Wendelstein 7-X shares a long-pulse fusion record with JET

GREIFSWALD, Germany, May 2025 — On May 22, 2025, the last day of its OP 2.3 campaign, the Wendelstein 7-X stellarator sustained a peak triple product for 43 seconds, a world-best for long plasmas that IPP now says it shares with the UK's JET tokamak. Here's what happened, what it cost, and what went wrong.

Cost and outcome (as of June 30, 2025)

Outcome: a joint record with JET for sustaining the triple product over long pulses. (IPP)
Duration: 43 seconds, refuelled by about 90 frozen hydrogen pellets. (IPP)
Also: 1.8 GJ of energy turnover in a 360-second plasma, up from 1.3 GJ in February 2023. (IPP)
Cost: not stated by IPP.
Status: a research result; no fusion power produced or claimed.

What a stellarator is

Stellarators hold plasma in a complex magnetic field inside a donut-shaped vacuum chamber. Tokamaks also use magnetic confinement but are "much better studied due to their simpler design," IPP says. W7-X is meant to show stellarators can deliver the properties theory predicts. (IPP)

European labs contributed too: Spain's CIEMAT ran simulations, Budapest's HUN-REN Centre for Energy Research filmed with ultra-fast cameras, and the microwave heating was developed with the Karlsruhe Institute of Technology and the University of Stuttgart. (IPP)

What the record is

The triple product, also called the Lawson criterion, multiplies plasma density, ion temperature and energy confinement time. Only above a threshold, which IPP gives as 3 × 10²¹ m⁻³ keV s, can a plasma produce more fusion power than the heating put in. (IPP)

Tokamaks still hold the highest values for short plasmas of a few seconds: Japan's JT-60U and JET. W7-X's record is for longer durations, which IPP calls important for a power plant. (IPP)

The JET correction

After the first release, members of the JET community told IPP about unpublished results from the end of JET's life. JET also achieved record triple products over long high-performance phases, "comparable to the best values achieved by Wendelstein 7-X," sustained for up to 60 seconds. The paper is under review. (IPP)

"If the as yet unpublished JET data are included, the Wendelstein 7-X stellarator and the JET tokamak are now joint leaders," IPP wrote. It called the parity remarkable because JET had three times W7-X's plasma volume. (IPP)

The pellet injector

Oak Ridge National Laboratory built the continuously operating pellet injector, in use since September 2024. During the record shot, about 90 millimetre-sized hydrogen pellets went in over 43 seconds while microwaves heated the plasma. Running the injector with variable pre-programmed pulse rates was a first, and IPP says the method could extend plasmas to several minutes. (IPP)

Princeton Plasma Physics Laboratory's X-ray spectrometer supplied ion-temperature data for the calculation. The plasma passed 20 million degrees Celsius, peaking at 30 million. (IPP)

Other campaign results

Plasma pressure reached 3% of magnetic pressure across the whole plasma for the first time, with the magnetic field cut to about 70%. A power plant would need 4 to 5%, IPP says. W7-X also slightly exceeded the energy-turnover best value from China's EAST tokamak in a 1,000-second discharge. (IPP)

"Elevating the triple product to tokamak levels during long plasma pulses marks another important milestone," said Thomas Klinger, head of W7-X operations. (IPP)

What went wrong

The first claim was overstated. IPP's original release said W7-X surpassed tokamaks for long pulses; its June 30 update made it a joint lead with JET. (IPP)
Short of a power plant. 3% plasma pressure ratio against the 4 to 5% a plant would need. (IPP)
JET's figures aren't published yet. They're under journal review, per IPP. (IPP)
What we don't know yet: the triple product value reached, which IPP's page shows only on a chart, and the campaign's cost.

Our follow-up question for the Max Planck Institute for Plasma Physics

Has the JET long-pulse paper been published, and does it change the joint-record standing?

Sources

Max Planck Institute for Plasma Physics, “Wendelstein 7-X sets new performance records in fusion research [UPDATE],” June 3, 2025, updated June 30, 2025. ipp.mpg.de

Disclosure: [not in copy] Editor placeholder