The superconducting magnet system of the stellarator Wendelstein 7-X

The stellarator fusion experimental device Wendelstein 7-X (W7-X) is presently under assembly at the Greifswald branch of the Max-Planck-Institut fur Plasmaphysik, Germany. The superconducting magnet system consists of 50 nonplanar coils, 20 planar coils, a superconducting bus system, and 14 current leads. It is organized in seven electrical circuits with ten coils, the bus system for the interconnection, and two current leads each. The magnet system is cooled by supercritical helium. It is enclosed in a cryostat with an outer diameter of 16 m formed by the plasma vessel and the outer vessel. There are five different types of nonplanar coils having dimensions of 3.5 × 2.5 × 1.5 meters in maximum 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 m 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 superconductor with 243 copper stabilized NbTi strands with an outer aluminum-alloy 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.