A startup chasing the dream of plentiful, safe, carbon-free electricity from fusion, the energy source of the Sun, has settled on a site, timetable, and key technology for building its compact reactor. Flush with more than $200 million from investors, including Bill Gates’s Breakthrough Energy, 3-year old Commonwealth Fusion Systems announced today that later this year it will start building its first test reactor, dubbed SPARC, in a new facility in Devens, Massachusetts, not far from its current base in Cambridge. The company says the reactor, which would be the first in the world to produce more energy than is needed to run the reaction, could fire up as soon as 2025.