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The bundle of magnets at the heart of the U.S. Department of Energy's Princeton Plasma ÌÇÐÄÊÓƵics Laboratory's (PPPL) National Spherical Torus Experiment-Upgrade (NSTX-U) is the star of the show.

Its magnets will produce the highest magnetic field of any large spherical torus, allowing for near steady-state conditions. They are critical to the design of NSTX-U. When it begins operating, it will be essential in determining 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 pilot plant.

The 19-foot toroidal field (TF) magnet carries up to 4 million amps of electric current to stabilize and confine the superhot plasma in fusion experiments. It will eventually connect to 12 TF coils on the outside of the vacuum vessel. Wrapped around it like a slinky is the ohmic heating (OH) coil, a 4-kilovolt magnet that induces an electric field, which drives an electric current into the vessel and helps to heat the plasma.

While the TF-OH magnet bundle is being assembled at Elytt Energy in Spain, the NSTX-U project team has found a clever, cost-effective way to prepare for its arrival by using an understudy for the star. It is made of red plastic and stands just 40 inches tall and 2 feet wide, but it is an exact replica of the top of the bundle.

"If it were a Hollywood set and you painted the TF-OH 3D print a different color, it would look just like the machine," said Tom Jernigan, a senior project manager on the NSTX-U project. "It's the best money we ever spent."

A strategy to reduce risk

The 3D model is part of a larger strategy to pre-fit all the components of the experiment in advance before beginning operations in 2026, said Dave Micheletti, the NSTX-U project director and deputy associate laboratory director for engineering.

"The use of 3D-printed prototypes has been instrumental toward reducing risk and accelerating the schedule," Micheletti said. "It allows us to positively confirm that components will fit together and eliminates the risk of rework once final assembly starts. It's saving both time and money."

The TF-OH model will allow the NSTX-U project team to ensure they have the perfect fit for 36 cooling water lines that extend out of the top of the bundle like a fountain. The water lines will keep the magnets cool when plasma heats to temperatures hotter than the sun during fusion experiments. The team will print another large piece of the TF-OH coil to perform similar fittings at the bottom of the coil.

More than 50 3D-printed components

The 3D TF-OH bundle surrogate is the largest 3D-printed component but far from the only one. After installing the center stack casing into the vacuum vessel, the team produced more than 50 3D-printed components. These included the 3D prints of the copper electrical bus bars, which supply power to the 12 poloidal field coils, as well as other brackets and supports. By installing these components early, the team can make any necessary modifications before producing the real thing. Technicians at PPPL have already begun the initial fabrication of the copper bus bars utilizing an OMAX water jet machine in PPPL's shop.

The preassembly process will give the project team a detailed plan when final assembly begins. "Everything has been put together, and it fits," Jernigan said.

Pre-fitting thousands of tiles

The project team also fitted the 2,000 plasma-facing component tiles that protect the machine from the extreme heat of plasma experiments on the upper part of the vacuum vessel. They'll soon perform a similar feat on the lower half.

"I think the team has done a fantastic job," Micheletti said. "The scope of this work includes installing thousands of tiles and positioning them within tolerances that are a few thousandths of an inch. After a huge effort by the team, everything is coming together very well."

Using prototypes to build the bundle

Prototypes have also been used extensively at Elytt Energy, where technicians tested each step of the process used to create the TF magnet at the center of the TF-OH bundle before building the actual magnet. Elytt Energy has now built four quadrants and will assemble them like .

The four quadrants are being compacted in a device with metal straps wrapped around it like bands. Next, the four quadrants will be wrapped in fiberglass, placed in a vacuum chamber and injected with hot resin through a process called vacuum pressure impregnation (VPI) to form one solid magnet.

"One of the biggest challenges in building the TF bundle is carefully aligning and compacting all four quadrants while keeping the interface insulation layers under even pressure," said Danny Cai, a senior engineer at PPPL. "We are happy that we have successfully achieved solid compaction, clearing this major hurdle."

When Elytt Energy finishes assembling the TF magnet, it will wrap the eight copper conductors of the OH magnet around it like thread around a sewing machine bobbin. The OH coil will then be wrapped in fiberglass and undergo the same VPI process to form one solid magnet.

When the TF-OH bundle is complete in the fall of 2025, it will be shipped back to PPPL, where it will be carefully placed into the center stack casing and hoisted back into the center of the NSTX-U vacuum vessel. When that happens, all the planning and preparations will come to fruition, and the real-time assembly can begin.