Scientists have achieved a remarkable breakthrough in the conceptual design of twisty stellarators, experimental magnetic facilities that could reproduce on Earth the fusion energy that powers the sun and stars. The breakthrough shows how to better shape the magnetic fields that enclose the plasma in stellarators to hold the fusion fuel together.

“The key thing was developing a piece of software that allows you to rapidly try out new design methods,” said Elizabeth Paul, a Princeton University presidential postdoctoral fellow at PPPL and co-author of a paper that details the finding in Physical Review Letters. The results produced by Paul and lead author Matt Landreman of the University of Maryland could boost the capability of stellarators to harvest fusion to generate safe and carbon-free electrical power for humankind.

Stellarators, invented by Princeton astrophysicist and PPPL founder Lyman Spitzer in the 1950s, have long taken a back seat to tokamaks in the worldwide effort to produce controlled fusion energy. But recent developments that include the impressive performance of the Wendelstein 7-X (W7-X) stellarator in Germany have created a renaissance of interest in the twisty machines.

Stellarators could produce laboratory versions of fusion without risk of the damaging disruptions that more widely used tokamak fusion facilities face. But the twisting magnetic stellarator fields have been less effective at confining the paths of the ions and electrons in plasma than the symmetrical, doughnut-shaped fields in tokamaks routinely do. Moreover, the complex coils that produce the stellarator fields are difficult to design and build.

The current breakthrough produces what is called “quasisymmetry” in stellarators to nearly match the confining ability of a tokamak’s symmetrical fields. While scientists have long sought to produce quasisymmetry in twisting stellarators, the new research develops a trick to create it nearly precisely.

The trick uses new open-source software called SIMSOPT (Simons Optimization Suite) that is designed to optimize stellarators by slowly refining the simulated shape of the boundary of the plasma that marks out the magnetic fields.

The breakthrough made some simplifying assumptions that will require enhancement. Also needing further development before the findings can be realized are new stellarator coils plus detailed engineering of the stellarator design.

But this Simons Foundation-sponsored finding combines the best features of stellarators and tokamaks to design a disruption-free facility with strong plasma confinement that reproduces a virtually unlimited source of fusion energy.

PPPL has proposed the source of the sudden and puzzling collapse of heat that precedes disruptions that can damage doughnut-shaped tokamak fusion facilities. Coping with the source could overcome one of the most critical challenges that future fusion facilities will face and bring closer to reality the production on Earth of the fusion energy that drives the sun and stars.

Researchers traced the collapse to the 3D disordering of the strong magnetic fields that bottle up the hot, charged plasma gas that fuels the reactions. “We proposed a novel way to understand the [disordered] field lines, which was usually ignored or poorly modeled in the previous studies,” said Min-Gu Yoo, a postdoctoral researcher at PPPL who has since become a staff scientist at General Atomics in San Diego.

The strong magnetic fields substitute in fusion facilities for the immense gravity that holds fusion reactions in place in celestial bodies. But when disordered by plasma instability in laboratory experiments, the field lines allow the superhot plasma heat to rapidly escape confinement and strike and damage fusion facility walls.

“In the major disruption case, field lines become totally [disordered] like spaghetti and connect fast to the wall with very different lengths,” said principal research physicist Weixing Wang, Yoo’s PPPL adviser. “That brings enormous plasma thermal energy against the wall.”

What hadn’t previously been known was the 3D shape, or topology, of the disarrayed field lines caused by turbulent instability. The topology forms tiny hills and valleys, Yoo explained, leaving some particles trapped in valleys and unable to escape confinement while others roll down the hills and impact the walls of the facility.

“This research provides new physical insights into how the plasma loses its energy toward the wall when there are open magnetic field lines,” Yoo said. “The new understanding would be helpful in finding innovative ways to mitigate or avoid thermal quenches and plasma disruptions in the future.”

The paradox startled PPPL physicists more than a dozen years ago. The more heat they beamed into a spherical tokamak, a magnetic facility designed to reproduce the fusion energy that powers the sun and stars, the less the central temperature increased.

