The Physics of Fusion

The basic physics of thermonuclear energy is seductively simple. Fission produces energy by atomic fracture, fusion by tiny acts of atomic union. Every atom contains at least one proton, and all protons are positively charged, which means that they repel one another, like identical ends of a magnet. As protons are forced closer together, their electromagnetic opposition grows stronger. If electromagnetism were the only force in nature, the universe might exist only as single-proton hydrogen atoms keeping solitary company. But as protons get very near—no farther than 0.000000000000001 metres—another fundamental force, called the strong force, takes over. It is about a hundred times more powerful than electromagnetism, and it binds together everything inside the atomic nucleus.

Getting protons close enough to cross this barrier and to allow the strong force to bind them requires tremendous energy. Every atom in the universe is moving, and the hotter something is the greater its kinetic agitation. Thermonuclear temperatures—in the sun’s core, fifteen million degrees—are high enough to cause protons to slam together so forcefully that they are united by the strong force. Hydrogen nuclei slam together and form helium. Helium nuclei slam together and form beryllium. The atoms take on more protons, and become heavier. But, strangely, with each coupling a tiny amount of mass is lost, too. In 1905, Einstein demonstrated, with his most famous equation, E=mc2, that the missing mass is released in the form of energy as the nucleus is bound together. The quantity of energy is awesome—in some cases, a thousand times what is needed to get atoms to bind in the first place. Without it, stars would not burn, and space would remain forever cold.

The sun is, essentially, a four-hundred-quintillion-megawatt thermonuclear power plant, fuelled by billions of years’ worth of hydrogen. Six hundred million tons of it is converted into energy every second. “If you go back, really far, you see the first caveman crawl out of his cave and be surprised every time the sun came up—that was the first time mankind encountered a fusion reactor,” Ned Sauthoff, a physicist at Oak Ridge National Laboratory, in Tennessee, who serves as iter’s American project manager, told me. “It was ninety-three million miles away. But, of course, the caveman was impressed by the warmth and the light, and, being human, he said, ‘How can I have one of those?’ ”

And why it's so difficult to accomplish on Earth.