NSTX-U magnet bundle is delivered to PPPL and prepared for installation

The central magnet bundle for the National Spherical Torus Experiment-Upgrade (NSTX-U) at Princeton Plasma Physics Laboratory has been delivered to the facility in New Jersey, the national lab recently reported. The school bus-sized, 23,000-pound magnet bundle, manufactured at Elytt Energy in Bilbao, Spain, consists of a toroidal field magnet system and an ohmic-heating magnet system.

PPPL Director Steven Cowley said of the event, "NSTX-U has capabilities found in no other plasma device anywhere in the world, and it will play a critical role in determining the future of commercial fusion. This is truly a moment to celebrate."

Spherical tokamak design: Dave Micheletti, the associate laboratory director for engineering at PPPL and the project director for NSTX-U, called the delivery of the magnet bundle "truly a momentous occasion. Now our full focus is on finishing machine reassembly and bringing this device to the world."

The bundle's toroidal field magnet system is what creates most of the magnetic fields that stabilize and confine the high-pressure plasma necessary for fusion reactions. Its ohmic-heating magnet system heats the plasma by creating electric current that flows through the plasma.

NSTX-U's "cored apple" design is characteristic of a spherical tokamak, and theoretically will allow the device to produce fusion plasma with less energy and at lower costs than "donut-shaped" toroidal tokamaks.

Researchers plan to use NSTX-U to determine the fuel conditions, measurement techniques, overall system shape, and AI control tools that can most efficiently generate commercial fusion power. The research could reveal how a spherical tokamak would perform as a fusion power plant.

Stefan Gerhardt, senior managing research physicist at PPPL, said, "I was thrilled to see the bundle brought into the building where it will be put to work. This is the culmination of years of work, and I'm eager to start doing some science!"

The research with NSTX-U will also help advance the Department of Energy's Fusion Science and Technology Roadmap, which "aims to usher a burgeoning U.S. fusion industry toward maturity on the most rapid, credible timeline." This road map establishes goals for "actions and milestones out to the mid-2030s, providing the scientific and technological foundation to support a competitive U.S. fusion energy industry."

Remaining work: The next step is installation of the magnet bundle, a process that first involves lowering a tall protective casing around it. The casing is made of a metal sheath studded with heat-resistant carbon tiles, which will protect the magnet from the intense heat of the plasma. Then, a crane will lift the shielded magnet bundle over a barrier wall and lower it through a circular opening in the top of NSTX-U into the interior of the device, where it will be secured in place.

After installation, the magnet bundle's power sources will be connected with 72 horseshoe-shaped flexbuses, which will allow it to interface with the other parts of the magnet system outside the vacuum vessel.

Other work that will then be carried out includes installation of cooling system hoses, vacuum vessel tiles, and a bakeout system that will heat internal components to remove unwanted elements that could interfere with plasma operations.

The final step before the start of operations will be the commissioning process, when NSTX-U will be fully tested to ensure that all systems are working together properly. Experiments with NSTX-U are expected to begin in 2027.