Design of the W7-X fusion device enables it to overcome obstacles

A key hurdle facing fusion devices called stellarators — twisty facilities that seek to harness on Earth the fusion reactions that power the sun and stars — has been their limited ability to maintain the heat and performance of the plasma that fuels those reactions. Now collaborative research by scientists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and the Max Planck Institute for Plasma Physics in Greifswald, Germany, have found that the Wendelstein 7-X (W7-X) facility in Greifswald, the largest and most advanced stellarator ever built, has demonstrated a key step in overcoming this problem.

Cutting-edge facility

The cutting-edge facility, built and housed at the Max Planck Institute for Plasma Physics with PPPL as the leading U.S. collaborator, is designed to improve the performance and stability of the plasma — the hot, charged state of matter composed of free electrons and atomic nuclei, or ions, that makes up 99 percent of the visible universe. Fusion reactions fuse ions to release massive amounts of energy — the process that scientists are seeking to create and control on Earth to produce safe, clean and virtually limitless power to generate electricity for all humankind.

Recent research on the W7-X aimed to determine whether design of the advanced facility could temper the leakage of heat and particles from the core of the plasma that has long slowed the advancement of stellarators. “That is one of the most important questions in the development of stellarator fusion devices,” said PPPL physicist Novimir Pablant, lead author of a paper describing the results in Nuclear Fusion.

His work validates an important aspect of the findings. The research, combined with the findings of an accepted paper by Max Planck physicist Sergey Bozhenkov and a paper under review by physicist Craig Beidler of the institute, demonstrates that the advanced design does in fact moderate the leakage. “Our results showed that we had a first glimpse of our targeted physics regimes much earlier than expected,” said Max Planck physicist Andreas Dinklage. “I recall my excitement seeing Novi’s raw data in the control room right after the shot. I immediately realized it was one of the rare moments in a scientist’s life when the evidence you measure shows that you’re following the right path. But even now there’s still a long way to go.”

Common problem

The leakage, called “transport,” is a common problem for stellarators and more widely used fusion devices called tokamaks that have traditionally better coped with the problem. Two conditions give rise to transport in these facilities, which confine the plasma in magnetic fields that the particles orbit.

These conditions are:

  • Turbulence. The unruly swirling and eddies of plasma can trigger transport;
  • Collisions and orbits. The particles that orbit magnetic field lines can often collide, knocking them out of their orbits and causing what physicists call “neoclassical transport.”

Designers of the W7-X stellarator sought to reduce neoclassical transport by carefully shaping the complex, three-dimensional magnetic coils that create the confining magnetic field. To test the effectiveness of the design, researchers investigated complementary aspects of it.

Pablant found that measurements of the behavior of plasma in previous W7-X experiments agreed well with the predictions of a code developed by Matt Landreman of the University of Maryland that parallels those the designers used to shape the twisting W7-X coils. Bozhenov took a detailed look at the experiments and Beidler traced control of the leakage to the advanced design of the stellarator.

“This research validates predictions for how well the optimized design of the W7-X reduces neoclassical transport,” Pablant said. By comparison, he added, “Un-optimized stellarators have done very poorly” in controlling the problem.

Further benefit

A further benefit of the optimized design is that it reveals where most of the transport in the W7-X stellarator now comes from. “This allows us to determine how much turbulent transport is going on in the core of the plasma,” Pablant said. “The research marks the first step in showing that high-performance stellarator designs such as W-7X are an attractive way to produce a clean and safe fusion reactor.”

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Materials provided by DOE/Princeton Plasma Physics Laboratory. Original written by John Greenwald. Note: Content may be edited for style and length.

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Cesium vapor aids in the search for dark matter

The hunt for dark matter is one of the most exciting challenges facing fundamental physics in the 21st century. Researchers have long known that it must exist, as many astrophysical observations would otherwise be impossible to explain. For example, stars rotate much faster in galaxies than they would if only ‘normal’ matter existed.

In total, the matter we can see only accounts for, at the most, 20 percent of the total matter in the universe — meaning that a remarkable 80 percent is dark matter. “There’s an elephant in the room but we just can’t see it,” said Professor Dmitry Budker, a researcher at the PRISMA+ Cluster of Excellence of Johannes Gutenberg University Mainz (JGU) and the Helmholtz Institute Mainz (HIM), explaining the problem he and many of his colleagues worldwide are contending with.

Dark matter could consist of extremely light particles

But so far no one knows what dark matter is made of. Scientists in the field are considering and researching a whole range of possible particles that might theoretically qualify as candidates. Among these are extremely lightweight bosonic particles, currently considered to be one of the most promising prospects. “These can also be regarded as a classical field oscillating at a specific frequency. But we can’t yet put a figure on this — and therefore the mass of the particles,” explained Budker. “Our basic assumption is that this dark matter field is coupled to visible matter and has an extremely subtle influence on certain atomic properties that would normally be constant.”

Budker and his team in Mainz have now developed a new method which they describe in the current issue of the leading specialist journal Physical Review Letters. It employs atomic spectroscopy and involves the use of cesium atom vapor. Only on exposure to laser light of a very specific wavelength do these atoms become excited. The conjecture is that minute changes in the corresponding observed wavelength would indicate coupling of the cesium vapor to a dark matter particle field.

“In principle, our work is based on a particular theoretical model, the hypotheses of which we are experimentally testing,” added the paper’s principal author, Dr. Dionysis Antypas. “In this case, the concept underlying our work is the relaxion model developed by our colleagues and co-authors at the Weizmann Institute in Israel.” According to the relaxion theory, there must be a region in the vicinity of large masses such as the Earth in which the density of dark matter is greater, making the coupling effects easier to observe and detect.

Previously inaccessible frequency range searched

With their new technique, the scientists have now accessed a hitherto unexplored frequency range in which, as postulated in relaxion theory, the effects of certain forms of dark matter on the atomic properties of cesium should be relatively easy to spot. The results also allow the researchers to formulate new restrictions as to what the nature of dark matter is likely to be. Dmitry Budker likens this meticulous search to the hunt for a tiger in a desert. “In the frequency range that we’ve explored in our current work, we still have not pinpointed dark matter. But at least, now that we’ve searched in this range, we know we don’t have to do it again.” The researchers still don’t know where dark matter — the tiger in his metaphor — is lurking, but they now know where it is not. “We just keep on targeting in more closely on the part of the desert where the tiger is most likely to be. And, at some point, we will catch him,” maintained Budker with confidence.

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Materials provided by Johannes Gutenberg Universitaet Mainz. Note: Content may be edited for style and length.

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IEEE Spectrum

All Engineering Knowledge Has an Expiration Date. The Trick Is to Know When

Engineering grads are facing an ever rising tide of knowledge