A plasma heating technique that uses high-frequency microwave beams tuned to the electron cyclotron frequency — delivering precise, localized power deposition deep inside the plasma.
Electron cyclotron resonance heating (ECRH) heats a magnetically confined plasma by launching high-frequency electromagnetic waves — typically in the microwave range of 100–170 GHz for modern tokamaks — at a frequency that matches the cyclotron frequency of electrons gyrating around the magnetic field lines. When the wave frequency equals the local electron cyclotron frequency (or one of its harmonics), the electrons absorb the wave energy resonantly, rapidly increasing their kinetic energy and thereby heating the plasma.[1]
Electrons in a magnetic field B gyrate at the electron cyclotron frequency fce = eB / (2πme), which for a 5 T field is approximately 140 GHz. An ECRH system generates millimeter-wave radiation at or near this frequency using high-power gyrotrons. The microwave beam is transmitted through evacuated or corrugated waveguides, launched through a steerable mirror into the plasma vessel, and absorbed in a narrow radial layer where the resonance condition is satisfied.[2]
Plasma heating: ECRH is routinely used for plasma start-up (ionizing gas and building initial current without the ohmic transformer), bulk heating, and reaching H-mode. Wendelstein 7-X, the world's largest stellarator, relies on up to 10 MW of ECRH as its primary heating system.[3]
Instability control: Steerable ECRH launchers can target neoclassical tearing modes and sawtooth oscillations by depositing current precisely at the resonant surface. This application is critical for ITER, which will deploy 20 MW of ECRH power from 24 gyrotrons operating at 170 GHz.
Current drive (ECCD): By launching the beam at an oblique angle to the magnetic field, net toroidal current can be driven non-inductively — an essential tool for steady-state tokamak scenarios.
Modern continuous-wave gyrotrons produce 1–2 MW per tube at efficiencies approaching 50% (with depressed collectors). Russia, Japan, and Europe have each developed gyrotrons for ITER, with the first units already delivered and tested. Advances in multi-frequency gyrotrons aim to provide flexibility for different magnetic field scenarios in future reactors.[2]