Nestled in the countryside of southern France is a sprawling industrial complex where scientists and engineers from around the world have converged to build the world's largest-ever fusion reactor: a doughnut-shaped vacuum chamber designed to contain temperatures 10 times hotter than the core of the Sun.At an estimated cost of $22 billion, the International Thermonuclear Experimental Reactor is the world's biggest bet on fusion energy: a project so daunting in scale that longtime geopolitical rivals have pooled their resources to share in its potential risks and rewards. ITER's central solenoid (left) is the largest magnet in the world. It will…