Watch as solid hydrogen is extruded to feed German stellarator
In May, the Wendelstein 7-X stellarator in Greifswald, Germany, concluded an experimental campaign by sustaining a plasma with a high triple product for 43 seconds. The machine far surpassed its own previous performance with a value that the Max Planck Institute for Plasma Physics (IPP) says “exceeds previous tokamak records for long plasma durations”—in part because of a fuel pellet injection system developed by researchers at Oak Ridge National Laboratory.
Watch ORNL’s video of that fuel pellet injection system—in use since September 2024—as it extrudes a column of frozen hydrogen and then cuts individual 3.2-millimeter-long pellets. The process, which takes just half a millisecond, was captured in slow motion by ORNL engineer Steve Meitner.
A still image captured from ORNL’s video of solid hydrogen extrusion. (Source: ORNL)
Larry Baylor and Steve Meitner of ORNL during the commissioning of the continuous pellet fueling system at the Wendelstein 7-X stellarator. (Photo: Jürgen Baldzuhn/Max Planck Institute for Plasma Physics)
ORNL’s role: The Wendelstein 7-X, currently the world’s largest stellarator, achieved its first plasma in 2015. The machine has excelled at reaching high temperatures (one factor of the fusion triple product), but researchers want to increase plasma density and confinement time. They did that by injecting more fuel—in the form of hydrogen pellets—directly into the stellarator, improving the stellarator’s plasma density and confinement.
“The W7-X device had previously achieved high performance for short durations, but they couldn’t sustain it at high plasma density,” said Meitner. “That’s where we came in.”
The system: The ORNL pellet fueling system designed for the W7-X device injects a continuous, high-speed, and customizable stream of solid hydrogen pellets—formed at 12 degrees above absolute zero—directly into the 30-million-degree plasma, according to ORNL. The device extrudes a filament of frozen hydrogen, from which 3.2-millimeter-long cylindrical pellets are cut and then fired into the plasma at speeds of 300 to 800 meters per second.
IPP said that its experimental record was achieved by injecting “about 90 frozen hydrogen pellets . . . while powerful microwaves simultaneously heated the plasma.” Those pellets were injected using “variable pre-programmed pulse rates for the first time. . . . This method is directly relevant for future fusion reactors and can potentially extend plasma durations to several minutes.”
Contested record: After IPP announced Wendelstein 7-X’s recent campaign in early June, researchers involved in the final experiments at JET, the long-serving Joint European Torus in the United Kingdom (decommissioned in 2023), described unpublished JET experiments that challenge IPP’s record triple product claims for Wendelstein 7-X. On June 30, IPP appended an update to its announcement:
After publishing this news item, members of the JET community informed us about previously unpublished results from the now decommissioned tokamak in the United Kingdom. At the end of JET's operating life, experiments were carried out there to maximize pulse lengths and energy conversion. In terms of the duration of the high-performance phase, JET also achieved a new world records [sic] for triple products. These are comparable to the best values achieved by Wendelstein 7-X. The record pulses were even sustained for up to 60 seconds. The corresponding technical article is currently being reviewed and will soon be published in a scientific journal.
What does this mean for the statements about Wendelstein 7-X? If the as yet unpublished JET data are included, the Wendelstein 7-X stellarator and the JET tokamak are now joint leaders in sustaining the triple product level for longer times. The fact that W7-X results are on a par with JET is remarkable because JET had three times the plasma volume of Wendelstein 7-X.
