Tokamak plasmas are basically symmetric around the major axis. They may have D-shaped rather than circular cross sections, but they still look the same from any direction. The figures here show that stellarators are far from symmetric. Instead of using the plasma current to shape the plasma, external coils are used, and these can produce shapes that cannot be formed by plasma current alone in a self-organized tokamak. It is precisely the lack of self-organization that gives stellarators their advantage [11]. Nonaxisymmetric shaping can be used to improve plasma stability, control ELMs, and eliminate disruptions. Indeed, the ELM coils being added to ITER to suppress ELM instabilities do so by spoiling the axisymmetry. In DEMO, the bootstrap current is relied on to supply at least 80% of the plasma current. This is extremely difficult to produce and control when self-organization is strong. In stellarators, a large plasma current is not at all necessary, since the rotational trans­form is generated by external coils.

In addition to their suitability for steady-state operation, stellarators have some unexpected advantages as reactors. Very small errors in the magnetic field (0.01%) have been found to cause problems with plasma confinement. Originally, stellara — tors’ problems were believed to be due to magnetic errors, but it has been found that once axisymmetry is broken, the wild shapes shown above are actually less sensi­tive to magnetic errors. Data from all stellarators have been found to follow a scal­ing law and fall on the same curve, as shown in Fig. 8.21 for tokamaks, so that extrapolation can be used to design larger machines. In addition, higher density and beta values have been achieved in stellarators. A purported benefit [11] of higher density is the formation of a MARFE (Multifaceted Asymmetric Radiation From the Edge), a “detached” layer which forms when plasma recombines before reach­ing the divertor. The energy is then radiated away before it reaches the divertor, sparing the divertor of the large heat load. The energy, however, has then to be taken up by the first wall. The advantages of stellarators come at a price: the dif­ficulty of making and assembling the weirdly shaped coils and vacuum chambers; but this technology has already been demonstrated.