Jon C. Hillesheim is a 🏷️#physicist and principal scientist at 📝Commonwealth Fusion Systems, where he works on the physics basis for 📝ARC, the company's planned fusion power plant. He is lead author of "Overview of the physics basis for the ARC fusion power plant," the framing paper of a five-paper collection published in the Journal of Plasma Physics on June 4, 2026.

Hillesheim joined CFS in 2022 after a decade at the UK Atomic Energy Authority, where his research centered on turbulence and transport — the mechanisms by which heat and particles leak out of a confined 📝plasma far faster than collisions alone would explain, and the reason a 📝tokamak must be built larger or run hotter than simple theory predicts. The overview paper he leads carries twenty co-authors and states ARC's design point: roughly 1.1 gigawatts of fusion power converted to 400 megawatts of net electricity delivered to the grid. Those figures are outputs of integrated modeling, not measurements. ARC has not been built, and 📝SPARC, the machine intended to demonstrate the physics they rest on, has not yet run.