Energy Generation of the Future is Approaching Closer Nuclear Fusion: Wendelstein 7-X Sets Rcord in Triple Product

At the Wendelstein 7-X nuclear fusion experimental facility, scientists achieved a world record in a key parameter of fusion physics—the triple product. This could be the next step on the path to a fusion power plant.

Nuclear fusion power plants promise a solution to all energy and environmental problems. Scientists have long focused on magnetic fusion, which must slowly transition into applied research. According to forecasts, magnetic fusion is expected to be the dominant technology by 2050. The two concepts, Stellarator and Tokamak, are being advanced at the European level. On the path to a fusion power plant, Stellarator-type facilities are one of the most promising options.

Now, the W7-X team from the Max Planck Institute for Plasma Physics (IPP) at the Wendelstein 7-X experimental power plant in Greifswald (Germany) announced a record: On May 22, 2025, scientists achieved a new peak value for the central key figure in fusion physics—the triple product—in the experimental campaign OP 2.3 with a plasma duration exceeding 43 seconds. This defines when more energy comes "out" of the fusion reactor than was "put in," meaning the fusion occurs. Thus, Wendelstein 7-X surpasses the best values of Tokamak-type fusion facilities at longer plasma durations.

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The highest values for the triple product were achieved by the Japanese tokamak JT60U (decommissioned in 2008) and the European tokamak facility JET in the UK (decommissioned in 2023). With short plasma durations of a few seconds, they remain the clear front-runners.

World's Best Triple Product for Long Plasma Durations

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 significantly facilitates achieving high temperatures in fusion facilities.

The world record for the triple product for long pulses was achieved through a new pellet injector, which shoots frozen hydrogen pellets into the plasma, enabling long plasma durations through "fuel replenishment." This highly complex and globally unique injector was developed by the Oak Ridge National Laboratory of the US Department of Energy (DOE) in Tennessee.

"The new record is a great achievement of the international team. It impressively demonstrates the potential of Wendelstein 7-X. Being able to raise the triple product to Tokamak levels during long plasma durations marks another important milestone on the path to a power-plant-suitable Stellarator," says Prof. Dr. Thomas Klinger, head of the operation of Wendelstein 7-X and the Stellarator Dynamics and Transport division at IPP.

This is How the Triple Product Experiments Were Conducted

In the record experiment for the triple product, about 90 frozen millimeter-sized hydrogen pellets were injected in rapid sequence over 43 seconds, while strong microwaves simultaneously heated the plasma. Heating and the pellet injector had to be precisely coordinated to achieve the optimal combination of heating power and fuel filling.

For this, the pellet injector was operated for the first time to work with different predefined pulse rates. This scheme is directly relevant for a future fusion reactor. It can potentially also be extended to longer plasma durations of several minutes.

The use of pellets was made possible thanks to the groundwork of several European laboratories, including simulation calculations by the Spanish fusion research laboratory CIEMAT and observations with ultra-fast cameras by the HUN-REN Centre for Energy Research 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 to bring the plasma to fusion-relevant temperatures.

During the record experiment, the plasma temperature was driven to over 36 million °F, peaking at even 54 million °F. The measurements for calculating the triple product were provided in part by the Princeton Plasma Physics Laboratory of the DOE, which operates an X-ray spectrometer to determine the plasma ion temperature at W7-X. The necessary measurement values for plasma electron density were obtained from the IPP's globally unique interferometer. The energy confinement time required for calculating the triple product was also determined with IPP measuring instruments.

Prof. Dr. Robert Wolf, head of the Stellarator Heating and Optimization Division at IPP, summarizes: "The records of this experimental campaign are more than just measurements. They represent an important advancement in the validation of the stellarator concept—enabled by excellent international collaboration."

Background: What is the Triple Product?

The triple product is the key measurement in a fusion power plant. Only above a certain threshold, as defined by the Lawson criterion, can a plasma in a facility generate more fusion power than the thermal power that needs to be invested. The energy balance then becomes positive, meaning more fusion energy is generated than heating energy is required to bring the plasma to temperature. Once the threshold is exceeded, the fusion reaction can sustain itself without further heating.

For a power plant, this threshold is:

n∙T∙ τ = 3 ∙ 10²¹ m^-3 keV s

The triple product results from the particle density of the plasma n, its temperature T (more precisely: the temperature of the ions, between which fusion reactions occur), and the energy confinement time 𝜏, which is the duration over which the thermal energy escapes from the plasma if not reheated. The confinement time is thus a measure of thermal insulation.

This is how the pellet injector works

Since September 2024, the new continuous-operation pellet injector has been successfully in use in Greifswald, Germany. The device continuously generates a strand of hydrogen ice (0.12 inches in diameter), from which 0.13 inches long cylindrical pieces (pellets) are cut at intervals of fractions of a second, to be shot into the plasma at high pressure like in a blowpipe. The pellets reach speeds of 670 to 1,790 mph. (kr)

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