World's largest superconducting fusion system will use American technology to measure the plasma within

World's largest superconducting fusion system will use American technology to measure the plasma within
by Rachel Kremen for PPPL
Plainsboro NJ (SPX) Oct 16, 2025

When the experimental fusion system known as JT-60SA comes online in 2026, it will be the world's largest fusion machine: a crowning achievement for Japan and Europe, which partnered to build it. Now, the research team has turned to the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) for critical measurement equipment.

The effort is part of a new agreement between PPPL, the National Institutes for Quantum Science and Technology (QST) of Japan and Europe's Fusion for Energy (F4E), allowing for broader collaboration between the researchers.

"PPPL is among the first U.S. institutions to have its equipment installed directly into JT-60SA," said Luis Delgado-Aparicio, head of advanced projects at PPPL. He leads the PPPL team working on the project along with PPPL Principal Research Physicist Masayuki Ono.

Under the agreement, the Lab will provide a measurement tool, or diagnostic, called an X-ray imaging crystal spectrometer (XICS). XICS will help scientists better understand and control the plasma inside JT-60SA. The four-ton tool will be installed in winter 2026 and will begin collecting data in that summer. Placing the XICS on the largest fusion tokamak in the world positions the U.S. and its strategic partnership with Japan at a new level.

The valve that will connect the XICS diagnostic to JT-60SA manufactured by Metal Technologies Company of Japan. The valve was specially designed by a team at PPPL, QST and MTC led by Luis F. Delgado-Aparicio and Masayuki Ono, who stand fifth and sixth from left. (Photo credit: Luis F. Delgado-Aparicio)

Precise measurements will be needed to produce commercial fusion

The XICS measures X-rays emitted by the plasma to determine critical information, including the temperature, speed and direction of flow of the plasma particles, as well as the density of impurities - unwanted particles that can cool the plasma. While in some ways this cooling can be beneficial, the plasma temperature needs to be carefully monitored to achieve maximum efficiency of the fusion system. These measurements are essential to keeping the fusion reaction stable and preventing the plasma from escaping its magnetic containment and damaging the inside of the fusion system.

Similar systems sometimes provide inaccurate measurements if the temperature shifts. But PPPL's XICS has an advanced calibration system that ensures every measurement is highly accurate, regardless of changes in density and temperature. This level of precision is crucial for achieving the stable, high-performance plasma conditions needed for commercial fusion power plants.

PPPL: Contributing to the world's most important fusion systems

JT-60SA represents a crucial stepping stone toward someday achieving commercial fusion energy. The machine uses superconducting magnets, which can operate continuously without losing energy to electrical resistance as long as the magnets are kept at an extremely cold temperature. In normal electrical systems, some energy is always lost as heat due to resistance. But when superconducting magnets are cooled to incredibly low temperatures, they lose all resistance and become highly efficient. This makes JT-60SA more similar to future power plants than older experimental machines.

It will be the most powerful tokamak before ITER is operational, the multinational fusion facility under construction in France. Despite being smaller than ITER, JT-60SA's power density - or power per unit volume - will be exceptionally high, allowing scientists to explore new plasma behaviors and test concepts for future power plants.

"This calibration scheme has never been implemented before at this scale," said Delgado-Aparicio. "Because JT-60SA will be such a powerful machine, we will access operating conditions that we have never achieved before. The measurements need to be very accurate for us to learn the science of those new regimes."

PPPL was the natural choice when JT-60SA's operators decided to seek international collaboration for their diagnostic systems, as the Lab pioneered and refined the diagnostic over the last two decades. The Lab also has a long history of developing diagnostic systems used around the world. PPPL's XICS system has already been installed on several fusion systems worldwide, including the Large Helical Device in Japan and Wendelstein 7-X in Germany.

"The XICS is essential. You need something like it to get the data from plasma and do the physics. That's one reason we were chosen to be the first U.S. institution to collaborate with JT-60SA," Ono said.

The collaboration extends beyond providing equipment for the tokamak or fusion system. PPPL scientists will operate the diagnostic locally and remotely, analyze the data and share findings with the international fusion community. The knowledge gained will inform the design and operation of similar diagnostics on ITER and future demonstration power plants.

"Taking advantage of facilities overseas is very important for fusion research in the U.S. to be world-class," said Ono.