Fusion Stellarator Starts Up
The construction of ITER, the 23 000-metric-ton tokamak-style fusion reactor is under way now in France. But a smaller reactor of a different design might be the key to its success. That reactor, the US $1.45 billion Wendelstein 7-X, was inaugurated yesterday, and researchers expect that it will ignite its first plasma a year from now.
Housed at the Max Planck Institute for Plasma Physics, in Greifswald, Germany, Wendelstein 7-X is a “stellarator,” a term coined in the 1950s by the Princeton astrophysicist Lyman Spitzer, who designed the first such machine for exploring the fusion reactions in stars. It’s a design that predates the tokamaks in use today but one that had fallen out of favor because the computers of the day weren’t able to properly model the 3-D magnetic field confining the plasma.
“The W7-X is the first large stellarator designed to get the physics right,” says Allen Boozer, an applied physicist at Columbia University, in New York City.
Although many view fusion power with skepticism, many others hope that one day we will harness the fusion of hydrogen atoms into helium atoms to produce electricity. The helium atoms have a slightly lower mass than the two hydrogen atoms that form them, and this difference in mass is released as energy, according to Einstein’s famous principle E=mc2. It is this reaction that powers the sun and, in an uncontrolled fashion, hydrogen bombs. However, for hydrogen atoms to fuse into helium atoms, they have to smash together with extremely high energies, corresponding to temperatures of at least 100 million °C. At these temperatures, electrons are entirely separated from the atomic nuclei and the gas becomes a plasma, but if the plasma touches a metal wall of the reactor, it immediately cools off, stopping any fusion reactions. Therefore, it has to be trapped in a “magnetic bottle,” a magnetic field created by electromagnets arranged in such a way that the plasma doesn’t touch the reactor vessel.
From the 1950s to the 1970s, these magnetic bottles were stellarators, based on Spitzer’s original design of a contorted-torus-shaped reactor vessel with toroidal coils surrounding it and a separate helical coil surrounding the torus. This helical coil was wound in such a way that it produced a magnetic field component perpendicular to that of the toroidal coils. The combination created the “helicoidal” magnetic field needed to confine the plasma. During the 1950s, Russian scientists developed what became a more successful design for magnetic confinement: the tokamak. It consisted of a torus-shaped vessel surrounded by toroidal magnets. Instead of the stellarator’s helical coil, however, a circular current through the plasma created the component of the magnetic field required to confine the plasma. The tokamak worked better than the stellarators of the time, and fusion researchers switched to the Russian design, demonstrating sustained fusion reactions with the Joint European Torus (JET), in Culham, England, in 1991. (Until ITER—the International Thermonuclear Experimental Reactor—is complete, JET remains the world’s largest magnetic fusion device. Its torus has an inner radius of 0.9 meters and an outer radius of 3 meters, which can confine a plasma with a volume of 100 m3.) But the required circular current through the plasma makes it prone to instabilities, resulting in stoppages called disruptions.
Stellarators don’t have this problem. Their difficulty was that the design of the machine required intensive 3-D simulation for determining the optimal shape of the magnetic field and the shapes of the helical and toroidal coils that created it. In the 1980s, supercomputers could do the job. At that time, the researchers in Greifswald decided on a new “bottom-up” approach, modeling the field using the fundamental equations of magnetohydrodynamics, says Thomas Klinger, who heads the W7-X project.
Although Wendelstein 7-X is not designed for actual fusion reactions, the experience of building and operating it will likely contribute valuable insights to the construction of ITER, which requires similar technology. The magnetic field in the machine will be created by superconducting magnets, cooled to a temperature of -269 °C, and carry a current of 18 200 amperes. The ITER design also calls for superconducting coils, and experience with this cryogenic technology will be important, says Klinger. “The fact that you have a cryogenic machine drives the quality requirements to a much higher level,” he says. “You have to meet high-level industrial standards as they exist in aviation or space science, and there we are contributing to ITER.”
What about a possible follow-up machine? Klinger says it’s too early. “First we have to gain experience running this machine. We have very good evidence that the W7-X is already very, very close to the design we need for a power station,” he says. “We first need success with our project. It has to prove to be fully competitive with the tokamak, and it has to be able to run steady state.” Steady-state operation is best for power generation, in part because you avoid thermal cycling and material fatigue. But it’s difficult to achieve in a tokamak.
Indeed, David Anderson, who directs research with the Helically Symmetric Experiment stellarator at the University of Wisconsin–Madison, believes that stellarators might be better suited for electricity generation. “They are much easier to operate than tokamaks,” he says.
That’s not to say stellarators are necessarily the way to go, according to Larry Grisham, a physicist who was with the Princeton Plasma Physics Laboratory until he recently became director of strategic development at the magnetometer firm Twinleaf, in Princeton, N.J. “The major disadvantage of a stellarator relative to a tokamak is that, for a given size, it is much more complicated to fabricate and assemble, and would presumably be even more complicated than a tokamak to repair in a radioactive environment,” Grisham says.
It will, of course, be many years before one design or the other can really prove its worth as a power generator.
About the Author
Alexander Hellemans covers science and technology in Europe. With Bryan Bunch, he is author of The History of Science and Technology: A Browser’s Guide to the Great Discoveries, Inventions, and the People Who Made Them from the Dawn of Time to Today (Houghton-Mifflin, 2004). In the May 2013 issue he reported on how nanowire transistors could save Moore’s Law.