenous.’ The fact that inspired our research is that the fusion community actually knew that it is possible to have magnetic fields that are quite far from being omnigenous but still display good plasma confinement (e.g., the Large Helical Device, an experimental device operating in Japan, and some old and recent numerical experiments in U.S.).”

In recent years, many physicists conducting research on nuclear fusion have focused their attention on omnigenous magnetic fields, as the properties of these fields are well-documented. As part of their study, Velasco and his colleagues set about to investigate less understood magnetic fields that could inform the design of future stellarator reactors.

“Our intuition was that, in these outliers, there was something interesting and useful to be learned about stellarator reactor design,” explained Velasco.

In a stellarator, the electric currents passing through the coils create a magnetic field organized into nested magnetic surfaces in the shape of a deformed doughnut. This magnetic field confines a plasma composed of deuterium and tritium, as well as the charged alpha particles resulting from fusion.

For fusion reactions to take place, allowing a reactor to generate electricity, the plasma inside stellarators needs to be hot enough. To heat up plasma to these high temperatures, physicists must carefully design the magnetic fields used to confine particles, a process known as “optimizing the stellarator.”

“Optimizing the stellarator to make it omnigenous ensures that the particles that make up the plasma stay, along their trajectories, close to the same magnetic surface,” said Velasco.

“Nevertheless, to achieve omnigenity, it is necessary to optimize the stellarator ‘as a whole.’ In our work we have found that similarly good confinement properties are obtained if one ‘splits’ each magnetic surface of the stellarator into several pieces and optimizes each of them separately. Hence the name ‘piecewise omnigenous.'”

The approach for optimizing stellarators proposed by Velasco and his colleagues could help to create optimized magnetic fields for nuclear reactors more effectively. In contrast with previously proposed approaches, it also does not rely on complex plasma shapes and the use of sophisticated coils.

“Designing and building an omnigenous field is not easy,” said Velasco. “In some cases, it may require complicated and expensive coils, which could endanger the whole project,” said Velasco. “An unfortunately extreme example of this was the National Compact Stellarator Experiment. Because there exists a vast variety of piecewise omnigenous magnetic fields, we are hopeful that some of them will be easier and/or cheaper to build.”

The recent work by this team of researchers could contribute to the future design of fusion reactors, by broadening the space of possible reactor configurations.

In their next studies, Velasco and his colleagues plan to systematically assess all the relevant physics properties of the piecewise omnigenous magnetic fields they uncovered, to determine whether they could compete with more conventional omnigenous magnetic fields.

“For instance, is the loss of energy due to turbulent processes too strong and how much simpler can we make the coils to comply with the technological requirements of a reactor?” added Velasco.

“Answering these questions will require a lot of work, drawing on the expertise (in several areas: theory, experiment, technology, engineering, etc.) of many colleagues of the National Fusion Laboratory at CIEMAT and of other collaborators abroad.”

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