Commonwealth Fusion Systems (CFS) is developing compact tokamak fusion reactors using high-temperature superconducting (HTS) magnets. The company was spun out of MIT's Plasma Science and Fusion Center in 2018 by a team led by CEO Bob Mumgaard, with MIT physics professor Dennis Whyte as a key scientific co-founder. Its central technical thesis — that a new class of superconducting materials enables magnets strong enough to make fusion reactors dramatically smaller and faster to build than previously possible — was experimentally validated in September 2021 when CFS achieved a 20 tesla magnetic field in a large-bore superconducting magnet, a world record that confirmed the scientific foundation of its approach.
CFS raised approximately $1.8 billion in a December 2021 Series B — one of the largest private clean energy raises in history — led by Tiger Global and joined by Google, Salesforce Ventures, Khosla Ventures, Breakthrough Energy Ventures, and others. The company is building its demonstration fusion reactor, SPARC, at a site in Devens, Massachusetts, with a target of achieving first plasma around 2027. The subsequent commercial power plant, ARC, is targeted for the 2030s.
Fusion energy — the same reaction that powers the sun — releases energy by fusing light atomic nuclei (typically deuterium and tritium, both isotopes of hydrogen) under extreme heat and pressure. The engineering challenge is confining a plasma at 100 million degrees Celsius, far hotter than the sun's core, long enough and densely enough to sustain a net-positive energy reaction. Tokamaks — donut-shaped magnetic confinement devices — are the most mature approach to achieving this. ITER, the international fusion project under construction in France, is a tokamak, and it is enormous: roughly 30 meters tall, 28,000 tonnes, costing approximately €20 billion.
CFS's insight is that fusion performance scales steeply with magnetic field strength — roughly as the fourth power of the magnetic field. Conventional fusion programs like ITER use low-temperature superconducting (LTS) magnets that max out at around 10–13 tesla. A new class of materials — rare-earth barium copper oxide (REBCO) tape, a high-temperature superconductor — can sustain superconductivity in much higher fields and at the more manageable temperature of liquid nitrogen (77 K rather than 4 K). CFS has developed the manufacturing and engineering techniques to wind REBCO tape into high-field magnets at scale. The result: a 20 tesla magnet demonstrated in September 2021, roughly double the field of conventional fusion magnets.
Because fusion performance scales so steeply with field strength, doubling the magnetic field allows plasma volume to shrink by approximately 16-fold for equivalent fusion performance. SPARC, the demonstration reactor, will have a plasma volume roughly 1/65th that of ITER while targeting a fusion energy gain (Q) greater than 2 — meaning it produces more than twice the energy it consumes to heat the plasma. If achieved, this would be the first demonstration of net energy gain from a fusion device. The compact form factor also means SPARC can be built in years rather than decades and at a cost in the hundreds of millions rather than tens of billions.
SPARC (Smallest Possible ARC) is a demonstration device designed to prove net energy gain from a compact tokamak. It is not a power plant — it will not be connected to the grid or generate commercial electricity. Its purpose is to validate the physics and engineering at scale: confine a burning plasma, achieve Q>2, and operate reliably enough to generate the performance data needed to design ARC. SPARC is under construction at a dedicated site in Devens, Massachusetts. CFS has published its design basis in peer-reviewed papers through the Journal of Plasma Physics, and the published physics have generally been validated by independent analysis.
ARC (Affordable, Robust, Compact) is the planned commercial fusion power plant that will follow SPARC. CFS envisions ARC producing roughly 200–500 MW of net electrical output — a size comparable to a mid-scale gas plant — with a modular design that could be replicated. Fusion fuel is effectively unlimited: deuterium is extracted from seawater, and tritium is bred from lithium inside the reactor. The waste product is helium, not long-lived radioactive material. ARC would be dispatchable, carbon-free, and not weather-dependent — a combination no current clean energy technology achieves. CFS has signed early letters of intent for power purchase from utilities and industrial buyers, though commercial contracts are contingent on SPARC demonstrating the underlying physics.
CFS operates on a faster timeline than any previous fusion program. The company has committed to first plasma on SPARC around 2027, a date that is ambitious by the standards of fusion engineering but plausible given that the primary technical risk — high-field magnet performance — has been experimentally resolved. The remaining work is engineering execution: manufacturing magnets at production volume, assembling the tokamak, and managing the plasma physics of a burning plasma for the first time outside a national laboratory.
The commercial proposition, if SPARC succeeds, is a fusion power plant that is small enough to be built at a manufacturing facility and shipped to site, firm enough to provide baseload power, and fueled by effectively inexhaustible materials. CFS is not alone in pursuing compact fusion — TAE Technologies, Helion Energy, and others have raised significant capital — but CFS has the most transparent technical program, the strongest independent validation, and arguably the most credible near-term milestone.
Fusion has been "20 years away" for 70 years. CFS has resolved the magnet problem, which was the key technical gating item for compact fusion. But a burning plasma — one that is self-sustaining from fusion reactions — has never been achieved in any private or public fusion device. SPARC will be the first attempt at net gain in a tokamak outside of the National Ignition Facility's laser-based approach. The gap between achieving net gain in a demonstration device and producing reliable, cost-competitive commercial power involves decades of engineering, materials science, and regulatory work that even optimistic timelines leave to the 2030s and beyond.
The tritium fuel cycle presents a significant engineering challenge. Tritium is produced in limited quantities as a byproduct of fission reactors and is radioactive with a 12-year half-life. Commercial fusion plants must breed their own tritium from lithium blankets inside the reactor — a technology that has never been demonstrated at scale. Getting the tritium breeding ratio above 1.0 (self-sustaining) is one of the key engineering milestones between SPARC and ARC. CFS and the broader fusion community are aware of this; it is simply one of many unsolved engineering challenges that stand between today's demonstrations and commercial power.
This profile was compiled from publicly available information including:
CFS Newsroom — Technical milestones, funding announcements, and reactor design updates.
Greenwald, M., et al. "SPARC physics basis." Journal of Plasma Physics, Special Issue, September 2020. Peer-reviewed design basis for the SPARC tokamak.
CFS 20 tesla magnet demonstration announcement, September 2021; Series B funding announcement, December 2021.
This profile is for informational purposes only and does not constitute investment advice, a recommendation, or a solicitation to buy or sell any security. CFS is a private company; financial data is limited to publicly disclosed information.