In principle, what is done to the ions can also be done to the electrons, but the technology is entirely different. The electrons’ cyclotron frequency is in the giga­hertz range, and huge microwave generators are required. The power or current input can be deposited accurately at specific places inside the torus by adjusting the microwave frequency to match the magnetic field at those places. Since micro­waves are carried through waveguides, which are specially sized and shaped pipes, they can be injected through holes in the first wall and do not require an antenna inside the vacuum chamber. The bad news is that electron cyclotron waves cannot penetrate into the plasma from the outside of the torus. A property of these waves is that they must be injected from a high magnetic field into a lower magnetic field. Since the magnetic field is highest in the hole of the torus, the launching waveguide must be located in the cramped space also occupied by the central solenoid and the inside blankets. Waves at twice the cyclotron frequency, which also resonate with the electrons’ gyrations, can get in from the outer, weak-field side; but the higher frequency is more difficult to generate.

The electron cyclotron heating system in ITER calls for 20 MW of power at 170 GHz. This frequency corresponds to the cyclotron frequency at 6.0 T (60,000 G), high enough to cover ITER’s magnetic field of 5.5 T at the inside radius. Although we use microwaves in everyday life, 20 MW at 170 GHz is an entirely different mat­ter. A microwave oven puts out 1 kW at 2.45 GHz using a magnetron so small that we are not aware of its presence. Powerful microwaves are generated by gyrotrons, which work by running ECRH in reverse. In a gyrotron, an energetic electron beam is first produced. It is then injected into a magnetic field, so that the electrons undergo cyclotron gyrations. In doing so, they emit microwaves at harmonics of the cyclotron frequency which are then channeled into a waveguide leading to the toka — mak. The microwaves get their energies from the electron beam, which loses part of its energy. In experiment, the remaining energy is captured in a beam dump as heat. In advanced gyrotrons, the beam can, in principle, be re-injected so that its remain­ing energy can be re-used. Note that the electron beam in a gyrotron cannot be injected directly into a tokamak to heat it because the electrons cannot get through the magnetic field. In a gyrotron, the electrons are injected into the magnetic field from the ends of the field lines. A tokamak, of course, has no such ends; hence the need to convert kinetic energy into microwave radiation and then injecting the radia­tion instead of kinetic energy directly.

High-power gyrotron research began in St. Petersburg, Russia, decades ago. Those that can operate continuously for ITER are being developed in Japan, Germany, and the USA. So far, 1 MW at 170 GHz in a long pulse has been shown to be possible. Figure 9.24 shows the size of such a gyrotron. ITER will need 24 of these to produce the required ECRH power. Figure 9.25 shows a design of a 2-MW gyrotron with superconducting magnets.

Since the gyrotron has to be under vacuum and the waveguide is at atmo­spheric pressure, windows have to be used to isolate the waveguide from the vacuums at both ends. At present, the only material that can transmit the wave power at that frequency is synthetic diamond. Windows 10 cm (4 in.) in diameter have been made and tested for proper cooling. In a reactor, gyrotrons and their windows have to run continuously without failure for months or years between maintenance shutdowns. This constitutes a large step in engineering that has yet to be done.