electron cyclotron resonance heating (ECRH) is 📝plasma heating by microwaves tuned to the electrons' gyration frequency.
Electrons spiral around magnetic field lines at a cyclotron frequency thousands of times higher than that of ions, so where 📝ion cyclotron resonance heating (ICRH) works in the tens of megahertz, ECRH works in the microwave band above 100 gigahertz — ITER's system runs at 170 gigahertz, delivered by megawatt-class gyrotrons. That frequency difference reshapes the entire system: instead of antennas pressed against the plasma edge, ECRH uses mirrors and waveguides to launch a beam through free space, and the beam can be steered.
Steerability is ECRH's defining advantage. Because the resonance occurs only where the wave frequency matches the local field strength, the power deposits in a small, precisely chosen volume — the most localized heating any 📝fusion system offers. That precision makes ECRH a control tool as much as a heater: aimed at a forming 📝magnetic island, driven current can stabilize the mode before it degrades confinement, and a short pulse can help ionize the gas at startup. Its trade-offs are the gyrotron itself — a frontier technology at these powers — and cutoff limits, since dense plasma can reflect microwaves before they reach the intended resonance.