Construction completed, PPPL is set to resume world-class fusion research later this fall

PPPL By John Greenwald September 25, 2015

Staffers who worked on the National Spherical Torus Experiment-Upgrade.

At the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL), world-leading fusion research resumes later this fall. After more than six years of planning and construction — including three years of building and 574,000 hours of labor — the National Spherical Torus Experiment-Upgrade (NSTX-U) is ready to play a critical role in the quest to develop fusion energy as a clean, safe and virtually limitless fuel for generating electricity.

The $94 million overhaul has made the machine the most powerful spherical tokamak in the world. The upgrade has doubled its heating power and magnetic field strength, lengthened its operation from one second to five seconds and increased its plasma performance by a factor of 10. The improvements create a flexible research platform that will enable some of fusion’s most outstanding puzzles to be directly addressed for the first time, thus supporting continued U.S. leadership in the quest for fusion systems that can form the basis of commercial fusion power. .

“This achievement signifies the completion of an extremely successful challenge, which opens the door to a decade or more of exciting research,” said PPPL Director Stewart Prager. “The world will now be watching to see if this experiment can serve to further improve our vision for future reactors.”

Passing stringent tests

The new machine passed stringent tests to reach construction completion. On May 11, operators produced 40,000 electron-volts from a second neutral beam — a device used to heat the plasma — to demonstrate the first step in doubling the heating power. Then, on Aug. 10, engineers produced a 100,000-amp plasma — the fuel for fusion reactions. The twin achievements easily met the Key Performance Parameters (KPP) that the project had to satisfy to be completed. “This is not a little spherical torus anymore,” said Al von Halle, the head of NSTX-U engineering and operations. “This machine has 10 times the capability of the original NSTX.”

Reaching this point required some 250 staff members, or more than half the Laboratory, to bring the project in on time within the DOE’s budget. “It took the work of physicists, engineers, technicians and many others to solve all the problems that cropped up along the way,” said Mike Williams, associate director of engineering and infrastructure, who is retiring this month after 39 years at the Laboratory. “This was a joint effort in every sense of the word.”

The many challenges reminded project head Ron Strykowsky of remodeling a house that’s already been built. “It’s easier to build a brand new house than to modify one that’s already standing,” Strykowsky said. “This forced us to adapt to what was there instead of building from the ground up.”

Strengthening every nut, bolt and support

Among the major hurdles was the need to strengthen every nut, bolt and support system throughout the machine to accommodate the higher magnetic fields. Doubling the fields created torque — or twisting forces — that could have destroyed the machine when it ramped up to full strength during operation. Analysts led by Pete Titus spent 28,000 hours supporting the Laboratory’s design and cognizant engineers by analyzing the components they designed to tolerate the higher currents and loads required for the upgrade. “This was quite a task,” said Titus, whose team began working three years before construction started and used a software program called ANSYS to analyze and reconfigure the support components.

There were many more challenges to face. The increased power of the machine will require constant monitoring to protect the magnets, and the overall vessel, from forces and stresses that could cause them to fail. To keep constant watch, engineers designed and built a Digital Coil Protection System that makes 1,200 computations every 200 microseconds to ensure that all is running smoothly.

Aligning the second neutral beam that pumps heat into the machine created another big challenge. Engineers headed by Timothy Stevenson had to cut a port into the tokamak vacuum vessel and aim the beam to within 80 thousands of an inch of the target inside. The spot the beam hit required reinforced armor to keep it from melting right through the vessel.

Pushing the envelope

“We had to push the envelope of everything we did,” Stevenson said of the overall upgrade, “and the review was highly complimentary.” Indeed, the DOE committee that conducted the closeout report recognized “the entire project team for their very high-quality work delivered over the course of the project, and resilience in overcoming expected and unexpected obstacles.”

PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit


Technicians inspect the new center stack that forms the heart of the NSTX-U.


The new neutral beam box arrives in the NSTX-U test cell.


The NSTX-U under construction with neutral beam boxes at left and tokamak, with American flag, at right.


Preparing to lower the center stack into the tokamak.


Plan for the NSTX-U upgrade