Bob Mumgaard, Commonwealth Fusion’s chief executive and one of the company’s founders, said a goal of the Sparc project was to develop fusion in time for it to play a role in mitigating global warming. “We’re really focused on how you can get to fusion power as quickly as possible,” he said.

Fusion, in which lightweight atoms are brought together at temperatures of tens of millions of degrees to release energy, has been held out as a way for the world to address the climate-change implications of electricity production.

Like a conventional nuclear fission power plant that splits atoms, a fusion plant would not burn fossil fuels and would not produce greenhouse-gas emissions. But its fuel, usually isotopes of hydrogen, would be far more plentiful than the uranium used in most nuclear plants, and fusion would generate less, and less dangerous, radioactivity and waste than fission plants.

But the hurdles to building a machine that can create and control a fusion plasma — a roiling ultrahot cloud of atoms that will damage or destroy anything it touches — are enormous.