Marathon Fusion builds fuel-cycle systems for fusion developers and is now constructing a compact beam-driven reactor in San Francisco to produce neutrons for that work. The company was founded in 2023 by Kyle Schiller, previously a science policy fellow at Schmidt Futures, and Adam Rutkowski, formerly a propulsion engineer at SpaceX. Its plasma centrifuge separates hydrogen and lithium isotopes by spinning a partially ionized plasma. In August 2026 Marathon demonstrated enrichment of both, a step toward recycling unburned tritium and toward a supply of lithium-6 that does not depend on the mercury-based chemical process used in Russia and China. The reactor fires a deuterium ion beam into a solid lithium tritide target so fusion reactions occur in a thin surface layer. Marathon lists 1 kilowatt of fusion power, a power density of 20 kilowatts per square meter, and a neutron flux of one trillion neutrons per square centimeter per second. The company plans to use those neutrons to qualify structural materials, to produce medical isotopes such as actinium-225, and to test the mercury blanket approach it described in 2025, in which neutrons on mercury-198 yield stable gold while the plant continues to breed tritium. The launch sits on a real supply constraint. Starting a commercial fusion plant takes kilograms of tritium, and breeding more of it takes enriched lithium-6 at volumes current production does not provide. Marathon is developing the separation and processing hardware other companies can buy before their reactors are finished, and building its own neutron source so materials tests and isotope production do not have to wait on someone else's machine.