The UK has powered up an unusual fusion reactor.

Science magazine takes us inside the Mega Amp Spherical Tokamak (MAST) Upgrade, an experimental fusion reactor which has been switched on after a 7-year build and revamping. Ultimately, it should provide pointers to a future source of abundant, cheap, clean energy – if we can get the containment fields right:

Tokamaks are the frontrunners in the decadeslong effort to generate energy by fusing light elements together. These doughnut-shaped vessels contain a superhot ionized gas—or plasma—of hydrogen isotopes that is constrained with powerful magnets and heated by microwaves and particle beams….

MAST is a variation on the standard tokamak; it is shaped more like a cored apple than a doughnut. Researchers believe that shape can confer greater stability in the roiling plasma than a doughnut-shaped tokamak, but it is less well understood than the traditional design. MAST first tested the concept on a large scale starting in 1999 and has now been upgraded with extra heating power, new technology for extracting heat from the plasma, and other improvements.