After a $94 million upgrade that took around four years to complete, a Princeton experimental fusion reactor recently broke. Scientists shut down the National Spherical Torus Experiment Upgrade (NSTX-U) after they discovered a malfunctioning coil. Since the NSTX-U shut down, only one fusion reactor is currently working in the United States.

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Fusion power could be a source of clean energy that “could power the world indefinitely.” But US fusion researchers are struggling after the upgraded reactor at the Princeton Plasma Physics Laboratory (PPPL) broke. As one other Department of Energy-funded fusion reactor at MIT was scheduled for safe shutdown at the end of September, as of now only a San Diego facility is operating.

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NSTX-U began operating last December at low power, and provided “high quality data” for 10 weeks before scientists realized they’d need to shut the reactor down. At first PPPL told Nature they were investigating the faulty coil, but later said even as the coil was being made there were questions “about the strength of the copper” in the coil. They said they investigated those questions at that time.

Non-profit research foundation Fusion Power Associates President Stephen Dean told Nature, “Mistakes like this do sometimes get made, but with all of the experience the fusion program has, it should not have happened this way.” He says it’s possible the problem could have been prevented through more analysis when questions over the copper’s strength were first raised.

Although scientists shut down the fusion reactor in July, the fact of the shutdown became known the end of September after the resignation of PPPL director Stewart Prager, who said he had considered leaving before the shutdown and will now step down so new leadership can fix the fusion reactor.

Jonathan Menard, program director of NSTX-U, said the faulty coil satisfied specifications of the laboratory and that a similar coil made with copper of the same grade functioned properly. The faulty coil could have malfunctioned through a flaw in the design or manufacturing process, but the scientists don’t yet know which explanation led to the malfunction. It’s not yet known how much it will cost to fix the fusion reactor but it will likely be around a year before it will work again.

Via Nature

Images via PPPL Princeton Plasma Physics Laboratory on Flickr (1,2)