Wendelstein 7-X sets new performance records in fusion research

A new fusion record was set thanks to researchers in Germany and the U.S. The international effort moves the world one step closer to a commercial fusion power plant, which will need to run continuously at temperatures hotter than the sun. 

The record-breaking machine, known as the Wendelstein 7-X (W7-X), is a stellarator. This twisty fusion system confines plasma using external magnets so the nuclei of atoms fuse together and release energy. W7-X is operated by the Max Planck Institute for Plasma Physics (IPP) in Germany and has systems designed and built by the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) and Oak Ridge National Laboratory (ORNL) with support from the Fusion Energy Sciences (FES) program in the DOE’s Office of Science.

“This world record marks the highest performing sustained fusion experiment that ran longer than 30 seconds, with record performance lasting for a full 43 seconds,” said Novimir Pablant, the division head for stellarator experiments at PPPL. Pablant said if they can reach this record for 30 seconds, there’s every reason to believe these plasma conditions could be sustained for weeks, months or even years because 30 seconds is long enough for the scientists to see the relevant physics at work. “This experiment ran long enough that nothing is changing any longer in terms of the plasma or experiment conditions.” However, there are technical challenges still to extending this discharge to, say, 30 minutes or more, related to the reliability of technology.

World’s best triple product for long plasma durations

The record is for something fusion researchers call the triple product, a key measure of whether a fusion system can sustain itself. It’s calculated by multiplying three factors: plasma density, temperature and the length of time the plasma is confined in the magnetic field. The new record-breaking result applies to stellarators and another, more common fusion system known as a tokamak. Tokamaks have a simpler design, but unlike stellarators, they require an internal electric current to sustain the plasma.

The previous triple product record holder for plasma pulses longer than 10 seconds was the Joint European Torus (JET) tokamak in the U.K., which was decommissioned at the end of 2023. At these longer durations — which are more relevant for future power plants — W7-X now leads, despite a plasma volume that is three times smaller than JET and five times less heating power. Larger plasma volumes make it significantly easier to achieve higher plasma performance. 

“The new record is a tremendous achievement by the international team. It impressively demonstrates the potential of Wendelstein 7-X. Elevating the triple product to tokamak levels during long plasma pulses marks another important milestone on the way toward a power plant-capable stellarator,” said Thomas Klinger, head of stellarator dynamics and transport at IPP.

W7-X is designed to demonstrate that stellarators can, in practice, achieve the outstanding properties predicted by theory and qualify as a concept for future fusion power plants. The fusion reaction must take place in a plasma — a soup of electrically charged particles heated to tens of millions of degrees Celsius. The plasma is trapped by a complex and powerful magnetic field, floating inside a doughnut-shaped vacuum chamber. 

Key to success: A new pellet injector system
PPPL played a key role in the record-setting experiment, developing the control system for ORNL’s pellet injector, which injects frozen hydrogen pellets into the plasma, enabling long plasma durations through continuous refueling. Pablant said the control system is a lot like the knobs on a stove. “It sets the parameters of the pellet injector.” Researchers use the control system to, for example, set the rate at which pellets will be injected, as well as their speed and the start and stop times. “It also controls all of the vacuum pumps for the pellet injector, reads the sensors and records the measurements of everything.”

During the record-setting experiment, about 70 frozen hydrogen pellets, each about a millimeter in size, were injected over 30 seconds, while microwaves heated the plasma. Precise coordination between heating and pellet injection was crucial to achieve the optimal balance between heating power and fuel supply. The key was operating the pellet injector with variable preprogrammed pulse rates for the first time — a scheme executed with impressive precision. This method is directly relevant for future fusion systems and can potentially extend plasma durations in W7-X to 30 minutes.

PPPL also operated — in collaboration with Tomas Gonda from Auburn University — an instrument that measured one of the key performance indicators for the record plasma: the temperature of the ions in the W7-X plasma. This diagnostic tool, known as an X-ray spectrometer, indicated that plasma temperatures exceeded 20 million degrees Celsius, with peaks reaching up to 30 million degrees. This work highlights one of the Lab’s core capabilities: diagnostics. PPPL provides measurement systems used worldwide, which are key in understanding the conditions needed for fusion.

This work was performed under the auspices of the U.S. DOE under contract number DE-AC02-09CH11466, and by UT-Battelle, LLC under DE-AC05-00OR22725. This work was also carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme Grant Agreement No. 101052200—EUROfusion. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them.

PPPL is mastering the art of using plasma — the fourth state of matter — to solve some of the world’s toughest science and technology challenges. Nestled on Princeton University’s Forrestal Campus in Plainsboro, New Jersey, our research ignites innovation in a range of applications including fusion energy, nanoscale fabrication, quantum materials and devices, and sustainability science. The University manages the Laboratory for the U.S. Department of Energy’s Office of Science, which is the nation’s single largest supporter of basic research in the physical sciences. Feel the heat at https://energy.gov/science and https://www.pppl.gov.