K. Riße

Wendelstein 7-X Commissioning of Superconducting Magnet System

The Wendelstein 7-X stellarator (W7-X), one of the largest stellarator fusion experiments, is presently being taken into operation at the Max Planck Institute for Plasma Physics in Greifswald. The main objective of the experiment is to prove the reactor relevance of the optimized stellarator concept. The W7-X experiment has a superconducting magnet system with 50 nonplanar and 20 planar coils grouped in five equal modules, which are electrically connected in 7 circuits with 10 coils of each type. The connections between the coils are made by superconducting the bus bars using the same NbTi Cable-in-Conduit Conductor as used for the superconducting coils. Particularly developed high-temperature superconducting current leads feed the current into the cryostat by bridging the temperature gradient from room temperature down to the 4-K level. Seven power supplies provide individual currents in the seven circuits. The quench detection system permanently checks the superconducting system regarding the occurrence of a quench. In case of a quench, the magnet safety system has to be activated, and a set of switches lead the current into the dump resistors. The commissioning of the magnet system was successfully performed until July 2015 with tests of the complete magnet system functionality needed for plasma operation at a magnetic field of 2.5 T.

Commissioning Results of the Superconducting Magnet System of Wendelstein 7-X

The superconducting fusion device Wendelstein 7-X went into operation in December 2015. One of the most important steps was the commissioning of the superconducting magnet system. It consists of 70 superconducting coils, 14 high temperature superconductor (HTS) current leads, and more than 100 superconducting bus bars. The system is divided into seven electrical circuits with ten coils, associated bus bars, and two current leads each. The commissioning of the magnet system was performed in two major steps: In the first step, the seven coil circuits were separately energized and operated at several current levels in a range between 2 and 12.8 kA. In the second step, all seven circuits were operated together. The adjustment of the quench detectors, the evaluation of the thermal and mechanical behavior, and the test of the magnet safety system were further work packages. Fast discharges were initiated to check the proper behavior of the magnet safety system. Steady-state operation of up to 8 h was performed to adjust the helium mass flows. The commissioning was successfully completed, the main goal of the magnet system is fully reached, which is also confirmed by magnetic flux measurements and the results of the first plasma operation.

Analysis of the Interlayer Joint Resistance of the Superconducting Coils of W7-X

Abstract The superconducting coils of the Wendelstein 7-X (W7-X) stellarator use a cable-in-conduit conductor that is wound as double layers. These double layers are connected by low-ohmic joints to limit ohmic heating. All joints were equipped with voltage taps to allow identification of the double layer causing a quench. During the current tests of the superconducting coils, the differences between adjacent voltage taps were measured, and the joint resistances between the double layers were estimated. The cryogenic tests of the 50 nonplanar and 20 planar coils provided a unique opportunity to analyze the variation of the resistance of 250 joints of the nonplanar coils and of 40 joints of the planar coils. The statistical analysis shows that the resistance of most of the joints was well below the specified value of 1 nΩ.