The people of 📝Commonwealth Fusion Systems are not confined to its payroll. The company's story runs through five groups: the MIT physicists who kept betting on high magnetic fields for fifty years, the founders who turned that bet into a company in 2018, the scientists and engineers who publish the case for the machines, the operators building and selling them now, and the partner executives who signed for power before a plant existed. This memo is organized by the part each group plays rather than alphabetically or by seniority, because the sequence — inheritance, founding, proof, execution, commitment — is the story. The third group has grown large enough to need subdividing, and it is subdivided by discipline rather than by employer: the people in it move between institutions, MIT and CFS staff co-author the same papers, and the thread a reader is following is a scientific question rather than an org chart.

The Alcator inheritance

The high-field approach predates the company by half a century. 📝Bruno Coppi conceived and named the Alcator program at MIT in the late 1960s, pursuing a small machine at extreme field while the international mainline built larger and weaker; he later took the same conviction down a different road with copper magnets. 📝D. Bruce Montgomery, who died in 2022, is why those fields were buildable, and his cable-in-conduit conductor became the baseline for nearly every superconducting 📝fusion device since. 📝Martin Greenwald set 📝Alcator C-Mod's experimental program for two decades, produced the 📝Greenwald limit that bears his name, and later framed 📝SPARC's physics basis for peer review. 📝Leslie Bromberg is the least-cited and most consequential: 📝ARC's demountable magnets trace to a paper he led in 2001.

The founding

The company began in a classroom. 📝MIT 22.63, the graduate design course taught by 📝Dennis Whyte, replaced lectures with collective machine design, and ARC was its deliverable — 📝Brandon Sorbom was lead author of the 2015 paper as a graduate student, with Whyte as senior author. 📝Bob Mumgaard studied how private finance could compress fusion's path to market and became the company's chief executive at its 2018 spinout. 📝Zachary Hartwig led the magnet program that reached 20 📝tesla in September 2021 — the result that converted a plausible thesis into a fundable one. 📝Dan Brunner was named chief technology officer at the 2018 launch and is a co-founder; he no longer holds that role, and his own public profile now lists a different company, so nothing here should be read as a current CFS position. What he contributed is measured rather than projected: on Alcator C-Mod he led the study of how narrow a strip the exhaust power arrives on at reactor-level 📝poloidal field, and that number still sizes the exhaust problem for both machines.

The people who do the science

The published case for SPARC and ARC is carried by scientists and engineers at several institutions, and the sections below sort them by what they work on rather than by who employs them. Most are at the 📝MIT Plasma Science and Fusion Center rather than at CFS; several moved between the two, or between a national laboratory and either; one is an independent academic at a university that publishes findings about ARC which CFS did not write. 📝Jon Hillesheim leads the ARC physics basis overview these disciplines are the constituent chapters of, and the full published record is collected in 📝The Science of Commonwealth Fusion Systems.

Magnets, cryogenics and the test campaign

The 20-tesla result of September 2021 is a hardware measurement, and producing it took a building. 📝Rui Vieira, Deputy Head of Magnetic Fusion Energy Engineering at the PSFC, led the paper on how the 📝Toroidal Field Model Coil (TFMC) was designed, fabricated and assembled; 📝Theodore Golfinopoulos wrote down the test facility built around it — power, 📝cryogenics, structure and instrumentation — in a paper titled "Building the Runway"; 📝Philip C. Michael built the supercritical helium loop that held the coil near 20 kelvin on closed-cycle cryocoolers rather than a liquid-helium plant; and 📝Vincent Fry is first author on the 50-kiloamp nitrogen-cooled demountable current leads that carried current across the cold boundary without carrying the room in with it. 📝Amy Watterson modeled those helium flows and the stresses they impose, and later measured what 2.5 hours in molten solder does to the 📝REBCO tape inside a PIT VIPER cable: 📝critical current uniform within experimental error. 📝Alex Zhukovsky is among that coil's authors and carries the longest thread in this section — a research engineer at the PSFC from 1988, a visiting scientist there since 2007 and a CFS consultant since 2019 — with published work running from 📝ITER 📝central solenoid joints to this magnet. 📝Erica Salazar, Senior Manager of Magnet Systems at CFS, demonstrated fiber-optic 📝quench detection on full-scale 📝VIPER cable at the 📝SULTAN facility, and 📝Charlie Sanabria is the cable itself. 📝Alexey Radovinsky designs high-field magnets at CFS and led the design of a pair of sub-two-ton REBCO end coils for a 📝magnetic mirror — a confinement geometry that is not a tokamak at all, and the clearest evidence the magnet platform is not tokamak-specific. Much of this work is gathered in 📝The TFMC Engineering Papers.

