image: NSTX-U’s central magnet bundle is delivered.
Credit: Photo credit: Michael Livingston / PPPL Communications Department
On June 3, a flatbed truck journeyed from Newark Liberty International Airport to Princeton, New Jersey, and around 8:30 a.m. trundled through the front gates of the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL). On its bed was a piece of machinery that could influence the course of energy innovation in the United States — the central magnet bundle for the National Spherical Torus Experiment-Upgrade (NSTX-U), a compact fusion system designed to be the most powerful of its kind in the world.
This was the final leg of its journey that began at Elytt Energy, a manufacturing company in Bilbao, Spain, specializing in the fabrication of powerful magnets for large scientific facilities, and included being shipped in a cargo plane across the Atlantic Ocean.
“This is truly a momentous occasion,” said Dave Micheletti, PPPL’s associate laboratory director for engineering and NSTX-U project director. “Now our full focus is on finishing machine reassembly and bringing this device to the world.”
NSTX-U will also play an important role in the DOE’s Fusion Science & Technology Roadmap, which targets actions and milestones to provide the scientific and technological foundation to support a competitive U.S. fusion energy industry. It will do so not only by providing important information about the viability of the spherical tokamak concept for fusion power plants, but also by helping scientists collect data to train artificial intelligence systems that can improve the operations of fusion machines.
Building the most powerful spherical tokamak fusion system in the world
One of NSTX-U’s most crucial components, the bundle is both substantial and important. It weighs 23,000 pounds and measures about 20 feet long — about the size of a school bus. And it consists of parts of two magnet systems. One, the toroidal field (TF) magnet system, creates the bulk of the magnetic fields that confine the plasma. The other, the ohmic-heating magnet system, helps heat the plasma by creating an electric current, the same phenomenon that occurs in kitchen toasters.
The fabrication process involved several steps. First, technicians created the TF magnet by combining 36 copper conductors, each measuring 19 feet long, into one unit using fiberglass tape, resin and a process known as vacuum-pressure impregnation (VPI). Next, to create the ohmic-heating magnet, technicians wound coils of copper conductor around the TF magnet like thread around a bobbin. The technicians then coated the finished magnet with insulating resin and used VPI to bind all of its separate components together.
Once installed in NSTX-U, the magnet bundle will enable plasma operations by creating two separate sets of magnetic fields. One, the toroidal magnetic field, circles the device’s apple-shaped vacuum circumference and stabilizes the plasma. The second magnetic field circles the device from top to bottom and is known as the ohmic-heating magnetic field. As this magnetic field’s strength changes over time, it creates an electric current that flows through the plasma. This current, in turn, provides both heating and a magnetic field to confine the plasma.
The bundle’s delivery means that very soon, a new generation of plasma physicists, engineers and technicians will have access to a brand-new fusion research system that will take its place as one of the premier systems in the world. “I welcome all fusioneers everywhere to this amazing research opportunity and encourage them to use this facility to help advance humanity’s understanding of plasma and fusion energy,” said Jonathan Menard, PPPL’s deputy director for research. “What a thrilling moment — for PPPL, the nation and the world.”
NSTX-U will have unique fusion research capabilities
Compact fusion systems like NSTX-U produce energy more efficiently than conventional tokamaks, making them easier and cheaper to build and replicate. When fully assembled, NSTX-U will allow scientists to conduct experiments demonstrating whether the spherical tokamak concept – a fusion device shaped more like a cored apple than the doughnut-like shape of conventional tokamaks – could be an ideal design for a future fusion power plant.
“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,” said Steven Cowley, PPPL director. “This is truly a moment to celebrate.”
PPPL’s primary fusion system, NSTX-U, is a DOE national user facility, a large-scale scientific research machine like a supercomputer or particle accelerator that scientists around the nation can use to conduct experiments after submitting research proposals.
The Lab comes together to welcome the magnet home
Once the truck passed through PPPL’s main entrance, it drove through the main campus toward a region in the back, called D-Site, where NSTX-U is housed in a large, multistory building. Staff used an overhead crane capable of lifting 15 tons to remove the bundle from the truck and transport it to a large space adjacent to NSTX-U known as the Fusion Research and Technology Hub, the former location of the Lab’s record-setting Tokamak Fusion Test Reactor that is now available for public-private research partnerships.
It was then installed horizontally on a piece of equipment known as the “tilt fixture.” After PPPL engineers perform routine adjustments over the next two months, they will slowly raise the bundle into a vertical position, resembling a small version of NASA’s Space Launch System, which carried the Artemis II mission into orbit around Earth. Both the tilt fixture and bundle will then be transported into a two-story room adjacent to the space housing NSTX-U.
“I was thrilled to see the bundle brought into the building where it will be put to work,” said Stefan Gerhardt, senior managing research physicist. “This is the culmination of years of work, and I’m eager to start doing some science!” Experiments using NSTX-U are expected to begin in 2027.
Completing the NSTX-U puzzle
Now that the bundle is in the adjacent room, the next step will involve lowering a protective casing around it. The casing is a tall metal sheath studded with heat-resistant tiles made of carbon — similar to those that once covered the bottom of the Space Shuttle — that will protect the magnet from the plasma’s intense heat. Once the bundle is shielded, the enormous D-Site crane will lift the magnet up and over a barrier wall and then lower it through a hole in the top of NSTX-U into the device’s interior, where staff will secure it in place. These lifts will take place over several days and involve more than a dozen PPPL staff engineers and technicians.
Once the magnet has been fully installed, technicians will connect it to its power sources using a total of 72 horseshoe-like components known as flexbuses. These parts will also allow the bundle to interface with the other parts of the magnet system outside the vacuum vessel. Staff will also hook up the cooling system hoses, complete the installation of the tiles within the vessel and set up a bakeout system that will heat the internal components to remove any unwanted elements that could interfere with plasma operations. After that, the final remaining step before beginning operations consists of what is known as commissioning — when staff test the entire device to make sure all the systems are working together properly.
A fusion moment for the ages
“The bundle’s arrival at PPPL is immensely important for the Lab,” Micheletti said. “It matters because this is the final step before beginning NSTX-U operations. We need to do the work safely and efficiently and with good quality. Now that it has been completed and delivered, we can see our way to the finish.”
PPPL is mastering the art of using plasma — the fourth state of matter — to solve some of the world's toughest science and technology challenges. Nestled on Princeton University’s Forrestal Campus in Plainsboro, New Jersey, our research ignites innovation in a range of applications, including fusion energy, nanoscale fabrication, quantum materials and devices, and sustainability science. The University manages the Laboratory for the U.S. Department of Energy’s Office of Science, which is the nation’s single largest supporter of basic research in the physical sciences. Feel the heat at https://energy.gov/science and http://www.pppl.gov.