View into the Wendelstein 7-X experimental hall at the Max Planck Institute for Plasma Physics in Greifswald, Germany. Credit: MPI for Plasma Physics, Jan Hosan.

The world’s most powerful stellarator, Wendelstein 7-X, has set the record in a key parameter of fusion physics: the triple product.

On the path toward a fusion power plant, stellarators are among the most promising concepts. In the future, they could generate usable energy by fusing light atomic nuclei.

This reaction must take place in a plasma — a hot gas of ionised particles heated to many tens of millions of degrees Celsius.

Stellarators use magnetic confinement to hold the plasma: the plasma is trapped by a complex and powerful magnetic field, floating inside a donut-shaped vacuum chamber.

With Wendelstein 7-X (W7-X), the Max Planck Institute for Plasma Physics (IPP) in Greifswald, with support from the European fusion consortium EUROfusion, is operating the world’s largest and most powerful experiment of its kind.

W7-X is designed to demonstrate that stellarators can, in practice, achieve the outstanding properties predicted by theory – and thus qualify as a concept for future fusion power plants.

View inside the vacuum vessel of the stellarator Wendelstein 7-X at the Max Planck Institute for Plasma Physics in Greifswald, Germany. Credit: MPI for Plasma Physics, Jan Hosan.

World-best triple product for long plasma durations

In the OP 2.3 campaign, which ended on May 22, the international W7-X team achieved a new world record for the triple product in long plasma discharges: on this last day, they sustained a new peak value of this key fusion parameter (see explanation below) for 43 seconds.

W7-X thus surpassed the best performances of fusion devices of the tokamak type for longer plasma durations.

Tokamaks also rely on magnetic confinement but are much better studied due to their simpler design. The highest values for the triple product were achieved by the Japanese Tokamak JT60U (decommissioned in 2008) and the European Tokamak facility JET in Great Britain (decommissioned in 2023).

With short plasma durations of just a few seconds, they remain the clear front-runners.  In terms of longer plasma durations, which are important for a future power plant, W7-X is now ahead, even though JET had three times the plasma volume.

Size makes it much easier to achieve high temperatures in fusion reactors.

Professor Dr Thomas Klinger, head of operations at Wendelstein 7-X and head of stellarator dynamics and transport at IPP, said: “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.”

In a future fusion power plant, a plasma with a high triple product (y-axis, logarithmic scale) must be maintained for long periods (x-axis). Previous fusion experiments only achieved high values for plasma durations of a very few seconds. On May 22, 2025, Wendelstein 7-X achieved the world record for plasma times of more than 30 seconds with a high fusion product. In this OP2.3 experiment campaign, further best values were achieved for plasma durations between 30 and 40 seconds. Tokamaks remain the record holders for short plasma times. Credit: MPI for Plasma Physics, Dinklage et al (to be published)/ X Litaudon et al 2024 Nucl. Fusion 64 015001.

Key to success

The new triple product world record for long pulses was made possible by the close collaboration between the European Wendelstein 7-X team in Greifswald and partners from the USA.

A key role was played by the new pellet injector (more details at the end of this article), which injects frozen hydrogen pellets into the plasma, enabling long plasma durations through continuous refuelling.

The US Department of Energy’s (DOE) Oak Ridge National Laboratory (ORNL) in Tennessee developed this highly sophisticated and globally unique injector and successfully put it into operation at Wendelstein 7-X.

During the record-setting experiment, about 90 frozen hydrogen pellets, each about a millimetre in size, were injected over 43 seconds, while powerful microwaves simultaneously heated the plasma.

Precise co-ordination 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 pre-programmed pulse rates for the first time — a scheme executed with impressive precision.

This method is directly relevant for future fusion reactors and can potentially extend plasma durations to several minutes.

In the record-breaking experiment, the plasma temperature was raised to over 20 million degrees Celsius, reaching a peak of 30 million degrees.

Measurements to calculate the triple product were provided, among others, by Princeton Plasma Physics Laboratory, which operates an X-ray spectrometer for ion temperature diagnostics at W7-X.

The necessary electron density data came from IPP’s worldwide unique interferometer. The energy confinement time required for the triple product calculation was also determined using diagnostic tools developed at IPP.

Additional highlights

During the OP 2.3 experimental campaign, Wendelstein 7-X achieved two further milestones:

• Energy turnover was increased to 1.8 gigajoules (plasma duration: 360 seconds).

The previous record from February 2023 was 1.3 gigajoules. Energy turnover is calculated as the product of injected heating power and plasma duration.

Maintaining continuous high-energy input and removing the generated heat are prerequisites for future power plant operation.

The corresponding best value for the 1,000-second discharge in the Tokamak EAST (China) was even slightly exceeded by Wendelstein 7-X.

• Plasma pressure relative to magnetic pressure reached 3% for the first time across the full plasma volume.

In a dedicated experiment series, the magnetic field was deliberately reduced to about 70%, lowering magnetic pressure and allowing plasma pressure to rise.

This ratio is a key parameter for extrapolating to a fusion power plant, where 4 – 5% across the volume will be needed.

The new record value was accompanied by a peak ion temperature of around 40 million degrees Celsius.

Professor Dr Robert Wolf, head of stellarator heating and optimization at IPP, summarised: “The records of this experimental campaign are much more than mere numbers.

“They represent a significant step forward in validating the stellarator concept — made possible through outstanding international collaboration.”