An overhead view of the JT-60SA reactor
The long trek toward practical fusion energy passed a milestone last week when the world’s newest and largest fusion reactor fired up. Japan’s JT-60SA uses magnetic fields from superconducting coils to contain a blazingly hot cloud of ionized gas, or plasma, within a doughnut-shaped vacuum vessel, in hope of coaxing hydrogen nuclei to fuse and release energy. The four-story-high machine is designed to hold a plasma heated to 200 million degrees Celsius for about 100 seconds, far longer than previous large tokamaks.
Last week’s achievement “proves to the world that the machine fulfills its basic function,” says Sam Davis, a project manager at Fusion for Energy, an EU organization working with Japan’s National Institutes for Quantum Science and Technology (QST) on JT-60SA and related programs. It will take another 2 years before JT-60SA produces the long-lasting plasmas needed for meaningful physics experiments, says Hiroshi Shirai, leader of the project for QST.
JT-60SA will also help ITER, the mammoth international fusion reactor under construction in France that’s intended to demonstrate how fusion can generate more energy than goes into producing it. ITER will rely on technologies and operating know-how that JT-60SA will test.
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Japan got to host JT-60SA and two other small fusion research facilities as a consolation prize for agreeing to let ITER go to France. The deal, spelled out in a 2007 Japan-EU agreement, called for upgrading Japan’s venerable JT-60 reactor, a research workhorse since the mid-1980s. The JT-60 building was retained while the reactor was rebuilt from the base up at an undisclosed cost.
At 15.5 meters tall, JT-60SA (for “superadvanced”), is roughly half the height of ITER and can contain 135 cubic meters of plasma, one-sixth the volume of its European cousin and about equal to a typical railroad tank car. Its plasmas will closely resemble those planned for ITER and should allow physicists to study plasma stability and how it affects fusion power output at long timescales, providing lessons that can be applied to the bigger tokamak, says Alberto Loarte, head of ITER’s science division.
Like many fusion projects, JT-60SA has experienced significant delays, stretching its timeline to more than 15 years. The original plan was for the reactor to come online in 2016. A redesign, procurement issues, and the March 2011 Tohoku earthquake all caused delays. Then, during testing in March 2021, a short circuit occurred in the cable supplying electricity to one of the superconducting magnetic coils. Traced to insufficient insulation in a critical wiring joint, the short damaged electrical connections and caused a helium leak that would have degraded cooling systems. The incident occurred while there was minimum current in the circuit. Had the current been higher, “damage to the coil could have been severe,” Shirai says. “We were fortunate.” To be safe, the JT-60SA team reworked the insulation in more than 100 electrical connections, a task that took 2.5 years. The accident has led engineers at ITER to plan more careful tests of their coils, too, Shirai says.
One limitation is that JT-60SA will only use hydrogen and its isotope deuterium in its experiments, not tritium—a third form of hydrogen that is expensive, scarce, and radioactive. Tritium is considered the most efficient option for energy production, so ITER plans to begin using deuterium-tritium fuel in 2035.
By 2050, Japan also hopes to build DEMO, a proposed demonstration power plant that would provide a stepping stone from the research of JT-60SA and ITER to commercial fusion power. Shirai says he is well aware of the competition from alternative approaches to fusion energy, fueled by an influx of private money into the field. But with competition comes opportunities for collaborations among those with new ideas. “Having many people coming into this area is a very good thing,” Shirai says.
Update, 3 November, 12:35 p.m.: This story has been updated to clarify Alberto Loarte’s comments.
