In the nineteen-eighties, tokamak performance had hit a ceiling because turbulence at the edge of plasmas was impossible to control: electromagnetic eddies carried energy outward from the superhot core in diffuse and unpredictable ways, abrading the tiles on the tokamak walls, sucking impurities into the plasma and cooling it. These instabilities seemed insurmountable until researchers in Germany stumbled upon a discovery: under the right heating conditions, the plasma contained itself by forming a steep, clean pedestal at its perimeter, with its inner temperature and density ballooning. At first, the effect was doubted. There was no theory to explain it, and plasmas had rarely offered gifts, only obstacles. But the pedestal was real, and it was christened H-Mode. It is now ubiquitous in tokamaks, though physicists still have only a general idea how it works, and maintaining it is hard: when the pressure behind the pedestal is too great, the plasma erupts into flares that must be quelled.

It is unclear whether ITER will have enough power to achieve H-Mode. The relevant heating systems on the largest existing tokamak are the size of five shipping containers; ITER’s will be three times larger, and will have to work in an unproved way, just as pliers the size of a skyscraper cannot be opened by hand. Even if the systems work, there might not be enough of them. Current extrapolations offer only a hazy guide to what ITER will require for the pedestal, with the range of uncertainty—what physicists call the error bar—remaining frustratingly large. Joe Snipes, a physicist at ITER’s headquarters, told me, “We tried and tried and tried—and when I say ‘we’ I mean the entire fusion community, experts from around the world working on different machines—we tried to reduce the error bar, but we really couldn’t do it; the H-Mode depends on so many different factors that we don’t understand.” Some engineers wonder if the relevant heating systems—hardware, costing a billion dollars, first developed for Reagan’s Star Wars Defense Initiative—have outlived their usefulness in tokamaks. Others believe that everything must be tried, because ITER ultimately remains an experiment: mapping the way is its purpose.

Snipes’s job will be to run the plasma. Not long ago, in the headquarters, he gave a lecture for engineers titled “Operational Limits on ITER.” Most of what he had to say involved the uncertainties of plasma behavior, but he reminded his colleagues that some limits might be imposed simply by the way ITER is built. While the Praetorian Guard was worrying over the gaps among components, trying to insure that there will be enough space to assemble the machine, the physicists were worrying over them, too. Neutrons are expected to pour out of ITER’s plasma like a tsunami. Because these particles have no charge, they will escape the grip of ITER’s magnets, advancing through any space that they can find, pushing into, or even through, obstructions—solid matter will not always stop them.

Early on, physicists understood that, as more gaps were introduced into the design, more neutrons would penetrate the machine, heating whatever absorbed them. To study the plasma’s effects on the structure, they purchased a million C.P.U. hours on MareNostrum, a supercomputer in Barcelona that is housed in a pristine glass box in the dimly lit nave of a nineteenth-century chapel. ITER’s magnets will be encased in a cryostat and continuously cooled with liquid helium. If they get warmer than negative two hundred and sixty-seven degrees, they will “go normal,” and lose the quality that makes them superconducting. At that point, the enormous electrical current running through them will look for an alternate outlet, like a dammed river. If all eighteen toroidal-field magnets were to experience this phenomenon at once, forty-one billion joules of energy would seek a new place to go. One scientist compared the outcome to two 747 airplanes simultaneously crashing into the machine.

Complex calculations are required to predict how many neutrons will hit the magnets, but gaps are being introduced faster than the analysis can be done. “The physicist responsible for this is constantly upgrading his models,” Snipes told me. “Every little gap causes him tremendous headaches. Now, it probably won’t be a problem—we will lower the plasma performance before we get to that dangerous state—but it will limit how high we can go.” In other words, even if ITER is able to produce record thermonuclear reactions, the machine may not be able to cope with them—an immensely frustrating prospect. Since the days of Dorland and Kotschenreuther, there have been far more encouraging computer models; one predicts that ITER could theoretically reach ignition. But, if the gaps proceed apace, even the project’s fundamental goals may be compromised.

“This is what happens when you are driven by a schedule that is not realistic, or when you are asked to build a machine with too few people, or too little money—so something has to give,” a scientist affiliated with the project said. “Whenever the director-general celebrates a milestone, he doesn’t acknowledge the shortcuts that have been taken to get to that milestone.” ITER is continually being reshaped to meet the demands of lower cost. The tokamak once had two exhaust components, called diverters. Now it has one. “And that is risky,” the scientist added. “That’s like building only one Space Shuttle, and expecting it to run for thirty years. If something happens to that one diverter, it could take five years to make another, so that might be the end of the project.” The compromises are a source of constant arguments, many of which go unresolved or are resolved cynically, people say, because Motojima fosters a culture antithetical to open science, because technical needs give right of way to diplomatic sensitivities, because ITER’s organizational structure is being modelled on that of a Japanese corporation—heavy on administration and intensely concerned with projecting an image of progress. “This project is supposed to be about hope, but fear runs rampant within it,” the scientist said. “Efforts are made on many levels to hide the problems, in part because people believe the situation can’t be remedied, and in part because some of the decision-makers will be dead by the time the big red button is pushed.”

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By summertime, the working atmosphere within the largest scientific collaboration in history was growing increasingly anxious. “ITER has always been a bit of a hectic place to work, eh?” Chiocchio had told me, but the frustrations were clearly mounting. In the previous year, ITER had met barely half its goals. The latest target date for turning on the machine—2020—was again slipping. Officials were now quietly talking about 2023 or 2024. What if the schedule continued to slide? Engineers operate in a world of strictly measured loads and heat fluxes, but political forces are impervious to precise measurement. Still, the ultimate repercussions were obvious: there would come a point, eventually, when frustrated politicians decided that ITER was simply not worth the increasing expense of delay.

In June, the ITER Council gathered in Tokyo, and it was evident that the organization was grappling with its own inner turbulence. At one point, the council member from Korea picked up his papers and stormed out. Ned Sauthoff, the U.S. project manager, bluntly made it known that he thought the project’s nuclear-safety culture was lacking. America’s involvement was growing more tenuous. The Department of Energy had cut funding for a tokamak at M.I.T. to help pay for ITER, and the decision had familiar implications; members of Congress were invited to view the inert machine, and they returned to the Hill expressing outrage. (“ITER is going to eat our whole domestic program.”) Official estimates of the U.S. contribution had doubled, to a billion dollars, and then rose again, to $2.4 billion, merely to get to “first plasma”—essentially, just turning on the machine. Before summer’s end, Dianne Feinstein, the chairwoman of the Senate subcommittee that handles appropriations for energy development, announced that she would discontinue all funding for ITER until the Department of Energy provided a detailed assessment of the total American financial commitment. The request was both logical and impossible to answer accurately; even people at ITER did not know. The department was reluctant to provide a number, and Sauthoff told me, “We are in unknown territory.”