In September 2021, Commonwealth Fusion Systems tested a large-bore high-temperature superconducting magnet to 20 tesla — proving that compact, high-field fusion devices are technically achievable.
On September 5, 2021, a team at Commonwealth Fusion Systems and MIT's Plasma Science and Fusion Center successfully tested a large-bore, high-temperature superconducting (HTS) magnet to a field strength of 20 tesla. The magnet, built using rare-earth barium copper oxide (REBCO) tape, was the most powerful HTS magnet of its size ever constructed and the strongest fusion-relevant magnet ever tested.[1]
Fusion power scales as the fourth power of the magnetic field strength. Doubling the field from 5 T (typical of conventional superconducting tokamaks) to 10 T increases achievable fusion power density by a factor of 16 in the same volume. The 20 T demonstration showed that fields far beyond the limits of legacy niobium-tin superconductors are accessible with HTS technology, enabling tokamaks that are dramatically smaller, faster to build, and less expensive than previous designs.[2]
The magnet used a D-shaped coil geometry representative of a toroidal field coil for the SPARC tokamak. It was wound from stacked REBCO tape conductors and cooled to approximately 20 kelvin using conduction cooling. The test demonstrated stable operation at full field, successful quench protection, and mechanical integrity under enormous electromagnetic loads — all critical requirements for a fusion power plant magnet system.[3]
The successful demonstration was widely regarded as one of the most significant fusion engineering milestones of the decade. It validated the compact high-field approach to fusion, attracted billions in private investment to CFS, and accelerated timelines across the broader fusion industry. Multiple other companies subsequently adopted HTS magnet technology for their own reactor designs.[1]