Brayton cycle is the thermodynamic cycle in which a working gas is compressed, heated at constant pressure, expanded through a turbine to produce work, and then returned to its starting state — the cycle that runs every gas turbine engine, and the power-conversion step proposed for 📝fusion plants whose heat arrives hot.
The cycle has four steps and no phase change: compression, constant-pressure heat addition, expansion through a turbine that produces more work than the compressor consumes, and heat rejection back to the starting state. In an open cycle — a jet engine or an industrial gas turbine — the heat is added by burning fuel in the gas itself and the exhaust is thrown away. In a closed cycle the working gas is sealed in a loop and heat enters through an exchanger, which is what allows a nuclear heat source to drive one. Thermal efficiency rises with pressure ratio and with turbine inlet temperature, so the cycle rewards a hot source. It is also called the Joule cycle, and is named for George Brayton (1830–1892), the American engineer whose Ready Motor of 1872 used a piston compressor and a piston expander rather than the turbomachinery that carries the name today.
The comparison that matters is against the Rankine cycle, which is what a conventional steam plant runs: there the working fluid boils and condenses, and the cycle's top temperature is bounded by practical steam conditions. Brayton keeps a single-phase gas throughout, so it can accept heat at temperatures where water would demand extreme pressures, and closed-cycle variants using helium or supercritical carbon dioxide as the working fluid are under development for high-temperature reactors because they promise compact turbomachinery at those temperatures.
In a fusion plant the electricity does not come from the 📝plasma directly but from the heat deposited in the 📝blanket by 📝14 MeV neutrons, so the power cycle's efficiency is precisely the conversion factor behind 📝gigawatt thermal vs gigawatt electric and a term in 📝Q_plasma vs Q_engineering. 📝The 2015 ARC Design Paper takes 📝FLiBe out of the blanket at roughly 900 kelvin and feeds a Brayton cycle with it. The cycle itself is the mature part of that chain: gas turbines are among the most widely deployed power machinery in the world, and nothing about the thermodynamics is speculative. What remains unproven is the fusion heat source upstream of it and the salt-to-gas heat exchange between them.