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.

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