The superconducting magnet system of the stellarator Wendelstein 7-X

The stellarator fusion experiment Wendelstein 7-X (W7-X) is presently under assembly at the Greifswald branch of the Max-Planck-Institut für Plasmaphysik (IPP), Germany. The superconducting magnet system consists of 50 non planar coils, 20 planar coils, a superconducting bus system and 14 current leads. It is organized in seven electrical circuits with 10 coils, the bus system for the interconnection and two current leads each. The magnet system is enclosed in a cryostat with an outer diameter of 16 meters formed by the plasma vessel and the outer vessel and cooled by supercritical helium. There are five different types of non planar coils having dimensions of 3.5×2.5×1.5 meters and a weight of about 5.5 tons. The two different types of the planar coils are nearly circular coils with a diameter of up to 4.5 meters and a weight of about 3 tons. The superconducting bus bar system connects the coils to each other and provides the connection to the current leads inside the cryostat. All types of coils and the bus system use the W7-X superconductor, a forced flow cable-in-conduit superconductors with 243 NbTi copper stabilized strands with an outer aluminum jacket. The current leads provide the transfer of the electrical current from the room temperature bus bar system outside the cryostat to the superconducting parts inside the cryostat. Their special feature is the upside down orientation with the cold end at the top.