A compact high-field tokamak design from MIT that demonstrated how high-temperature superconducting magnets could dramatically shrink fusion power plants — directly inspiring SPARC and Commonwealth Fusion Systems.
ARC (Affordable, Robust, Compact) is a conceptual fusion power plant design developed at MIT's Plasma Science and Fusion Center (PSFC), first published in 2015. The design showed that newly available rare-earth barium copper oxide (REBCO) high-temperature superconducting (HTS) tapes could produce magnetic fields strong enough to confine a burning plasma in a device far smaller than ITER. ARC became the intellectual foundation for the SPARC experiment and for Commonwealth Fusion Systems (CFS), which spun out of MIT to commercialise the technology.[1]
ARC's defining insight is that fusion power density scales as the fourth power of the magnetic field strength (Pfus ∝ B4). By employing REBCO HTS magnets capable of producing fields above 20 tesla on the plasma axis (compared to ITER's ~5.3 T), ARC achieves equivalent fusion performance in a device with roughly half the linear dimensions and a fraction of the volume. This dramatically reduces cost and construction time.[2]
The ARC design specifies a major radius of 3.3 m, a minor radius of 1.1 m, a toroidal field of approximately 9.2 T on axis (with peak fields at the coil exceeding 23 T), and a fusion power of 525 MW thermal. It incorporates demountable HTS magnets that can be separated for maintenance access — a significant engineering advantage over monolithic superconducting coil systems. The design also features a liquid immersion blanket using FLiBe (a lithium-beryllium fluoride salt) for tritium breeding and neutron shielding.[3]
While ARC itself remains a paper design, it catalysed real-world action. In 2018, CFS was founded with MIT collaboration and venture funding to first build SPARC (a smaller, net-energy experiment validating ARC physics) and then proceed to ARC-class commercial plants. In 2021, CFS demonstrated a 20 T large-bore HTS magnet — the key enabling technology. SPARC, under construction in Devens, Massachusetts, is designed to achieve Q > 2 and validate the path to ARC-scale power production.
ARC's publication marked a turning point in fusion energy's trajectory. It demonstrated that advances in superconductor technology could bypass decades of incremental scaling, and it attracted billions of dollars of private investment into the fusion sector. The compact high-field approach has since influenced numerous other fusion concepts and helped establish the modern private fusion industry.