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. Wendelstein 7-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, Wendelstein 7-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.
"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," says Prof. Dr. Thomas Klinger, Head of Operations at Wendelstein 7-X and Head of Stellarator Dynamics and Transport at IPP.
Key to success: the new pellet injector from Oak Ridge National Laboratory
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 refueling. The U.S. 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 millimeter in size, were injected over 43 seconds, while powerful microwaves simultaneously 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 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.
The use of pellets was made possible thanks to preliminary work carried out by several European laboratories, including simulation calculations by the Centre for Energy, Environmental and Technological Research (CIEMAT) in Spain and observations using ultra-fast cameras by the HUN-REN Centre for Energy Research in Budapest. The microwave heating system (more precisely: electron cyclotron resonance) was developed in collaboration with the Karlsruhe Institute of Technology (KIT) and a team from the University of Stuttgart. It is considered the most promising method for bringing plasma to temperatures relevant for fusion.
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 from the OP 2.3 campaign
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 1000-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. Prof. Dr. Robert Wolf, Head of Stellarator Heating and Optimization at IPP, summarizes:
"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."
More information about the triple product
The triple product — also known as the Lawson criterion — is the key metric for success on the path to a fusion power plant. Only when a certain threshold is exceeded can a plasma produce more fusion power than the heating power invested. This marks the point where the energy balance becomes positive, and the fusion reaction can sustain itself without continued external heating.
For a fusion power plant, the required threshold is:
n∙T∙???? = 3 × 10²¹ m⁻³ keV s
The triple product is derived from three factors:
the particle density of the plasma n,
its temperature T (more precisely: the temperature of the ions between which fusion reactions take place) and
the energy confinement time ???? (pronounced: tau), i.e. the time it takes for the thermal energy to escape from the plasma if no additional heat is supplied. The confinement time is therefore a measure of the thermal insulation.
More information about the pellet injector
Since September 2024, the new continuously operating pellet injector has been successfully in use.
It was developed at Oak Ridge National Laboratory, a research center of the U.S. Department of Energy, specifically for Wendelstein 7-X, and it sets a global benchmark in its category.
The pellet injector ensures a steady supply of hydrogen particles into the plasma — a crucial requirement for future fusion power plants.
The device continuously forms a 3-millimeter-diameter strand of frozen hydrogen, from which 3.2-millimeter-long cylindrical pellets are cut at intervals of fractions of a second and fired into the plasma at speeds of 300 to 800 meters per second.
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