Edge-localized modes (ELMs) were described in Chap. 8. They are instabilities of the H-mode pedestal which can release plasma suddenly to the wall. Although most of these particles should flow to the divertor, the sudden burst of heat can erode and damage the divertor’s surfaces. The H-mode pedestal constrains one-third of the plasma’s energy, and 20% of this or as much as 20 MJ can be dumped into the divertor in a fraction of a second [16]. The preferred method to suppress ELMs is to impose a rippled magnetic field at the surface of the plasma, near the pedestal. The idea is to break up instabilities that tend to be aligned with the magnetic field. The pattern of currents in the ELM coils can be varied slowly to follow changes in the magnetic field lines. This method has been tested in the DIII-D tokamak in San Diego, California, and thorough calculations have been made to design the sizes and spacings of the coils for ITER [17]. A panel of ELM coils is shown in Fig. 9.27. Figure 9.28 shows what the surface of ITER will look like with these coils installed. It will take 2.6 MW of power to drive these coils. Being in-vessel components, the coils have to withstand intense heat and neutron bombardment. In ITER, the coils are protected from the plasma by a 50-cm thick, water-cooled, nonbreeding blanket whose only function is to attenuate the neutrons.4

In DEMO, there would be no place for ELM coils, since breeding blankets have to cover the machine to capture as many neutrons as possible. Locating the coils behind the blanket would probably be too far. ELM coils are ad hoc, temporary

solutions not included in the original design of ITER since the problem had not yet arisen. The physics of ELMs has to be understood better to find passive methods for their control, but there is time to do this.

Once the ELM coils have been installed, they can also be used for other purposes. By applying a small current at a low frequency like 50 Hz, a weak insta­bility called the RWM can be controlled. A differently spaced DC current can also be added to help prevent disruptions (described in detail in the next section).