The reason that a toroidal magnetic bottle has to have twisted field lines is that ions and electrons drift vertically in opposite directions, as explained in Chap. 4. This drift arises from the fact that the magnetic field is necessarily weaker on the outside of the torus than on the inside, near the hole in the doughnut. An obvious idea to get a larger volume of plasma without changing the drifts is simply to make the tokamak taller, without changing its radius. This is shown in Fig. 7.11a. The sharp corners have very bad curvature, so they have to be rounded off. A machine built at General Atomics in San Diego, the Doublet, is shown in Fig. 7.11b. This looks like two merged tokamaks, one on top of the other, connected by a region with good curvature. The bean-shaped cross section studied at Princeton University has good curvature on the inside of the torus and shown in Fig. 7.11c [4]. It turns out that it is not necessary to curve the inside surface; keeping it straight is almost as

good, since the magnetic field naturally gets stronger as the plasma tries to escape toward the inside of the doughnut. We then have the D-shape shown in Fig. 7.11d. The outside of the D still has bad curvature, but it curves more gently than in a circular tokamak because of the elongation. Figure 7.12 is a D-shaped toroidal-field coil shown during the construction of the ASDEX tokamak in Germany. This was one of the first large tokamaks of the time (ca. 1980) but is small compared with those operating today.

The D-shape is not all gravy: the bad curvature at the corners of the D is very sharp, but at least it occurs in only a small part of the total surface. Actually, this part of the D can be used for a necessary function — that of plasma exhaust. A product of D-T fusion is helium (alpha particles). This “ash” has to be taken out since confining it would use up the magnetic confinement capability reserved for the DT. Furthermore, the normal escape of DT plasma, though slow, still carries out

Fig. 7.12 A D-shaped ASDEX coil more heat than the walls of the chamber can stand. By channeling the escaping plasma into the corners of the D, special devices called divertors can be placed there to handle the heavy heat load. Figure 7.13 shows a diagram of the cross section of a D-shaped tokamak with divertors. The last closed magnetic surface is changed with locally placed coils so that the field lines leave the surface and lead outwards into the divertor. Plasma diffusing to that surface then enters the divertor, where it is captured by high-temperature, rapidly cooled materials.