A third type of wave that can be used for heating and current drive is the so-called lower — hybrid wave. This wave is particularly useful for current drive because it can control the current profile near the outside of the plasma. The lower-hybrid frequency lies between the cyclotron frequencies of the ions and electrons, or about 5 GHz in ITER. Klystrons are used to generate frequencies in this range. The wave has a long wavelength in the direction of the magnetic field, so to launch it requires a large “grill,” meters in size, as shown in Fig. 9.26. Each of the openings is a waveguide fed by one or several klystrons, each with its own vacuum window. The phase of the wave emanating from each wave­guide is set so that the total grill, including some dummy waveguides, forms the wave that deposits its energy in the right place. Since the launcher lies close to the plasma surface, its materials must sustain the heat and neutron damage that that implies.

In summary, the physics of auxiliary heating and current drive is well under­stood, but the engineering of the power supplies and the wave launchers present some difficult problems.