Research on closed magnetic bottles started with stellarators such as the figure-8 stellarator shown in Fig. 4.18. In 1969, the Model C Stellarator in Princeton, the largest at the time, was converted to a tokamak because of the good results coming from Russia with their configuration. Soon almost all new machines were tokamaks. This was because of the self-healing properties of tokamaks, as described in Chap. 7. When the temperature profile in the plasma became too peaked, sawtooth oscillations would arise and smooth it out to maintain stability. All this is now changed, and stellarators have come back as a hope for the future.

The difference between these two toroidal devices, tokamaks and stellarators, is the way the poloidal magnetic field (the component that gives the field lines their helical twists) is generated. In tokamaks, a large current in the plasma generates that field. In stellarators, external coils generate that field, and no large plasma cur­rent is necessary. But these external poloidal-field coils are hard to make. Present — day Advanced Tokamaks no longer rely on the self-healing features that were initially useful. We have learned how to shape the plasma current with radiofre­quency and microwave power to keep the plasma stable. In fact, sawtooth oscilla­tions are now deliberately eliminated. Since self-organization is no longer necessary, we can reconsider stellarators. Stellarators are less subject to effects such as disruptions that are connected with the large plasma current. In effect, they elimi­nate a source of energy that allows a plasma to self-organize destructively in its efforts to escape from confinement. Furthermore, since transformer action to drive the plasma current is not necessary, stellarators are more suitable for steady-state, continuous operation.