The U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) has achieved a major milestone in the upgrade of PPPL’s main experiment, the National Spherical Torus Experiment-Upgrade (NSTX-U), with the arrival in Spain of several 19-foot long copper conductors that will form the basis of the inner magnet of the fusion device called the toroidal field coil or “TF coil.”  

The magnet bundle at the center of the fusion device consists of two magnets, the TF coil and the ohmic heating coil or OH coil. The combined magnets must be completed for PPPL to begin reassembling the NSTX-U. This will allow NSTX-U to resume experiments aimed at exploring whether the cored-apple-shaped spherical tokamak would provide a viable model for future fusion energy pilot plants. 

“This is a major step in the project,” said Dave Micheletti, the director of major science and engineering projects and the NSTX-U Recovery Project director. “The magnet bundle is at the heart of the project — literally and figuratively.”
 

19-foot conductors arrive in Spain 

The 19-foot conductors began arriving at Elytt Energy in March 2024. They will be used to create the TF coil, which connects with red vertical magnets outside the NSTX-U. The TF coil confines and stabilizes the superhot plasma to produce the conditions necessary for fusion experiments.

Testing the process 

Staff at Elytt Energy have been preparing for the elaborate process to create the magnet for months. Under the oversight of PPPL engineers and quality assurance staff, they designed and created equipment for each step of the process. They have tested the equipment by putting mock conductors through a kind of dress rehearsal to ensure the process works. 

The copper conductors are world travelers that were fabricated in Finland, where the copper was heated up and stretched into wedge-shaped copper bars, each weighing 500 pounds. They were then shipped to the U.S., where technicians drilled, heat treated, machined and welded the conductors in a multiple-step process in Maine, Michigan and Ohio.

Enough conductors to create a quadrant 

Elytt Energy currently has 13 conductors, enough to create one quadrant of the coil with four spares. 

“If they can squeeze the nine together and vacuum pressure impregnate them successfully, that’s a huge milestone — that’s the technical challenge,” said Danny Cai, a mechanical engineer who is the cognizant engineer on the project. “If we pass that hurdle, we’ve basically passed the biggest hurdle on the technical side.”
 

Arrival begins a lengthy process 

The arrival of the conductors means Elytt Energy has enough conductors to complete one major step in a lengthy process to assemble the TF coil. Elytt Energy will eventually mold four quadrants with nine conductors each — a total of 36 conductors — to create the cylindrical magnet that produces a powerful electromagnetic field in NSTX-U.

The TF coil is made up of four wedge-shaped quadrants, each of which will be individually constructed in a process called “vacuum impregnation” before they are melded together using the same process. Elytt Energy will construct two TF coils so that PPPL has a backup for this essential component of the NSTX-U.  

Step 1: Grit blasting 

Before work starts, the conductors will be grit blasted, which entails exactly what it sounds like: They will be blasted with a sand-like substance that makes the surface rough enough for priming. 

Step 2: Priming (preparing the conductors) 

Next, the conductors will be primed, much as one would apply a prime coat of paint on a house before painting it. They will then be heated to make sure the priming adheres. 

Step 3: Wrapping the conductors (insulating the conductors) 

The next crucial step is to wind strips of a special fiberglass tape around the conductors to provide a layer of insulation around them.

Step 4: Placing the assembled conductors in a vacuum mold 

The wrapped conductors are then carefully stacked together and placed in a mold.  

Step 5: Compressing the conductors 

Technicians adjust the mold to compress the conductors to meet exact specifications. They then measure the compressed conductors carefully. They must ensure they are the right dimension to move on to the final and most important step: the process known as vacuum pressure impregnation (VPI).

Step 6: Injecting the resin (VPI) 

During VPI, technicians pump resin into the vacuum mold to fill all the voids in the fiberglass and meld them all together. 

