Among the top puzzles in the development of fusion energy is the best shape for the magnetic facility -- or "bottle" -- that will provide the next steps in the development of fusion reactors. Leading candidates include spherical tokamaks, compact machines that are shaped like cored apples, compared with the doughnut-like shape of conventional tokamaks. The spherical design produces high-pressure plasmas -- essential ingredients for fusion reactions -- with relatively low and cost-effective magnetic fields.

Spherical tokamaks could be excellent models for a possible next step known as a Fusion Nuclear Science Facility (FNSF). Such a device could develop the materials and components for a fusion reactor and could precede a pilot plant that would demonstrate the ability to produce net electricity.

The FNSF could provide a pathway leading from ITER, the international tokamak under construction in France to demonstrate the feasibility of fusion power, to a commercial fusion power plant. With different magnet technology, the spherical tokamak could potentially serve as a pilot plant as well.

The two most advanced spherical tokamaks in the world today are the National Spherical Torus Experiment-Upgrade (NSTX-U) at PPPL and the Mega Ampere Spherical Tokamak (MAST) that is being upgraded at the Culham Centre for Fusion Energy in the United Kingdom. "We are opening up new options for future plants," said Jonathan Menard, head of NSTX-U Research and lead author of a paper in the journal Nuclear Fusion that discusses the fitness of both spherical machines as possible models.

The ability of these machines to create high plasma performance within their compact geometries determines their fitness as possible models for next-step fusion facilities. For now, the increased capabilities of the NSTX-U and the soon-to-be-completed MAST-U facility moves them closer to the capability of a commercial plant that will create safe, clean and virtually limitless energy.