Ion Cyclotron Resonant Heating

Cyclotron motion of a plasma ion around a magnetic field line

In magnetically confined plasmas, particles (ions and electrons) rotate around magnetic field lines with frequencies that depend only on three quantities: charge and mass of the particle, and magnetic field strength. Other parameters like temperature or density play no role at this ‘cyclotron’ frequency. Therefore, if an electromagnetic wave with cyclotron resonant frequency is launched into the plasma, all the targeted particles (defined by mass and charge) are heated, provided that the magnetic field complements the resonant condition. In tokamaks, the magnetic field decreases with distance from the tokamak major axis. This allocates the resonant region to a narrow vertical layer, thus giving us a simple control over deposition of the cyclotron resonant wave.

To accommodate complicated wave propagation rules, multiples of the base cyclotron frequency, called ‘higher harmonics’ are mostly applied in practice. The effect of higher harmonic resonance relies on space variations in the wave intensity, so that such a resonance is stronger for particles with larger orbits. That is, higher harmonic heating is more significant for fast particles than for slow particles, which introduces temperature dependancies as well as distortion in thermal distribution due to the heating.

Ion cyclotron resonant heating (ICRH) is routinely applied on JET. It is resonant with the second harmonic (i.e. double) frequency of ion gyration of main JET plasma ions (deuterium) or with a base frequency of gyration of a minority species (e.g. tritium, helium…). The available resonant frequencies at JET are in the range of 23-57 MHz (megahertz, or million of oscillations per seconds) which correspond to the length of the vacuum electromagnetic wave from 13m (at 23 MHz) down to 5m (at 57 MHz). This is a “shortwave” frequency, which is not very popular in the air due to many fades, blackouts and interferences (FM radio frequencies are just a bit higher, around 100 MHz). In total, the installed power of the JET ICRH system is as much as 32 MW (megawatts = million watts), and in practice only part of this potential is sufficient for the JET experiments. This is a huge power compared to radio or TV broadcast, where a 50 kW (kilowatts = thousand watts) transmitter is already considered powerful.