A team of researchers has restarted the world's largest fusion experiment—the Joint European Torus (JET) reactor, near Oxford, U.K. The move is a step forward in the quest for practical nuclear fusion.

The project was put on hold while a new lining was installed. This lining mimics the planned configuration of the International Thermonuclear Experimental Reactor (ITER), a full-scale experimental fusion reactor now under construction in southern France. As a consequence, the new undertaking at JET is being called the ITER-Like Wall Project.

The JET team says the lining, made of tiles of the light metal beryllium, should be better able to withstand the extreme conditions needed for a self-sustaining fusion reaction than the carbon-fiber composite tiles used before. The lining will also allow for laser-driven fusion experiments, similar to those underway at the National Ignition Facility in California.

JET is a tokamak—a device for carrying out magnetic confinement fusion. Its doughnut-shaped reactor contains plasma made from hydrogen that's squeezed by powerful magnetic fields. Eventually, magnetic pressure and heat force the hydrogen nuclei to fuse into helium, releasing a burst of energy and freeing high-energy neutrons.