In a stellarator the magnetic cage is produced with
a single coil system – without a longitudinal net-current in the
plasma and hence without a transformer. This makes stellarators suitable
for continuous operation, whereas tokamaks without auxiliary facilities
operate in pulsed mode.
Dispensing with the ring-shaped plasma current means, however,
abandoning the axial symmetry present in tokamaks. As the helical twisting
of field lines is achieved solely with external coils, the later have
to be twisted accordingly: the magnet coils and plasma have a complicated
shape. This, however, affords additional freedoms in shaping the magnetic
field and making it properties accessible to optimisation.
In order to overcome the deficiencies of previous stellarators,
IPP conducted a systematic search for the optimum magnetic field. For
more than ten years the Stellarator Theory Group investigated the wide
area of possible stellarator configurations. The outcome is the optimised
magnetic field of Wendelstein 7-X: The quality of plasma equilibrium and
confinement will be on a par with that of a tokamak.
For a fusion power plant stellarators could provide a technically
simpler solution than what tokamaks might achieve. This question cannot
be answered by theoretical means; clarifying it experimentally is the
objective of the Wendelstein experiments at IPP.