In the world of fusion energy, scientists and engineers study the fourth state of matter known as plasma in an effort to design and build a new type of power plant. Relying on the heat produced by two small atoms smashing together, a network of such facilities would help create a novel source of stable electricity and help ensure America’s energy independence. And while scientists in this endeavor are devoting their attention to complex machinery and temperatures hotter than the surface of the sun, they are also trying to determine the best designs for such a power plant by focusing on geometry.

In fusion systems, shape matters. The earliest device designed by Lyman Spitzer Jr., the founder of the U.S. Department of Energy ‘s Princeton Plasma Physics Laboratory (PPPL), was shaped like a figure eight. A later system, known as the tokamak, was developed in the 1960s and shaped like a doughnut in an effort to keep the plasma confined by creating a central electrical current that formed vital confining magnetic fields. Other fusion devices were shaped like straight lines or twisty crullers.

Additionally, some fusion systems look like cored apples. Known as spherical tokamaks, they resemble doughnut-like tokamaks that have been compressed, making the hole down the center far narrower than before. Scientists have found that spherical tokamaks have properties that could confine plasma energy more efficiently than conventional tokamaks. These properties could help generate a plasma with the necessary temperature and density for a sufficient amount of time to create a fusion reaction that heats itself, like a mini star on Earth.