Baking the perfect pie one piece at a time: PPPL makes progress on central magnet of NSTX‑U

Written by
Jeanne Jackson DeVoe
Oct. 25, 2024

The U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) achieved a major milestone recently when it completed the intricate process of building the first quadrant of the magnet at the heart of the National Spherical Torus Experiment-Upgrade (NSTX-U).

PPPL is assembling two high-current magnets to create the toroidal field-ohmic heating coil (TF-OH) bundle. The magnets make up the core of the NSTX-U, similar to the core of an apple. They are designed to produce the highest magnetic field strength of any large spherical torus.  

The toroidal field (TF) coil is a 19-foot tall inner magnet resembling a telephone pole that carries up to four megaamps of electric current or 4 million amps to stabilize and confine the hot plasma in fusion experiments. The outer magnet, the ohmic heating (OH) coil, is a 4-kilovolt magnet that wraps around the TF coil like thread around a bobbin. It uses up to 24,000 amps to induce an electric field that drives an electric current within the vessel and helps heat the plasma. 

“These magnets are critical to the NSTX-U experiment, and the team has been laser focused on this assembly,” said Steve Cowley, Laboratory director. “Constructing the first quadrant is a big achievement.” 

The quadrants are going through a complex process called vacuum pressure impregnation (VPI), in which pieces of the TF coil “pie” are baked together into one very tall, solid piece of pie. Technicians at Elytt Energy in Bilbao, Spain, finished making the quadrant using this process in July. Preliminary electrical tests on the quadrant in August showed the process was successful. 

“Our team has been tirelessly working to make this happen,” said Dave Micheletti, division director of major science and engineering projects. “We’re happy the process was successful and are looking forward to when the entire magnet is complete.”
 

The key step before NSTX-U reassembly

The NSTX-U Recovery Team, made up of scientists, engineers and technicians charged with rebuilding the NSTX-U, has reconstructed many of the key components of the device. Once the center stack magnets are installed, the team can begin focusing on reassembling and testing the NSTX-U. 

Scientists from all over the U.S. and the world will collaborate on NSTX-U experiments, which will help scientists determine the necessary conditions and engineering solutions required to produce cost-effective fusion energy. NSTX-U’s compact design makes it an ideal candidate to serve as the model for a fusion pilot plant followed by a commercial fusion reactor.

PPPL engineers have carefully designed the TF coil and OH coil so the powerful magnets will be strong enough to confine the plasma during plasma experiments, which can occur every 20 minutes when the experiment is operating. 

The 19-foot conductors that are the main ingredient of the TF coil magnets were prepared at facilities all over the world and the U.S. before being shipped to Elytt Energy. There, engineers and technicians have custom-built equipment to construct the magnets and have tested the equipment with prototypes in a kind of dress rehearsal for the actual construction. PPPL engineers and quality assurance staff have carefully monitored the entire process.
 

Baking the pie in quadrants 

To understand how the TF coil magnet is constructed, one could think of four tall pieces of pie being baked separately. The four pieces are then baked together to create the magnet. 

The step-by-step “recipe” to construct the TF coil magnet is as follows:
 

Preparing to bake the piece of pie

Illustration of conductors forming quadrant

Left: One of nine conductors form a quadrant. Right: A primed conductor (a); Technicians wrap a conductor in fiberglass tape (b); Tacking two conductors (c). (Illustration credit: Kiran Sudarsanan / PPPL Communications Department; Photo credit: NSTX-U Recovery Team)

Step 1: After the 19-foot copper conductors arrive in Spain from the U.S., they are grit blasted and primed. Just as a painter might sand and prime a house before painting it to ensure the paint adheres to the house, the conductors are grit blasted and primed so that the fiberglass and resin used to make the coil adheres to the conductors. 

Step 2: Next, each conductor is wrapped in fiberglass tape that resembles a gauzy white material. The fiberglass is a very thin, strong glass that will insulate the conductors and meld with the resin when the resin is heated up and cured. 

Step 3: After the conductors are wrapped, nine of them are carefully stacked together. 
 

Baking the piece of pie

illustration of quadrant heating in mold

Left: Heating the conductors. Right: The conductors are compressed in a mold fitted with cylinders (a); The machinery to inject resin into the conductors (b); The conductors, which have now been injected with resin through vacuum pressure impregnation, are heated and wrapped in an insulation blanket to retain the heat (c). (Illustration credit: Kiran Sudarsanan / PPPL Communications Department; Photo credit: NSTX-U Recovery Team) 

Step 4: The conductors are placed in a metal mold with cylinders on top that compress the conductors together. Technicians measure the stacked conductors and compress them to ensure the assembly has the correct dimensions before going to the next step.

