The next generation of fusion energy research at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) is approaching. Crews recently finished building the central magnet assembly for the National Spherical Torus Experiment-Upgrade (NSTX-U), an experimental device known as a spherical tokamak, which is designed to be the most powerful of its type in the world. On Oct. 27, this assembly was hoisted over the NSTX-U device and lowered into the heart of the machine.
The completion of these steps mean that NSTX-U is nearing project completion. "I was pleased with how everything went," said Stefan Gerhardt, senior managing research physicist for NSTX-U. "From lifting the assembly off the ground to lowering it into the machine, the process went flawlessly - straight up and straight in.
"The reason it went so well is that the team had prepared," Gerhardt said. "They had performed dry runs, evaluated the tight clearance points and planned out everything that could have happened. The success of this step shows our strength in creating a strategy and executing complicated, large-scale engineering operations."
The next steps involve ensuring the central magnet assembly is centered exactly within NSTX-U. Having the assembly in a centered position ensures that NSTX-U can produce the precise magnetic fields necessary for scientific research. The crews will use laser-based measuring tools to determine the assembly's position. They will then achieve the centering by using shifting and tilting mechanisms engineered in the magnet assembly base.
"Once we complete this step - currently scheduled for November - and confirm that all of the vacuum seals are tight, we will be able to say that the insertion of the central magnet assembly into NSTX-U is finished," Gerhardt said.
Technicians lifted and moved the central magnet assembly using an industrial crane. (Photo credit: Michael Livingston / PPPL Communications Department)
Building on months of engineering successes
These steps follow the delivery of the completed magnet bundle in June - a significant moment in the construction of NSTX-U when all the components needed to complete the machine were on the PPPL campus. Measuring 20 feet long - the length of two midsized sedans placed end to end - the bundle, which makes up the core of the assembly, consists of two separate magnet systems. One, the toroidal field magnet system, creates the bulk of the magnetic fields that confine the plasma. The other, the ohmic-heating magnet system, helps heat the plasma by creating an electric current, the same phenomenon that occurs in kitchen toasters.
After performing tests and adding components like cooling systems and structural interfaces, technicians loaded the bundle on a movable fixture and transported it to a space adjacent to NSTX-U. There, they installed additional structural components, lower and upper divertor magnets, and the protective inner vacuum boundary known as the center stack casing. This work completed the central magnet assembly, which was installed as described above.
Enabling research that could improve energy resiliency
When fully assembled, NSTX-U will allow scientists to conduct experiments demonstrating whether the spherical tokamak - a fusion system that is more compact than traditional tokamaks - could be an ideal concept to produce cost-effective commercial fusion. NSTX-U is the largest spherical tokamak in the U.S. and is designed to be the most powerful in the world. As the U.S. moves toward the development of a fusion pilot plant, NSTX-U is essential in determining which fuel conditions, measurement techniques, overall system shape and control tools can most efficiently produce commercial fusion power.
Crews carefully monitored the insertion of the central magnet assembly to ensure it progressed smoothly. (Photo credit: Michael Livingston / PPPL Communications Department)
Beginning a new phase of work
Once the insertion is complete, NSTX-U crews will begin a new phase of work. They will be able to start connecting hoses that bring cooling water to the magnets, installing more protective tiles within the vacuum vessel and attaching the copper bus bars that connect the central magnet assembly to the magnet components outside the vessel. "Basically, there will be a flurry of activity," said Gerhardt. "It will be go, go, go!"
The final machine assembly is expected to conclude by early 2027, setting the stage for operations later that year.
Technicians helped the central magnet assembly pass through an opening with clearances of a quarter of an inch. (Photo credit: Michael Livingston / PPPL Communications Department)
