The director of the flagship US fusion research laboratory was forced to resign in the wake of a mishap that has caused an extended shutdown of one of the world’s top fusion experiments. The failure two months ago of a magnet at the National Spherical Torus Experiment Upgrade (NSTX-U) is expected to take the machine out of commission for a year.
“The recent technical setback in the NSTX-U facility unexpectedly and suddenly defines a moment that seems to me appropriate” to resign, said a statement from Stewart Prager, who has led the Princeton Plasma Physics Laboratory (PPPL) in New Jersey for eight years. “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.”
A spokesperson at the Department of Energy’s Office of Science, which oversees PPPL, wouldn’t comment on personnel matters. 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.
Spherical tokamaks are shaped like cored apples, compared with the doughnut shape of conventional tokamaks. The compact shape is designed to confine highly pressurized plasma with weaker magnetic fields than those required at other tokamaks, so the spherical reactors are potentially more cost-effective. The NSTX-U began operations last December and was formally dedicated by Energy secretary Ernest Moniz in April.
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.
Prager will continue to conduct research at the laboratory. Terry Brog, who moved from Pacific Northwest National Laboratory in July to become PPPL deputy director for operations, will serve as acting director.