ion cyclotron resonance heating (ICRH) is 📝plasma heating by radio waves tuned to the ions' gyration frequency.
Every charged particle in a magnetic field spirals around the field lines at a characteristic cyclotron frequency set by its charge, its mass, and the field strength. Launch a radio wave at exactly that frequency and the resonant ions surf it, absorbing energy pass after pass and sharing it with the rest of the plasma through collisions. For ions in fusion-grade fields the resonance falls in the tens of megahertz — ordinary radio frequencies — so ICRH systems are built from high-power transmitters and antennas mounted inside the 📝vacuum vessel. ITER's system will deliver 20 megawatts at 40 to 55 megahertz.
Its near-neighbor is electron cyclotron resonance heating, which targets the electrons instead: because electrons are thousands of times lighter, their resonance sits at microwave frequencies near 100 gigahertz and requires entirely different hardware. ICRH's advantage is that it puts power directly into the ions — the population whose temperature sets the 📝fusion rate — and it scales naturally to the high magnetic fields of compact machines. Its classic difficulty is the antenna: coupling megawatts across the plasma edge without driving impurities into the core. 📝Yijun Lin led the paper that solves that problem for 📝SPARC — twelve four-strap antennas near 120 megahertz, straps rotated to cut impurity generation — inside 📝The SPARC Physics Basis (2020).