SAINT-PAUL-LES-DURANCE, France—Teams working on two continents have marked similar milestones: They’ve each produced very impressive magnets. On Thursday, scientists at the International Thermonuclear Experimental Reactor (ITER) in southern France took delivery of the first part of a massive magnet so strong its American manufacturer claims it can lift an aircraft carrier. Almost 60 feet tall and 14 feet in diameter when fully assembled, the magnet is a crucial component in the attempt by 35 nations to master nuclear fusion. Massachusetts Institute of Technology scientists and a private company announced separately this week that they, too, have hit a milestone with the successful test of the world’s strongest high temperature superconducting magnet that may allow the team to leapfrog ITER in the race to build a ‘sun on earth.’ Rather than splitting atoms, fusion mimics a process that occurs naturally in stars to meld two hydrogen atoms together and produce a …