Historical Context & Technical Milestone
On September 5, 2021, Commonwealth Fusion Systems and the MIT Plasma Science and Fusion Center successfully energized a full-scale toroidal-field coil prototype using rare-earth barium copper oxide (REBCO) high-temperature superconducting tape, achieving a steady 20-Tesla field on the coil's inner surface. It was the highest sustained magnetic field ever produced by a large-bore superconducting magnet of fusion-relevant geometry.
The demonstration validated the central engineering hypothesis behind CFS's SPARC tokamak: that REBCO HTS tape, operating at 20 K rather than the 4 K required by conventional niobium-tin magnets, could deliver fields high enough to make a compact, net-energy tokamak buildable on a private-capital timescale. SPARC's design target of Q ≥ 10 follows directly from the B⁴ scaling of fusion power with magnetic field strength.
The September 2021 test triggered a wave of investment that took CFS's cumulative funding past $3 billion and re-established fusion as a venture-fundable category. By 2026, every major magnetic-confinement developer — Tokamak Energy, Type One Energy, Realta, Proxima, and Pacific Fusion — has incorporated HTS magnet technology into its baseline design.