The next step in the Wendelstein stellarator line is the large superconducting device Wendelstein 7-X, currently under construction in Greifswald, Germany. Steady-state operation is an intrinsic feature of stellarators, and one key element of the Wendelstein 7-X mission is to demonstrate steady-state operation under plasma conditions relevant for a fusion power plant. Steady-state operation of a fusion device, on the one hand, requires the implementation of special technologies, giving rise to technical challenges during the design, fabrication and assembly of such a device. On the other hand, also the physics development of steady-state operation at high plasma performance poses a challenge and careful preparation. The electron cyclotron resonance heating system, diagnostics, experiment control and data acquisition are prepared for plasma operation lasting 30 min. This requires many new technological approaches for plasma heating and diagnostics as well as new concepts for experiment control and data acquisition.
Abstract-The stellarator fusion experimental device 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 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 meters formed by the plasma vessel and the outer vessel. There are five different types of non planar coils having dimensions of 3.5 x 2.5 x 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 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 superconductor with 243 copper stabilized NbTi strands with an outer aluminumalloy 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.
The Karlsruhe Institute of Technology (which is a merger of former Forschungszentrum Karlsruhe and Karlsruhe University) is responsible for the design, construction and testing of the high temperature superconductor (HTS) current leads for the stellarator Wendelstein 7-X (W7-X) which is presently under construction at the Greifswald branch of the Max-Planck-Institute for Plasma Physics. The current leads are of the binary type, the HTS part covering the temperature range between 4.5 K and 60 K while the heat exchanger covers the range between 60 K and room temperature being cooled by 50 K He. In total 2 prototypes and 14 series current leads are required with a nominal current of 14 kA and a maximum current of 18.2 kA. The paper describes the design and first test results of the prototype HTS current leads.Index Terms-Current leads, fusion magnets, high temperature superconductor, Wendelstein 7-X.
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