The structure composed of single coils allows the magnetic field to be shaped in detail. A great deal of theory and computation effort was invested to optimise the magnetic field for Wendelstein 7-X so as to overcome the disadvantages of previous classical stellarators. Its predecessor, Wendelstein 7-AS (1988 - 2002), the first device of this new generation of Advanced Stellarators, had already subjected elements of the concept to first experimental testing.
The further developed successor, Wendelstein 7-X, is now to investigate the new stellarator’s suitability for a power plant. It is expected that plasma equilibrium and confinement will be of a quality comparable to that of a tokamak of the same size. But it will avoid the disadvantages of a large current flowing in a tokamak plasma: With plasma discharges lasting up to 30 minutes, Wendelstein 7-X is to demonstrate the essential stellarator property, viz. continuous operation.
In November 2011 the interiour of Wendelstein 7-X was still open: Visible was the plasma vessel, one of the stellarator coils, a planar coil, part of the support structure and the cryostat together with a lot of cooling pipes and power supply lines.
Photo: IPP, Wolfgang Filser
A stellarator is a device used to confine a hot plasma with magnetic fields in order to sustain a controlled nuclear fusion reaction. It is one of the earliest controlled fusion devices, first invented by Lyman Spitzer in 1950 and built the next year at what later became the Princeton Plasma Physics Laboratory. The name refers to the possibility of harnessing the power source of the sun, a stellar object.
Stellarators were popular in the 1950s and 60s, but the much better results from tokamak designs led to them falling from favor in the 1970s. More recently, in the 1990s, problems with the tokamak concept have led to renewed interest in the stellarator design, and a number of new devices have been built. Some important modern stellarator experiments are Wendelstein 7-X, in Germany, the Helically Symmetric Experiment (HSX) in USA and the Large Helical Device, in Japan.
Stellarators, unlike tokamaks, do not require a toroidal current, so that the expense and complexity of current drive and/or the loss of availability and periodic stresses of pulsed operation can be avoided, and 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.
SOURCES - wikipedia