On an overcast day in early December, a yellow earth mover scooped dirt from the edge of a deep pit in Devens, Massachusetts, on the site of an old Army base some 50 miles outside of Boston. 

This is the future home of SPARC, a prototype fusion reactor that, if all goes as hoped, will achieve a goal that’s eluded physicists for nearly a century. It will produce more energy from fusing together atoms, the same phenomenon that powers the sun, than it takes to achieve and sustain those reactions.

By some point in 2025, the scientists at Commonwealth Fusion Systems expect, their machine will blow past that threshold, generating 10 times more energy than it consumes. That demonstration, they say, will enable the startup to develop full-size facilities capable of delivering as much electricity as a small coal plant by the early 2030s.

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