The stellarator concept, proposed as early as 1951 by Lyman Spitzer of Princeton University, was initially researched in experiments in the 1950s and 1960s in the USA and Europe. Its name suggests a ‘star machine’, producing energy from fusion. This star machine uses strong magnetic fields to confine the plasma in a torus-shaped vessel, so in this respect, it is similar to a tokamak.
The difference is that stellarators rely entirely on magnetic fields produced by external coils to produce their magnetic confinement, eliminating the need for a toroidal plasma current but requiring a more complex shape for the coils than in tokamaks. The concept is then intrinsically able to maintain the confinement configuration without the use of the current drive systems which are needed in tokamaks. Disruptions, instabilities and other plasma events associated with the free energy of a large (several million amp) toroidal current either do not occur or are strongly reduced.
Simulated magnetic fields for the Wendelstein 7-X Image: IPP
A stellarator is much more complex than a tokamak to design and build – only in relatively recent times have the theoretical understanding, computing power and fabrication techniques been developed to the level needed to construct large stellarators with a performance comparable to tokamaks. This line of investigation is now being pursued vigorously, since researchers have realised that plasma confinement without a large plasma current could have major long-term and economic benefits for the steady-state operation of a fusion power reactor, in particular, substantially increasing the plasma stability and thus simplifying control and monitoring of the burning fusion plasma.
Stellarators offer an intrinsic potential for steady-state, continuous operation, so research in this area is the prime contribution to concept improvement studies in Europe. The work also helps our understanding of the physics of plasma confinement systems. In Europe, two Stellarator projects have been built, TJII in Spain and W7-AS in Germany. TJII is the only stellarator currently in operation but a new larger stellarator, W7-X , is under construction in Germany.
W7-X magnets have a complex configuration Image: IPP