Sixty percent of the plasma exhaust is designed to go into the “divertor,” thus sparing the first wall from the major part of the heat load. Materials and cooling methods can be used in the divertor that cannot be used for the first wall. Figure 9.4 shows how this is done. Special coils located at the bottom of the chamber bend the outermost field lines so that they leave the main volume and enter the divertor. Plasma tends to follow the field lines, so that most of it leaves the chamber by striking the surfaces of the divertor rather than the first wall. Only

the plasma that migrates across the magnetic field hits the first wall. The heat load on the first wall can be larger than average when there is an instability such as an ELM or a disruption that takes plasma across the field lines suddenly. The first wall of ITER will have to withstand such heat pulses, but DEMO must be built to avoid such catastrophes.

As can be seen in the diagram, the boundary layer of diverted field lines is very thin, only about 6 cm in ITER. In the divertor, these field lines are spread out over a larger area, and the surfaces which the plasma strikes are inclined almost parallel to the field lines so that the heat is deposited over as large a surface as possible. Tungsten can be used for these surfaces, and even carbon compounds can be used in spite of their tritium retention. The divertor parts are easier to replace than the first wall, so the tritium can be removed periodically. The heat load on the divertor surfaces is huge, some 20 MW/m2, so the cooling system is an important part of the design. Water cooling is possible in ITER, but helium cooling at higher temperatures would have to be used in DEMO and FPPs. The conditions inside a divertor are so intense that they are hard to imagine. Ions with tens of keV energy stream in along the field lines, accompanied by electrons that neutralize their charges. When the ions meet a solid surface, they recombine with electrons to form neutral atoms. There is a dense mixture of plasma with neutral gas made of deuterium, tritium, helium, and impuri­ties, which later have to be separated out in an exhaust processing unit. The neutral gas has to be pumped away fast by vacuum pumps before it flows back into the main chamber and gets ionized again into ions and electrons. To trap the neutrals inside the divertor, a dome-shaped structure has to be added. Figure 9.5 shows the main parts of a divertor designed for ITER. The plasma impinges at a glancing angle onto the high-temperature surfaces made of tungsten and CFC. A heat-sink material, CuCrZr, transfers the heat to water-cooled surfaces.

Water cooling, which is limited to about 170°C, would be insufficient for DEMO and FPP, and cooling by helium gas would have to be used. The helium

would be injected at 540°C and be heated to 720°C, while the tungsten and CFC tiles would get to 2,500°C [3]. The coolant would be injected under pressure to cool a small dome as illustrated in Fig. 9.6. These domes are then assembled into nine-finger units, and these units then form a uniformly cooled surface.

Divertor technology is in better shape than other problem areas because divertors are small, and they have already been extensively tested. For instance, meter-sized tungsten and CFC divertor segments (Fig. 9.7) have been tested in Karlsruhe, Germany, up to heat fluxes of 20 MW/m2. In that large laboratory, divertor materials

have been neutron-irradiated, and their manufacturing and assembly techniques have been worked out. Even remote handing techniques for replacing divertors have been tested. It seems possible to design water-cooled divertors for heat fluxes up to 20 MW/m2 and helium-cooled divertors up to 15 MW/m2 [31].