Spherical devices such as the National Spherical Torus Experiment-Upgrade (NSTX-U) at PPPL are shaped more like cored apples than doughnut-shaped conventional tokamaks and are candidates to become models for a fusion pilot plant.

“Normally, the more beam power you put in, the higher the temperature gets,” said Stephen Jardin, head of the theory and computational science group and lead author of a proposed explanation published in Physical Review Letters. “So this was a big mystery: Why does this happen?”

Recent high-resolution computer simulations uncovered the source of the problem. “What we now think is that when raising the injected beam power, you’re also increasing the plasma pressure, and you get to a certain point where the pressure starts destroying the magnetic surfaces near the center of the tokamak,” Jardin said. “That’s why the temperature stops going up.”

This might be a general mechanism in spherical tokamaks, he said, and the possible destruction of surfaces must be considered when future spherical tokamaks are planned.

Scientists have discovered the beneficial impact of reversing a standard procedure for  improving fusion results. The procedure addresses locked tearing modes, a problem that  occurs in all modern donut-shaped tokamaks. These instability-caused modes rotate with the hot, charged plasma and tear holes called islands in the magnetic field that confines the gas, allowing the leakage of necessary heat.

The islands grow larger when the modes stop rotating and lock into place — a growth rate that increases the heat loss, reduces the plasma performance and can cause disruptions that allow the energy stored in the plasma to strike and damage the tokamak’s inner walls. To avoid such risks, researchers now beam microwaves into the plasma to stabilize the modes before they can lock.

However, the PPPL findings strongly suggest that researchers stabilize the modes in large, next-generation tokamaks after they have locked. In today’s tokamaks, “these modes lock more quickly than people had thought, and it becomes much harder to stabilize them while they’re still rotating,” said Richard Nies, a doctoral student in the Princeton Program in Plasma Physics and lead author of a Nuclear Fusion paper that lays out the surprising findings.

Also, in large future tokamaks like ITER, the international facility under construction in the south of France, “the plasma is so huge that the rotation is much slower, and these modes lock pretty quickly when they’re still pretty small,” Nies said. “So it will be much more efficient in big future tokamaks to let them first lock and then stabilize them.”

That reversal could facilitate the fusion process, which scientists around the world are seeking to reproduce. The process combines light elements in the form of plasma to release vast amounts of energy. “This provides a different way of looking at things and could be a much more effective way to deal with the problem,” said Allan Reiman, a distinguished research fellow and co-author of the paper. “People should take more seriously the possibility of allowing the islands to lock,” he said.

Updating a mathematical model could improve the design of future tokamak fusion facilities dramatically, PPPL researchers found. The update incorporates a process called “resistivity” that inhibits the flow of electricity.

Adding resistivity to models that predict the behavior of plasma, the soup of electrons and atomic nuclei that fuels fusion reactions, can increase understanding of how stable  the fuel and its ability to produce reactions will be. “It’s kind of like incorporating the viscosity of a fluid, which inhibits things moving through it,” said physicist Nathaniel Ferraro, one of the collaborating researchers. “For example, a stone will move more slowly through molasses than water, and more slowly through water than through air.”

Researchers intend to use the upgrade to develop a model that predicts before experiments whether the plasma will be stable, said Andreas Kleiner, a PPPL physicist and lead author of a paper reporting the results in Nuclear Fusion. “Basically, in this research, we saw that resistivity matters, and our models ought to include it.”

Scientists need stable plasma because instabilities can lead to plasma eruptions that can damage internal components of the tokamak, requiring frequent repairs. Future fusion reactors will have to run without stopping for repairs for months at a time.

“We need to have confidence the plasma in these future facilities will be stable without having to build full-scale prototypes, which is prohibitively expensive and time-consuming,” said Ferraro. “You want a model that is simple enough to calculate but complete enough to capture the phenomenon you are interested in. Andreas found that resistivity is one of the physical effects that we should include in our models.”

Future research will focus on determining why resistivity produces these types of instabilities in spherical tokamaks. “We do not yet know which property causes the resistive modes at the plasma edge to appear. It might be a result of the spherical torus geometry, the lithium that coats the insides of some facilities, or the plasma’s elongated shape,” Kleiner said. “But this needs to be confirmed with further simulations.”