From PPPL: “PPPL Director Stewart Prager Steps Down”


September 26, 2016
Larry Bernard

Prof. Stewart Prager, a world renowned plasma physicist and passionate voice for a future of clean, abundant and benign energy fueled by fusion, has stepped down today from directorship of the national laboratory he has headed for the last eight years.

[But two sources who declined to be identified said DOE had forced Prager’s resignation; one source called the move an example of the agency’s “buck stops here” contractor management philosophy. A second source corroborates that account: “This was a firing as much as anything else.”]

Prager denies that he was asked to resign. “I never spoke to the Office of Science,” he says, adding he had only discussed his resignation with officials at Princeton University, which operates PPPL for DOE. Prager, who is also a professor of astrophysics at Princeton, says he had been considering stepping down since January.

The NSTX-U has been shut down since the end of July, when one of the machine’s 14 magnets, a poloidal field coil, shorted out after 10 weeks of running time, says Mike Zarnstorff, PPPL deputy director for research. Replacing it, and a second identical coil on the opposite side of the tokamak chamber, will take about a year, he says. The machine was nearly due for a scheduled six-month shutdown for maintenance, he notes.

The NSTX-U, a $94 million upgrade of a reactor built in 1999, is the world’s most powerful spherical tokamak, with a design field strength of 1 T and heating power of 10–12 MW. It is a variant of the mainstream tokamak technology at laboratories such as the UK’s Joint European Torus and the ITER international fusion collaboration in France. Tokamaks use magnetic fields to bottle up plasmas of hydrogen isotopes, which heats the confined particles to tens of millions of degrees and causes them to fuse into helium.

Zarnstorff says the root cause of the coil failure is still unknown, although he suspects it was a manufacturing problem. A source close to the lab says the copper wire used in the coil may have been too stiff to accommodate a particular bend in the winding.

The tokamak was operating at half its maximum design field strength when the problem occurred, Zarnstorff says, and full field operation wasn’t planned until next year. The lab has identified four design simplifications to mitigate risks in the replacement coils. A second defect, a damaged copper cooling tube, was discovered when the machine was taken apart. “We know that copper was an unwise choice,” Zarnstorff says. “It should have been made of stainless steel.”

During the shutdown, the lab will review the entire NSTX-U design to determine how to implement beneficial design changes and mitigate further risks. “We’ll have to wait and see and do the [analysis] work,” Zarnstorff says.
David Kramer, Physics Today]

“It has been a wonderful experience being lab director of the Princeton Plasma Physics Laboratory. But, at the seven-year mark last January I began to think about moving on to the next phase of my life,” Prager said. “The recent technical setback in the NSTX-U facility unexpectedly and suddenly defines a moment that seems to me appropriate for that transition.


It is best for new, continuing leadership to shepherd the rebuilding of the facility and the engineering changes that will be needed over the next year.”

Prager, the sixth director in the 65-year history of PPPL, joined the Lab in the fall of 2008 after a long career at the University of Wisconsin. A pioneer in plasma physics, he is internationally known for experiments that contribute to the fundamental knowledge of fusion energy and the design of devices that will produce it.

A professor in the department of astrophysical sciences at Princeton University, Prager will continue research activity at PPPL in fusion energy and plasma physics, beginning with a year’s sabbatical.

Prof. Dave McComas, Princeton vice president for PPPL, joined Princeton University President Christopher L. Eisgruber in thanking Prager for his dedication to fusion and plasma science. “We are truly grateful to Stewart for his many outstanding contributions and dedicated leadership over the past eight years,” McComas said.

McComas praised Prager for his leadership in several key initiatives. Among them: founding the Max Planck-Princeton Center for Plasma Physics; increasing joint faculty appointments between Princeton and the Lab; increasing ties between University departments and the Laboratory; developing a fusion communications strategy; broadening the Laboratory’s engagement in low-temperature plasma physics, plasma nanomaterials, laboratory astrophysics, computational fusion science and high energy density physics; and launching a campus plan to modernize the PPPL infrastructure.

Prof. A.J. Stewart Smith, the former Princeton Vice President for PPPL, said:

“For more than seven years it has been my great honor and pleasure to work with Stewart, and give whatever assistance I could. In 2008 Stewart towered above the other candidates when we were searching for Rob Goldston’s successor, so we were simply thrilled when he agreed to head the leadership team we proposed to DOE in the competition for the PPPL contract. Stewart’s intelligent but gentle leadership and tireless efforts on behalf of PPPL and the U.S. fusion community have greatly improved communication and cooperation among U.S. fusion institutions. He has also been and continues to be a constant driving force in building an exciting new U.S. research effort to establish stellarators as a strong alternative to tokamaks for practical fusion reactors. I wish him all the best, and great success as he resumes his distinguished scientific career after so many years of dedication to the laboratory. He will be a most difficult act to follow.”

Dr. Terry Brog, who recently joined the laboratory as deputy director for operations and chief operating officer (COO) at PPPL, will be interim director of the Lab. Dr. Stacia Zelick, chief information officer and head of information technology, will be interim deputy director of operations and COO as the global search for Prager’s successor begins. Dr. Mike Zarnstorff, deputy director for research, will continue to serve in that role.

PPPL is a DOE national laboratory operated by Princeton University, and the only national lab dedicated to fusion research and plasma science. NSTX-U is a spherical tokamak, or fusion device, producing plasmas many times hotter than the center of the sun to explore the development of fusion as a safe, clean, and abundant energy source to generate electricity for the world. The compact facility confines high pressure plasma in relatively low and cost-effective magnetic fields, and could serve as a model for the next major step in the U.S. fusion program.

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Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University. 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