The stellarator fell out of favor in the late 1960s. The device, a magnetic-confinement fusion reactor named for the sun, was shoved to the side after Soviet scientists revealed their tokamak design to the world in 1968. The tokamak has been the preferred design for fusion reactors ever since, but the stellarator might be making a comeback.
German scientists at the Max Planck Institute for Plasma Physics (IPP) built a stellarator called the Wendelstein 7-X that was switched on for the first time in 2015. Previous tests pushed the plasma in the reactor to higher temperatures and densities than ever before achieved in a stellarator, and now the IPP reports that it has broken its old records in a new test with upgraded components on the Wendelstein 7-X.
A stellarator is similar to a tokamak in that both devices use large superconducting magnets to suspend hydrogen plasma and heat it to the temperatures and pressures required to fuse the material into helium. (The Wendelstein 7-X consists of 50 superconducting magnet coils about 3.5 meters high.) The stellarator, however, traps the plasma in a twisting and spiraling shape, rather than the (doughnut shape) of a tokamak. The twisting path of a stellarator is designed to cancel out instabilities present in the suspended hydrogen plasma.
In a , IPP researchers pushed the Wendelstein 7-X to a record stellarator "fusion product," which is a measure of the ion temperature, density of the plasma, and energy confinement time. This value provides an indication of how close the device is to hitting sustainable nuclear fusion (generating more energy than is initially required to start the reaction). The Wendelstein 7-X reached an ion temperature of about 40 million degrees and a density of 0.8 x 1020 particles per cubic meter, producing a fusion product of 6 x 1026 degrees x second per cubic meter—a new world record for stellarators.
"This is an excellent value for a device of this size, achieved, moreover, under realistic conditions, i.e. at a high temperature of the plasma ions," said Thomas Sunn Pederse, director of the Stellarator Edge and Divertor Physics Division at the IPP, in a .
To achieve improved efficiency, the Wendelstein 7-X was outfitted with a new interior wall of graphite tiles, allowing higher temperatures to be reached. This interior lining, known as a divertor, protects the twisting chamber walls and allows technicians to pump more plasma in at higher temperatures, providing more control over the density and purity of the hydrogen plasma as well. "First experience with the new wall elements are highly positive," Sunn Pedersen says.
Previous experiments achieved pulses of plasma lasting about six seconds. The new divertor lining has allowed the researchers to jack up the plasma pulse time to 26 seconds. Heating energies introduced to the system were also increased to about 18 times those of previous experiments, up to 75 megajoules of energy.
In addition to introducing the new graphite interior lining, the Wendelstein 7-X team was able to optimize the reactor based on data from the previous experiments, which were analyzed in a recent published in Nature Physics. Plasma experiments will resume with the reactor in July, and in the future, the IPP plans to replace the graphite tiles of the divertor with water-cooled components, allowing plasma pulses of up to 30 seconds.
The Wendelstein 7-X, which is an experimental reactor not designed to generate power, continues to get closer to its fusion optimization goals. Along with stellarator experiments in the U.S. and Japan, the work is bringing this unique and complex fusion reactor design back from the dead. And with experiments at places like MIT and Google using new helium-3 fuels and eddy current plasma confinement, the world continues to inch closer and closer to the promise of half a century ago: virtually limitless energy generated by the same method that powers the sun.