Next, technicians weld the metal cover of the casing closed and pressure test it to ensure there are no leaks. They then pump all of the air out of the mold to create a vacuum. 

Step 5: Resin is pumped into the vacuum casing through a tube and assisted by the vacuum. The resin is a plastic material that is liquid when heated and solidifies when it cools. With no air in the mold, the resin flows into the mold, coating every inch of the wrapped conductors. 

Step 6: The combined conductors wrapped in fiberglass and coated with resin are baked at 170 degrees Celsius (338 degrees Fahrenheit) for a few days and then gradually cooled back to room temperature.
 

Assessing the piece of pie

illustration of quadrant undergoing conductivity test

Left: Testing the quadrant. Right: The conductors, which have been taken out of the mold, are electrically tested (a); After the conductors have been assembled into a quadrant, the process will be repeated with three more quadrants to create the toroidal field coil (b). (Illustration credit: Kiran Sudarsanan / PPPL Communications Department; Photo credit: NSTX-U Recovery Team) 

Step 7: After the conductors have cooled, technicians take the piece of pie or quadrant out of the mold and perform electrical tests to ensure the integrity of the insulation. They then carefully measure it to ensure it will fit inside the center stack casing after being joined with the other three quadrants.

Step 8: Once the first piece of pie has been tested, work can begin on the next pieces until all four quadrants are completed.
 

Putting the pieces of pie together

illustration of 4 quadrants coming together to form TF bundle

Left: The four quadrants come together. Right: Pieces of a test quadrant, which was disassembled, with four quadrants put together in the bottom right (a); A mock-up of a complete toroidal field coil (b). (Illustration by Kiran Sudarsanan / PPPL Communications Department; Photo credit: NSTX-U Recovery Team) 

Step 9: When all four quadrants have been constructed, it’s time to put the pie together. Technicians wrap the four pieces with fiberglass tape to create one large unit and place it in a large mold. They then inject the mold with resin, heat it up, cool it and follow the steps above to create one complete toroidal field magnet.
 

Creating & winding the second magnet: The OH coil

illustration of OH Coil winding

Left: The ohmic heating coil is wound around the toroidal field coil. Right: Copper coils for the ohmic heating (OH) coil in Finland (a); a mock-up of the wrapped ohmic heating coil being wrapped around the OH coil (b); A mock-up of the ohmic heating coil in its casing in preparation for the VPI process. (Illustration credit: Kiran Sudarsanan / PPPL Communications Department; Photo credit: NSTX-U Recovery Team) 

When the whole pie is complete, Elytt Energy will begin assembling the other magnet: the OH coil. The conductors for the OH coil are spiral-shaped copper conductors that came from Finland and were grit blasted and primed in Italy before being shipped to Spain. 

Eight of the OH coils will be wrapped in fiberglass and then wound around the TF coil to create one continuous, 600-foot coil. Elytt Energy technicians have been carefully practicing this process on a mock-up prototype using the custom-built fabrication equipment they will use to build the real thing.
 

Completing the OH coil

With the winding complete, the TF-OH combined magnets will be placed in another mold, which will follow the same VPI recipe to complete the OH coil, with the TF coil sitting inside the OH coil like a nesting doll.
 

center stack casing that magnets will be installed into

Once complete, the TF-OH coil will be fitted into the center stack casing. (Photo credit: Michael Livingston / PPPL Communications Department)

Installing the magnet bundle in the center stack casing & into the vacuum vessel

When the VPI process is complete and the combined TF-OH bundle has been thoroughly tested, it will be shipped to PPPL. The NSTX-U team will use a large crane to lift the magnet bundle and carefully place it inside the center stack casing. The crane will then lift the entire assembly and place it inside the NSTX-U vacuum vessel.

The lift will be carefully choreographed by the NSTX-U Recovery Team, which will practice the lift in advance to prepare for this moment. The team plans to do a test lift of the center stack casing in the fall of 2024.

When the TF-OH bundle has been placed inside the NSTX-U vacuum vessel like the core of an apple being placed back inside the apple, the work of the NSTX-U team is far from over. Many services, such as gas, water and electricity, must then be connected to the vessel and center stack. They will ensure all the components are ready for operations by carefully testing each component on the experiment and the safety systems. Then, NSTX-U can resume full operations.  


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.