This method heats ions by pushing them with a rotating electric field whose direction follows the ions’ cyclotron motion as they revolve in their nearly circular Larmor orbits. It is sometimes more efficient to heat a minority species, such as helium-3 rather than deuterium or tritium, because of the way the energy is coupled into the plasma. The cyclotron frequency depends on the magnetic field strength, so the applied electric field has to be of a specific frequency, depending on magnetic field at the location where the ions are to be heated. In ITER, this frequency is in the range around 50 MHz. This is too low a frequency to be transmitted through a pipe, so an antenna has to be placed inside the vacuum chamber. The antenna is outside the field lines leading to the divertor (see Fig. 9.4), but it is so close to the plasma that it will be bombarded by ions. These ions will sputter antenna material into the plasma, and such contamination usually cools the plasma. ITER is to have 20 MW of ion cyclotron heating. The power is not the main problem; the problem here is to design antennas which will not affect the plasma deleteriously.