The boundary: divertor and exhaust

Exhaust is the problem of getting fusion power back out of the machine without destroying the surface it lands on. 📝Brian LaBombard, a senior research scientist at the PSFC, joined Alcator C-Mod at the start of its construction, built its edge diagnostics, and set out the 📝X-point target divertor in the 2015 ADX proposal — the long-legged geometry ARC's 📝divertor descends from. 📝Adam Kuang and 📝Thomas Eich took the exhaust problem for SPARC and ARC respectively. 📝Tom Looby, a scientist at CFS, wrote HEAT, the code that traces magnetic field lines from the 📝scrape-off layer onto three-dimensional CAD geometry to predict where the power lands; two of its physics modules were checked in 2025 against 📝ASDEX Upgrade infrared and thermocouple data, matching in 📝L-mode and disagreeing in the ELMy cases those runs did not model. 📝Thomas Body spent years validating edge turbulence codes against 📝TCV and ASDEX Upgrade before spending that lineage on the 2026 ARC exhaust paper, which projects 📝divertor detachment at roughly 0.9 % argon seeding in the divertor — a modeled prediction for a machine that does not exist yet.

Core transport, pedestal and heating

📝Phil Snyder, Vice President of Plasma Physics at CFS, originated EPED, the model the field uses to predict how high a tokamak's 📝pedestal will stand; measured pedestals on six tokamaks agree with it to within 20–25 %, and a neural-network surrogate trained on it sets ARC's core boundary condition at a projected pedestal top near 360 kilopascals, which SPARC is meant to test at reactor field. 📝Jerry Hughes, a co-author on the 2011 EPED paper, carried the same framework into SPARC's pedestal projection. 📝Amanda Hubbard, a principal research scientist at the PSFC, is central to I-mode — the C-Mod regime with 📝H-mode-like energy confinement and L-mode-like particle transport, so heat stays in while impurities do not accumulate — and mapped the conditions for entering it. Those are measurements on a machine that ran. 📝Nathan Howard leads the transport modeling that sets how much power ARC would make; 📝Pablo Rodriguez-Fernandez did the equivalent for SPARC in 2020. 📝John Rice reads 📝ion temperature and rotation out of x-ray line emission, and his 2007 inter-machine comparison of intrinsic 📝plasma rotation — the toroidal spin a 📝plasma develops with no external torque — pooled data from 📝JET, C-Mod, Tore Supra, 📝DIII-D, JT-60U and TCV. Heat has to get in as well as out: 📝Yijun Lin designed SPARC's only external heating system, and 📝Steve Scott computed whether its alpha particles stay confined long enough to matter.

Disruption, runaway electrons and machine protection

📝Ryan Sweeney owns 📝disruptions on both machines. 📝Alex Tinguely, group leader for energetic particle physics at the PSFC, is first author of the 2021 paper modeling complete prevention of 📝runaway electron beam formation in SPARC by a passive three-dimensional coil the disrupting plasma drives itself, needing no detection and no trigger, and of the 2024 synthetic 📝synchrotron radiation diagnostic that works out what a camera would see before there is a machine to point it at. Both are modeled predictions. 📝Robert Granetz, a principal research scientist at the PSFC, supplies the measured half: his 1996 📝halo current work on C-Mod established the magnitude and toroidal asymmetry of the loads a 📝vacuum vessel must survive, and he helped assemble the cross-machine disruption databases the field now designs against. 📝Nils Leuthold brings the ARC 📝magnetohydrodynamics stability analysis from Columbia, and 📝Carlos Paz-Soldan — an associate professor at Columbia University and founding director of the 📝Columbia Fusion Research Center, not a CFS employee — co-authored it. That distinction is the value: CFS sponsors research at Columbia, and Columbia published the ARC stability finding under its own name as an independent check. Paz-Soldan's own runaway electron work is experimental, done on DIII-D and JET, where secondary 📝deuterium injection terminated megaampere-level runaway currents without measurable first-wall heating.

Diagnostics and neutronics

Three people make the science checkable rather than merely stated. 📝Matt Reinke leads the diagnostic set that will measure SPARC. 📝Prasoon Raj is first author of the 2024 overview of its 📝neutron diagnostics — four complementary subsystems targeting 10 percent uncertainty on fusion power across more than eight orders of magnitude of dynamic range, a requirement written down for a machine that has not yet produced a neutron. 📝Andrea Saltos, neutronics methods technical lead at CFS, works the same neutrons in the opposite direction: how many get through meters of 📝shielding and through the penetrations cut in it to admit those very diagnostics, calculated with deterministic solvers benchmarked against experiments whose answers were measured.

Building and selling the machines

📝Alex Creely wrote the paper that let the field check SPARC's design and now serves as chief engineer for ARC. 📝Rick Needham carries the commercial argument physics cannot make, and 📝Lorence Kim brings the experience of financing a science company before its product was proven. 📝Kristen Cullen led the public case for siting the first plant in Virginia; 📝Jennifer Ganten carries two decades of energy diplomacy into the regulatory questions that decide where plants get built. 📝Joe Paluska leads marketing, and 📝Stephen Shankland — a science journalist of twenty-five years — reports on the machines from inside the company.

The people who signed

Commitments from outside are what separate a funded experiment from a business. 📝Claudio Descalzi backed the company through 📝Eni in 2018 and signed for ARC's power in 2025. 📝Michael Terrell committed Google to 200 megawatts, half the first plant's output. 📝Edward H. Baine put 📝Dominion Energy Virginia's name behind the 📝Fall Line Fusion Power Station site. Beyond any single company, 📝Andrew Holland of the 📝Fusion Industry Association led the campaign behind the 2023 decision to regulate fusion separately from fission — the ruling that cleared the licensing path for every plant now planned in the United States.

Contexts

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