Current Leads for the Wendelstein 7-X Superconducting Magnet System

The stellarator fusion experiment Wendelstein 7-X (W7-X) is presently under assembly at the Greifswald branch of the Max Planck Institute for Plasma Physics, Germany. The W7-X superconducting magnet system basically consists of 50 nonplanar and 20 planar coils, including the interconnecting bus bars, and the support structure. The seven electrical circuits with 10 coils each in a series are supplied by 14 current leads (CLs) operating between the cryogenic and ambient temperature environments. A special feature of these feedthroughs is their upside-down orientation to save space in the vicinity of the machine. Basic electrical CL requirements are maximal steady-state currents of 18.2 kA and voltage strengths of 13 kV to ground. A W7-X CL consists of a copper conductor that also acts as a heat exchanger at the warm end side, in its continuation of a high-temperature superconductor part, and at the cold end side of a copper bar with integrated Nb3Sn rods. All components are fully contained within a CL vacuum chamber that is separated from the main W7-X cryostat vacuum. The high-voltage (HV) electrical insulation is built up of a glass tape epoxy resin system. Mechanical support of the CLs is achieved by a warm and cold glass fiber reinforced plastic flange. There are three He cooling circuits: 1) one for the bus bar and contact cooling; 2) one for the cold end of the CL (both at about 5 K); and 3) one for the CLs heat exchanger (entrance temperature about 50 K). After intensive tests of two prototypes, the series production has been established and completed. The CLs were tested at room temperature (HV, helium leaks, instrumentation, etc.) and with electrical currents up to the maximum current at cryogenic temperatures. This paper gives an overview of the basic CL design requirements and layout as well as on the fabrication and acceptance tests. Furthermore, a description of the assembly progress is given.