Harnessing fusion energy requires seeing deep inside the plasma that fuels the reaction to understand its behavior. But it's challenging to catch a glimpse. Custom technology is needed to measure particles hotter than the sun, many times per second.
A new international project will add powerful new X‑ray imaging systems to fusion experiments in France and Japan, along with a multi‑energy camera system in France, to make those measurements and help guide the design of future fusion systems.
The effort is led by the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL), a global leader in fusion research, working with partners at Massachusetts Institute of Technology (MIT), the University of Tennessee, Knoxville (UTK) and host laboratories overseas. R-V Industries (RVI), a private company based in Honey Brook, Pennsylvania, built and tested many of the system's parts, including the vacuum chambers, stands, mounts and bellows.
"This investment marks a critical step toward advancing our U.S. Fusion Science & Technology Roadmap and the Genesis Mission," said Jean Paul Allain, Director of the Office of Fusion at DOE. "The high-quality data generated will be invaluable for model validation and verification, while also advancing our efforts to converge artificial intelligence and fusion data, supporting the DOE's Genesis Mission through the AI-Fusion Digital Convergence Platform."
Staff from PPPL, RVI and the National Institutes for Quantum Science and Technology meet to do final vacuum tests of the X‑ray imaging system at RVI in Honey Brook, Pennsylvania. (Photo courtesy of Sunny Nyhus / PPPL)
DOE has provided $12.5 million in funding for the project, with PPPL staff stationed abroad for several years. International partners often turn to PPPL for the Lab's unparalleled theory, computation and diagnostic techniques, adding rich value to the overall fusion landscape. As PPPL marks its 75th anniversary this year, the project highlights how the Lab's legacy of discovery continues to shape the future of fusion energy around the world.
"This is a strong example of scaling up the capability of the Lab and the U.S. program through international partnership on a major international facility," said Matthew Lanctot, acting research division director for the DOE's Fusion Energy Sciences.
Seeing the whole plasma
At the tungsten (W) Environment in Steady-state Tokamak (WEST), PPPL and MIT are adding two new X-ray Imaging Crystal Spectrometer (XICS) systems to look through the top and bottom of the plasma, adding to an existing French system that looks through the center. Because these new views avoid the central axis of the doughnut-shaped plasma, scientists call them 'off-axis' - and they're essential for seeing the full picture. The additional systems will let researchers look at the plasma from more angles and with greater precision. Such a view is critical for understanding how plasma behaves and, ultimately, how to produce a sustained fusion reaction.
"If you think of the plasma like a human body, if you only look at the belly button, then you don't know what's happening with the head or the feet," said PPPL's head of advanced projects Luis Delgado-Aparicio, who leads the project. "Now we will be completing the picture, so we can study the entire body."
Ultimately, the expanded and improved view provided by XICS will allow for a better understanding of how plasma behaves inside a fusion system like WEST, which is operated by France's Alternative Energies and Atomic Energy Commission in partnership with the EUROfusion consortium. It is one of many fusion systems worldwide known as a tokamak: a doughnut-shaped device that confines a plasma using magnetic fields. WEST is particularly interesting to study because its walls are made of tungsten, a material many fusion researchers believe is the best choice in terms of longevity and plasma management.
MIT is implementing the two off-axis XICS systems, which will show how temperature, rotation and tungsten impurity levels vary across the entire plasma - not just at one point, but mapped from the plasma's core to edge. "This is crucial information for all heat, momentum and impurity transport studies," said John Rice, a senior research scientist at MIT's Plasma Science and Fusion Center.
DOE Under Secretary for Science Darío Gil is pictured (fifth from left) in Japan, with the opened crates containing PPPL's X-ray Imaging Crystal Spectrometer. (Photo courtesy of Sunny Nyhus / PPPL)
Managing heat for future fusion systems
Delgado‑Aparicio and PPPL staff research scientist Tullio Barbui are also designing a new vertical multi-energy soft X-ray camera to pair with an existing horizontal camera on WEST. Much like XICS, the vertical multi-energy camera will provide insights into managing the heat inside a tungsten-clad tokamak.
"Using the data produced by the multi-energy suite and by XICS, we're going to all work together to understand particle transport, plasma confinement and radiation management and, ultimately, manage power loss so that fusion systems can run efficiently," said Delgado‑Aparicio.
Livia Casali, an assistant professor, Zinkle Fellow and ITER scientist fellow at UTK, will design and execute experiments to test impurity behavior. The measurements from the new PPPL spectrometer will provide detailed constraints on radiation and impurity transport. Casali will then use her novel computer code, SICAS, to analyze the experimental data gathered in WEST and the tokamak JT-60SA, which is in Naka, Japan. "Impurities affect radiation and temperature, which, in turn, modify plasma conditions that then alter impurity behavior," Casali said. "SICAS captures this feedback loop consistently, producing a clear and unified view of the whole plasma system." Casali's code simulates ion and impurity transport across the entire plasma system within an integrated framework that allows each region to dynamically influence the others.
Livia Casali, an assistant professor, Zinkle Fellow and ITER research scientist fellow at WEST, is pictured in Cadarache, France. (Photo courtesy of Livia Casali / UTK)
Testing advanced scenarios on JT‑60SA
JT‑60SA, a tokamak operated by Japan's National Institutes for Quantum Science and Technology in collaboration with Europe's Fusion for Energy, will also receive a 3.3‑metric‑ton XICS system designed and built by PPPL. The XICS system has already been packed into seven large crates for shipment and will be installed and tested over the next two years, with the first data expected in September 2026.
The project will involve significant international collaboration and data sharing, with PPPL researchers working in Japan for the next four years. The project is just one way that PPPL continues to amplify its impact through partnerships with companies, universities and labs across the U.S. and the world.
"This project ties together what we learn on WEST and JT‑60SA and feeds it directly into PPPL's broader tokamak program," said Rajesh Maingi, head of tokamak experimental science at PPPL, who serves as the project's formal monitor. "It's a model for how U.S. laboratories can contribute high‑impact diagnostics to international facilities."
