Stellarators are weird-looking fusion devices. The reason is that ions and electrons are better confined when the magnetic field is “twisting” around the centre-line of the device called the magnetic axis.

In tokamaks, the vacuum field generated by the evenly-positioned toroidal field coils is somewhat “flat”, as shown on the figure below.

Vacuum field from a set of 12 Toroidal Field (TF) coils in a tokamak. Field-lines are “flat”, i.e. not “twisting”.

In order to generate “twisting”, a strong toroidal plasma current (in red in the figure below) is induced by time-varying currents in coils that are stacked through the centre of the doughnut. The toroidal plasma current produces its own “poloidal” component and makes the field-lines “twist” around the centre of the doughnut (yellow lines in the figure below). The axisymmetry and the strong “twist” combine to make tokamaks very efficient at confining ions and electrons (low transport).

The need for induced currents however limits the duration of tokamak discharges to finite times (as long as 15 minutes in JET); there is always a maximum current that a centre-stack coil can be ramped-up to.

Stellarators do not induce currents, so much so that the challenges of plasma control and the deleterious effect of plasma instabilities are bypassed altogether. The question is then how to produce sufficient magnetic “twist” from outside (external coils). There is no other way than to break the harmonious axisymmetry of tokamaks and make the coils wrap tightly around it in strange shapes.

There are in fact two main ways to achieve high magnetic “twist”: 1) more turns in the helical coils yields a faster rotating plasma boundary , 2) non-planar shape of the centre-line yields “writhing” of magnetic field-lines.

High twist due to rotating plasma boundary, but keeping the centre-line flat.
High twist due to non-planar centre-line, but little rotating of the boundary.