BYLINE: Jeanne Jackson DeVoe
As the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) looks forward to beginning operations on the National Spherical Torus Experiment-Upgrade (NSTX-U), the project team is putting together the intricate jigsaw puzzle of components inside the vacuum vessel months before they will be permanently reassembled.
While preassembly might not make sense for putting together a jigsaw puzzle, project engineers say it makes all the sense in the world for the NSTX-U. This process allows the team to identify and solve any potential problems with fitting the pieces together, which could save time down the road. The timeline for reassembly of the components typically takes place several months later just before commissioning and operations of the NSTX-U begin, according to project team members.
The NSTX-U project was 90% complete as of February 2025. When the NSTX-U begins operating in 2026 and producing scientific results, the experiment will be essential to proving whether spherical tokamaks, which are smaller and more compact than traditional doughnut-shaped tokamaks, could provide a more efficient and cost-effective model for a fusion energy pilot plant.
“The team has done an amazing job pre-fitting many of the components,” said Dave Micheletti, NSTX-U project director and deputy associate laboratory director for engineering. “This will save us significant time when we reassemble the machine prior to beginning operations.”
The project team’s first task in November 2024 was placing the center stack casing into the exact center of the NSTX-U vacuum vessel. A giant crane hoisted the 20-foot, 2,800-pound center stack casing over a wall and into the center of the vacuum vessel. The task required weeks of detailed planning and preparation and was a perfect fit.
“It landed exactly where it’s supposed to be,” said Tom Jernigan, senior project manager. “That was just so cool, and I was very excited because it allowed us to install all these components. I’m excited about getting this work done.”
Fitting thousands of tiles
The next step for the project team was installing approximately 1,000 tiles onto the center stack casing before the lift and another 3,000 tiles inside the vacuum vessel. These custom-designed graphite tiles protect the magnets and the inner walls of the experiment from the intense heat during fusion experiments, which can reach temperatures of up to 100 million degrees — seven times hotter than the sun.
An innovative process
The project team used a sophisticated process to fit other components as well, including copper bus bars, which conduct electricity. Rather than trying to lift heavy and unwieldy metal pieces that hold the components in place in the vacuum vessel, the team used 3D-printed components for the trial fitting. These hard plastic components are lighter and easier to manipulate and manufacture. This allowed the team to test whether the components fit before using the real materials.
Another major activity for the project team was successfully testing the electrical system and the magnets, or coils, before commissioning and operations. To do this, the team powered up each of the two powerful motor generators, which have 730-ton flywheel rotors that can store up to 2,250 megajoules of energy each. That energy is delivered to the field coil power conversion (FCPC) system, which converts this energy to pulsed direct currents to power the magnets for early coil testing. The FCPC team did months of maintenance and testing on these high-powered converters leading up to the tests with the magnets.
“Through the testing, we powered all of the magnets to levels that will be seen during research operations, and in so doing, we both confirmed the condition of the existing magnets and also demonstrated that all of the supporting systems are fully functional,” Micheletti explained.
This was no simple task. The team first turned on the water systems to cool the power equipment and the coils. Then, the motor generator used for that day’s run had to be started, which took about 20 minutes. The personnel safety system was then started, which locked up the test cell and other areas along the high-power route. The FCPC team then performed preoperational electrical tests on the FCPC system of the power circuits.
The Plant Instrumentation & Control group, which provides integrated, distributed control and monitoring of the NSTX-U systems during operations, collects research data and operates systems that can shut down experiments if there is a problem, has also been upgrading equipment and performing tests with the power systems and integrated operations group since the fall of 2024. Members of the Plant Instrumentation & Control team were among several staff members gathered in the NSTX-U control room with the chief operating engineer and power control engineer in charge to assist with the tests. Other staff were stationed at the FCPC, as well as the motor generators and cooling water systems. The project team performed a total of 230 shots over two and a half weeks.
“It was an important milestone for NSTX-U. It was the first time since 2016 that the coils have been pulsed. “This early coil testing activity is a big risk reducer for the commissioning of NSTX-U later in the year,” said John Dellas, head of power systems and test director for early coil testing. “It’s really an orchestrated effort. We had a lot of people working closely together to make this a success.”
Testing and modeling the central magnets
Meanwhile, the project team is taking a similarly meticulous approach to building the two magnets that are bundled together in the center stack casing as they complete a major step toward completing the inner toroidal field (TF) magnet at Elytt Energy in Bilbao, Spain, more than 3,500 miles away from Princeton, New Jersey.
In early March 2025, the Elytt Energy team completed the final quadrant of the 19-foot-tall inner TF coil of the toroidal field-ohmic heating (TF-OH) bundle. This is the fourth of four quadrants, which are like four pieces of pie, each made up of nine copper conductors.
The fourth quadrant will be carefully measured and analyzed. The Elytt Energy team will then put the four quadrants together and wrap them in fiberglass. The quadrants will then be placed in a larger vacuum mold, undergoing the same process of injecting resin, baking and cooling to form one solid magnet used to create the quadrants.
“We’ve made significant progress with the completion of the fourth quadrant, and we’re looking forward to completing the TF-OH magnet bundle,” Micheletti said.
Each step to construct the magnets was carefully mapped out by PPPL and Elytt Energy engineers. The Elytt Energy team also uses mock-ups of the magnets to rehearse each step of the manufacturing process and test out all of the custom-built equipment, including some equipment sent to Spain from PPPL. This process allows the team to identify and resolve any issues before they begin constructing the TF-OH bundle.
While the TF coil is being completed, the Elytt Energy team is using a mock-up of the OH coil to continue perfecting the process of constructing the combined TF-OH magnet. The OH coil consists of eight copper conductors, which are wrapped in fiberglass and wound around the TF coil like thread around a bobbin on a sewing machine. The combined magnets will be placed in another vacuum mold, where the OH magnets will be injected with resin, baked and cooled to form the TF-OH bundle.
A plan to disassemble and then reassemble the puzzle
Before the TF-OH bundle arrives at PPPL in the summer of 2025, the project team will disassemble the tiles and remove the center stack casing so the magnet can be installed inside. The team will then install the final pieces of the device, including the six smaller poloidal field coil magnets, two sets each, that are used to shape the plasma and that are nestled inside the vacuum vessel at the top and bottom of the center stack.
Once reassembly is complete, the project team can look ahead to the next phases of the NSTX-U project: commissioning the experiment and beginning operations in 2026.