A tokamak provides the required twist to the magnetic field lines, not by manipulating the field with external currents, but by driving a current through the plasma itself.

The field lines around the plasma current combine with the toroidal field to produce helical field lines, which wrap around the torus in both directions.

Although they also have a toroidal magnetic field topology, stellarators differ from tokamaks in that they are not azimuthally symmetric. Instead, they have a discrete rotational symmetry, often fivefold, like a regular pentagon.

It is generally argued that the development of stellarators is less advanced than that of tokamaks, although the intrinsic stability they provide has been sufficient for active development of this concept. The three-dimensional nature of the field, plasma and vessel makes it much more difficult to carry out either theoretical or experimental diagnostics with stellarators.

Even harder to design is the divertor – the section of wall that receives the exhaust power from the plasma in a stellarator.

The out-of-plane magnetic coils, commonly found in many modern stellarators, and possibly all future ones, are also much harder to manufacture than the simple, planar magnetic coils which suffice for a tokamak, and the utilization of the magnetic field volume and strength is generally poorer than in tokamaks.

Unlike tokamaks, however, stellarators do not require a toroidal current, so that the expense and complexity of current drive and the loss of availability and periodic stresses of pulsed operation can be avoided.

Additionally, there is no risk of toroidal current disruptions. It might be possible to use these additional degrees of design freedom to optimize a stellarator in ways that are not possible with tokamaks.