How do tokamaks work to make nuclear fusion possible?

A tokamak is a device that uses magnetic fields to trap very hot plasma inside a ring-shaped vacuum chamber. Plasma, a fourth state of matter, is a gas whose atoms have been stripped of their electrons, leaving free electrons and charged nuclei. The word comes from a Russian acronym describing a toroidal (ring-shaped) chamber with magnetic coils.

The best-known fusion energy project in the world, the International Thermonuclear Experimental Reactor (ITER), uses this design. Nuclear energy today comes almost entirely from fission, the controlled chain reaction of uranium atoms, but fusion has always been theoretically possible and a subject of consistent long-term investigation by scientists. When light atomic nuclei such as deuterium and tritium, two isotopes of hydrogen and the best-studied candidates, join, they form helium and a fast neutron.

But even this simple merger requires temperatures hotter than the sun’s core, which no earthly material can contain. In physics, this is called the problem of containment.