Step 7: Heating the conductors in the vacuum mold (final step of VPI) 

Next, the bundle is slowly heated to 170 degrees Celsius (338 degrees Fahrenheit) to meld the resin to the conductors. The bundle is then just as slowly brought back to room temperature inside the mold. 

The layers of glass and the resin together form a thick layer around the coils to insulate each one of the conductors so they can each conduct electricity to create a magnetic field without shorting out.

Step 8: Removing the bundled quadrant and testing 

Once the process is complete, technicians remove the quadrant from the mold. Elytt Energy engineers and technicians will perform a series of tests on the quadrant under the supervision of PPPL engineers and quality control staff. These include dimensional tests to ensure the quadrant is the exact size and electrical and mechanical tests to determine that each conductor is functioning properly electronically and mechanically. The entire process takes about one week.

Assembling the TF coil 

• Step 1: Constructing the four quadrants
  The four quadrants are each constructed separately with nine conductors each, like four equal slices of a pie using the same process. Each bundle of nine conductors is grit blasted and primed, wrapped in special fiberglass, compressed, injected with resin in a vacuum pressure mold and then gradually heated and cooled to insulate the conductors and meld them into one quadrant.
   
• Step 2: Assembling the four quadrants into one bundle and wrapping them in fiberglass
  The four quadrants, comprising 36 conductors, are assembled into what will become the TF coil, and the bundle is wrapped in another layer of fiberglass to insulate it.
   
• Step 3: Placing the bundle in the mold
  The wrapped bundle of quadrants is placed in a large vacuum pressure mold.
   
• Step 4: Compressing the bundle
  The bundle of quadrants is compressed in the mold to the exact required dimensions.
   
• Step 5: VPI of the bundle
  Technicians inject the bundle of quadrants with resin under vacuum pressure during VPI to further insulate the bundle.
   
• Step 6: Heating and cooling the TF coil
  The bundle of quadrants, now melded into one TF coil, is heated and then cooled down again over several days.
   
• Step 7: Removing the coil and testing
  The completed TF coil is removed from the bundle and is carefully measured and tested.
   

The other part of the TF-OH bundle: The OH coil is prepared in Italy 

In contrast with the straight conductors that form the TF coil, the copper conductors in the OH coil resemble spools of wire. Like the TF conductors, the copper conductors for the OH bundle were processed in Finland. They were then shipped to the Innovation and Consulting on Applied Superconductivity, Società a Responsabilità Limitata (ICAS) in Italy, where the next step will be to grit blast and prime them. The conductors will then be transported by truck to Elytt Energy in Spain, where they will undergo the same VPI process to insulate the coil. 

Elytt Energy has been preparing for the OH coils by doing mock-up trials of the process in which the conductors are wrapped in fiberglass tape before they are wound around the TF coil. They are using machinery that was designed and built at PPPL before it was shipped to Elytt Energy for that purpose. 

Once assembled, the OH coil will consist of about 600 feet of conductors in one continuous coil with two conductors on each of the four layers. The OH coil will be placed over the TF coil like a spool of thread around a bobbin on a sewing machine to create the TF-OH bundle. 

The poloidal field coils are ready to go at PPPL 

PPPL is also preparing two sets of three magnets called poloidal field (PF) coils at PPPL’s coil shop. PPPL technicians machined the magnets and built a support structure for them.

Sigmaphi Accelerator Technologies built the six magnets and a set of six backup magnets in 2020 to replace an NSTX-U magnet that failed in 2016, along with five similar magnets. The two sets of A, B and C poloidal field coils nestle against the top and the bottom of the center stack. The magnets are used to shape the plasma during fusion experiments.

Three executives from Sigmaphi Accelerator Technologies visited the Lab to see the magnets and tour NSTX-U. “It was nice to see the NSTX-U,” said Sébastien Longelin, president and CEO. “It’s an exciting and very impressive mission.” 

The PF coils will be installed after the TF/OH bundle is placed inside the center stack and the center stack is dropped into the middle of the vacuum vessel.