Astrophysics professor Stewart Prager stepped down as director of the Princeton Plasma Physics Laboratory on Sept. 26th, just before news broke of a malfunction at the National Spherical Torus Experiment Upgrade fusion experimental facility.
Sources are conflicted over the connection between Prager’s resignation and the malfunction. Nonetheless, the resulting maintenance will likely pose a significant setback not only to PPPL, but also to the international fusion community as a whole.
News of the malfunction appeared a day after Prager announced his decision to step down as PPPL director, raising questions regarding the impact that this incident had on his decision.
Physics Today reported that two sources, both of whom declined to be identified, claimed Prager was asked to step down by the Department of Energy. One source reportedly said “This was a firing as much as anything else.” Prager, however, denied that he was fired, telling Physics Today, “I never spoke to the Office of Science,” and added that he had been considering stepping down since January.
Prager will remain as an astrophysics professor at the University, though he will be on a leave for a year.
In PPPL’s press release, Prager addressed the situation’s impact on his decision.
“The recent technical setback in the NSTX-U facility unexpectedly and suddenly defines a moment that seems to me appropriate for that transition,” Prager said in the press release. "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.”
The recent shutdown of the NSTX-U occurred in July, when one of the reactor’s 14 magnets shorted out, according to information obtained by Physics Today from Michael Zarnstorff, the PPPL deputy director for research.
Further inspection of the machine revealed that another, identical coil on the opposite end of the machine would also need to be replaced before operations could continue. Furthermore, according to Physics Today, another issue with a copper cooling tube was discovered while disassembling the machine.
The entire repair process could take up to one year, which is more than double the scheduled six-month maintenance period for which the project was nearly due.
When asked to comment, PPPL released a statement to the ‘Prince’, confirming in an email that research that was expected to start in 2017 “will be delayed by 6 or more months.”
Given how much time will be lost during repairs, some researchers wondered if these problems might have been precluded during design or fabrication.
“The shutdown is a loss of experimental time that is significant,” Prager wrote in an email. He wrote that though acquiring experimental results is delayed for roughly a year, planned research projects “will move forward but with that delay.”
Prager wrote in an emailed statement to the Daily Princetonian that the loss of experimental time “is a significant opportunity cost.” He said that though replacing the failed components “is not expected to require additional funds…. funds that would have been used for experimental operations will now be used for the repairs.”
According to Physics Today, Zarnstorff said that using copper for some parts was “an unwise choice,” and that they should have instead used stainless steel.
Additionally, Nature quoted a former Princeton researcher who said that the copper used may have been stronger than necessary, which might have caused problems during manufacturing.
“The failures were preventable and reflect insufficient rigor in the design process,” Prager wrote. He also wrote that the repairs require “substantial disassembly” and time.
According to the PPPL website, research on magnetic fusion began at Princeton in 1951 under the code name Project Matterhorn. Since then, Princeton has been at the forefront of plasma physics research, including the record-breaking Tokamak Fusion Test Reactor , which operated from 1982 to 1997.
More recently, researchers have been studying plasma physics at the National Spherical Torus Experiment, which began operation in 1999. This approach proved successful enough that the Department of Energy in 2011 called for a $94 million upgrade of the facilities to equip them for future fusion research. This project, designated the NSTX-U, was completed in 2015, and began operating in December of that year, according to Physics Today.
Like its predecessors, NSTX-U uses strong magnetic fields to concentrate hydrogen plasma, which will fuse into helium given sufficient heat and pressure. This fusion releases massive amounts of energy, which could potentially serve as an alternative to other forms of power generation in the future. Public servants and figures such as Norman Augustine ’57 GS ’59 have long spoken of the benefits of fusion, which in theory could provide clean, CO2 emission free energy.
The greatest challenges in fusion often have to do with the sheer amount of energy required to start and maintain a fusion reaction, which is often just as great as the heat generated by fusion. Other challenges arise from the way the plasma is contained and concentrated, which is essential not only to safety, but also to the creation of conditions that allow for fusion to occur. This second aspect is what sets the NSTX-U apart.
The NSTX-U instead produces plasma in the shape of a sphere with a hole through the center, which, according to the PPPL, allows for the confinement of a higher plasma pressure per unit of magnetic field strength. In other words, this change in shape could allow reactors to operate more efficiently and economically.
According to the PPPL, the recent upgrade allows the NSTX-U to test high performance plasma under extreme heat and pressure. These conditions provide valuable data on their own, and could also influence the design of future reactors.
The Department of Energy and PPPL will investigate the incident over the course of repairs. A PPPL statement noted that “This is a complex device with a unique design and significant engineering challenges, so there is risk of failure.”
He also noted that “We have a goal to keep the repair costs within the current funding for NSTX-U. This will minimize the financial implications."
Prager said that design changes to prevent a recurrence are “well-known and well-understood.”
“There is very high confidence that such component failure will not recur,” Prager said. “In a facility as complex and one-of-a-kind as NSTX-U it is not uncommon for such component failures to occur, particularly at the onset of operations of a new facility. However, the consequence of the failures in NSTX-U is highly consequential because the components are situated in the facility in a highly inaccessible location.”
Media representatives at the Department of Energy did not respond to a request for comment.