Spherical tokamaks could provide path to limitless fusion energy

Creating “a star in a jar” – replicating on Earth the way the sun and stars create energy through fusion – requires a “jar” that can contain superhot plasma and is low-cost enough to be built around the world. Such a device would provide humankind with near limitless energy, ending dependence on fossil fuels for generating electricity.




Physicists at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) say that a model for such a “jar,” or fusion device, already exists in experimental form – the compact spherical tokamaks at PPPL and Culham, England. These tokamaks, or fusion reactors, could provide the design for possible next steps in fusion energy – a Fusion Nuclear Science Facility (FNSF) that would develop reactor components and also produce electricity as a pilot plant for a commercial fusion power station.



Spherical tokamaks are compact devices that are shaped like cored apples, compared with the bulkier doughnut-like shape of conventional tokamaks. The increased power of the upgraded PPPL machine and the soon-to-be completed MAST Upgrade device moves them closer to commercial fusion plants that will create safe, clean and virtually limitless energy without contributing greenhouse gases that warm the Earth and with no long-term radioactive waste.

「球状トカマクは、扱いにくいドーナツ型の従来のトカマクと比べると、芯を抜いたリンゴのような形をしたコンパクトなデバイスです。性能向上されたPPPLマシンとまもなく完成予定のMAST(Mega Amp Spherical Tokamak) Upgrade(MAST-U)装置のさらに高いパワーが、それらを、地球を暖めるグリーンハウスガスに寄与する事なしに、また長期にわたる放射性廃棄物を伴うこと無く、安全でクリーンでほぼ無限のエネルギーを作り出す商用核融合プラントへとさらに近づけます。」



The devices face a number of physics challenges. For example, they must control the turbulence that arises when superhot plasma particles are subjected to powerful electromagnetic fields. They must also carefully control how the plasma particles interact with the surrounding walls to avoid possible disruptions that can halt fusion reactions if the plasma becomes too dense or impure. Researchers at PPPL, Culham, and elsewhere are looking at ways of solving these challenges for the next generation of fusion devices.




For pilot plants, the authors call for superconducting magnets to replace the primary copper magnets in the FNSF. Superconducting magnets can be operated far more efficiently than copper magnets but require thicker shielding. However, recent advances in high-temperature superconductors could lead to much thinner superconducting magnets that would require less space and reduce considerably the size and cost of the machine.




Included in the paper is a description of a device called a “neutral beam injector” that will start and sustain plasma current without relying on a heating coil in the center of the tokamak. Such a coil is not suitable for continuous long-term operation. The neutral beam injector will pump fast-moving neutral atoms into the plasma and will help optimize the magnetic field that confines and controls the superhot gas.




Taken together, the paper describes concepts that strongly support a spherical facility to develop fusion components and create on Earth “a star in a jar”; the upgraded NSTX and MAST facilities will provide crucial data for determining the best path for ultimately generating electricity from fusion.