At the heart of the National Spherical Torus Experiment-Upgrade (NSTX-U) are two powerful magnets — the toroidal field (TF) coil and the ohmic heating (OH) coil — that are bundled together in the center stack of the cored-apple-shaped fusion energy device at the Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL).

The TF coil is the inner magnet used to stabilize the plasma, which is necessary to produce the conditions necessary for fusion experiments. It connects with the red vertical magnets outside the NSTX-U.

Wrapped around the TF coil and separated by just a fraction of an inch is the OH coil, which is used to drive current into the plasma. The two magnets will be bundled together inside the gleaming new center stack of the NSTX-U.

PPPL sent out international requests for bids for the TF/OH coil and has turned to expert fabricators from across the ocean and all over the United States for each step of the process.

Journey of TF/OH coils began at PPPL

One might say the TF and OH coils were originally conceived at PPPL. Before their journey even started, PPPL engineers redesigned the TF/OH bundle on-site. PPPL has a great deal of expertise in this area, having designed and built the TF/OH coils at PPPL in the past. "It’s a simpler design with fewer parts," said Dave Micheletti, director of the NSTX-U Recovery Project.

"This is an extraordinary magnet bundle," said Les Hill, the NSTX-U Recovery Project manager. "We’ve adopted a very unique, phased approach."

Design improvements and testing prototypes

The new design has "a number of targeted but various impactful design improvements," explained Stefan Gerhardt, deputy director of the NSTX-U Recovery Project. The "most noteworthy" of these improvements, he said, was simplifying the way the magnets are cooled.

The new design streamlined the process by having the cooling channels drilled into the conductors rather than previous designs in which they were soldered on, and plastic tubes were inserted to cool the conductors. The other significant change was creating a tenth-of-an-inch air gap between the two coils so that they don’t touch each other.

Just as the components for these magnets traveled around the world, so did PPPL staff, who visited each site to oversee each process. And while the conductors are being prepared, Elytt Energy — a fabricator of magnets in Bilbao, Spain — has been hard at work designing and testing the complex process that will take place at the end of the magnet’s journey.

Along the way, each component was tested with prototypes, especially when it came to assembling the conductors into the TF and OH bundles. "If we’re seeing problems during prototyping, we’re testing to get it right," Micheletti explained. "We want to make sure the bundle will go the distance when it’s delivered to PPPL."

Journey started in Finland

One might say the TF conductors were born in Finland, where the 19-foot copper conductors were fabricated. There, the copper was heated up and extruded or stretched into wedge-shaped copper bars, each weighing 550 pounds. It’s a process that engineer Arthur Bortz, who’s the assistant project manager in charge of the TF/OH bundle, compares to a Play-Doh machine churning out square tubes.

Luvata, a copper manufacturer, has created more than 100 copper conductors, which will be used for a TF coil and a backup coil, as well as testing coils. The company is also processing each of the circular coils used for the OH bundle, which resembles a gleaming copper slinky.

Many stops in the U.S.

The TF conductors were then flown back to the U.S., where they were prepared and transported by truck to Dearborn Bortec in Fryeburg, Maine, for gun drilling. This process involved drilling tiny holes from end to end of the 24-foot conductors. These channels will be used to water cool the TF coils when the NSTX-U is operating, using a much simpler yet more robust process.

The conductors then travel to a machining company called Visioneering in Auburn Hills, Michigan, where they undergo an extra step added to the process. PPPL engineers realized that after machining, the conductors warped. They attributed this to a residual stress inside the conductor. They added the extra step of heat treatment to reduce the residual stress before machining. The conductors are heated at 355 degrees Celsius (671 degrees Fahrenheit) and baked for about nine hours.

The conductors remain at Visioneering for machining before being sent to the Edison Welding Institute in Ohio to attach copper "flags" to each end of the conductors using a special process called "friction stir welding" that uses heat from friction to weld together two pieces facing each other. The "flags" provide the attachment location for electrical connections during operation. Then, the coils return to Visioneering in Michigan for final machining before being flown to Elytt in Bilbao, Spain’s Basque country.

Final stop: Bilbao, Spain

The first conductors will likely arrive at Elytt in November, and Elytt has been preparing for the big moment for months. Under the watchful eye of PPPL Recovery Project engineers and quality assurance staff, Elytt has been developing specially built machinery and testing each step of the process with prototypes of the coil. Recovery Project engineers have traveled to Bilbao to check on the project, and PPPL has hired a Spanish and Basque-speaking engineer to oversee the process.

A complex process

The TF coil consists of 36 conductors. Elytt will first assemble four quadrants of nine conductors each. The conductors will be wrapped in fiberglass tape and will bind the quadrant together using a process called vacuum pressure impregnation. Each quadrant is placed in a mold inside a vacuum chamber, filled with a resin and baked at a high temperature until it creates one unit. After the four quadrants are completed, the process is repeated to assemble all four quadrants into one circular magnet.

Elytt has been perfecting this process at each step by creating models of the quadrants and the assembled coil and then dissecting them to ensure that the resin has permeated each area of the magnet.

Once that process is perfected, Elytt will also turn to the OH coil, which will undergo a similar process. The OH coil is also being processed at Luvata in Finland, but it will never leave Europe.

Its next stop will be ICAS in Italy for grit blasting and priming, and from there, it will be transported via truck to Elytt, where it will also be molded together through the vacuum pressure impregnation process and will eventually be placed around the outside of the TF coil.

Journey ends at PPPL

The journey of the FH-OH coil will come full circle when it returns to PPPL, where it will be installed into the center stack casing by the end of the year.

"We’ve made a lot of progress," Hill said. "... We are excited about the progress we’re making."

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 https://www.pppl.gov.