Magnet coils

The hot plasma is confined in a magnetic cage composed of specially shaped magnet coils.

The sophisticatedly shaped coils for stellarator devices are fashioned somewhat differently from the pancake coils for tokamaks. Instead of rigid copper rails one uses here more flexible copper strands embedded in winding forms. Mechanical strength is provided by fibre-glass bands and synthetic resin.

One of the 50 non-planar magnet coils of Wendelstein 7-X – about 3.5 metres high and weighing six tons – is being strung onto the plasma vessel.

The Wendelstein 7-X stellarator of IPP is equipped with superconducting coils made of niobium-titanium. They are to produce a field of 6 tesla (on the coils, i.e. 3 tesla on the magnetic field axis.). The superconducting material is embedded as thin strands in copper wires braided to form a cable. Liquid helium for cooling to 4 kelvin flows between the individual wires through the cable cavities. For this purpose the cable is enclosed by a helium-tight aluminium sheath.

During the fabrication and the winding process the sheath material is soft and flexible and can be annealed afterwards. This – together with fibre-glass and synthetic resin reinforcement – also guarantees the necessary mechanical strength. It is thus possible to make direct use of the experience in winding the copper coils gained from Wendelstein 7-AS, the predecessor experiment.

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