A plasma heating method using radio-frequency waves in the 30–120 MHz range tuned to the ion cyclotron frequency — capable of preferentially heating fuel ions or minority species deep in the plasma core.
Ion cyclotron resonance heating (ICRH) — also called ion cyclotron radio-frequency (ICRF) heating — delivers energy to a magnetically confined plasma by launching fast magnetosonic waves at frequencies matching the cyclotron frequency of the plasma ions or their harmonics. For typical tokamak fields of 2–5 T, the relevant frequencies lie in the radio-frequency (RF) band between roughly 30 and 120 MHz. The waves are generated by high-power RF transmitters, fed through transmission lines, and coupled into the plasma via in-vessel antennas positioned close to the plasma edge.[1]
ICRH can heat the plasma through several absorption scenarios:
Minority heating: A small concentration (typically 1–10%) of a minority ion species (e.g., hydrogen in a deuterium plasma, or helium-3 in a deuterium-tritium plasma) is chosen so that its cyclotron frequency falls within the wave's frequency range. The minority ions absorb the wave energy efficiently and then collisionally transfer it to the bulk plasma. This is the most commonly used ICRH scenario.[2]
Second-harmonic heating: Waves at twice the cyclotron frequency of the majority species can heat that species directly, though the absorption is weaker and requires higher plasma temperatures to be effective.
Mode conversion: Under certain conditions, the fast magnetosonic wave converts to a short-wavelength ion Bernstein wave or kinetic Alfvén wave near the ion-ion hybrid resonance layer, depositing energy on electrons.
ICRH antennas are among the most challenging in-vessel components. They must operate within centimeters of the plasma edge, withstand intense heat loads, and maintain reliable RF coupling despite changes in plasma position and edge density. Modern antenna designs — such as the "ILA" (ITER-like antenna) tested on JET and the traveling-wave antenna concept — use arrays of current straps with Faraday screens to minimize impurity generation from RF sheath effects.[3]
ICRH has been deployed on JET (up to 6 MW), ASDEX Upgrade, Alcator C-Mod, WEST, and EAST. ITER will install 20 MW of ICRH power, using two port-plug antennas operating between 40 and 55 MHz. The system is designed for both hydrogen-minority heating during the non-active phase and helium-3-minority heating in D-T plasmas.[1]