The current leads of 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 (IPP), Germany. The W7-X superconducting magnet system basically consists of 50 non planar and 20 planar coils including the interconnecting bus bars, and the support structure. The seven electrical circuits with ten coils each in series, are supplied by 14 current leads (CL) operating between the cryogenic and ambient temperature environments. A special feature of these feed troughs 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 current lead consists of a copper conductor which 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 which is separated from the main W7-X cryostat vacuum. The high voltage electrical insulation is built up of a glass tape epoxy resin system. Mechanical support of the current leads is achieved by a warm and cold glass fiber reinforced plastic flange. There are three He cooling circuits: one for the bus bar and contact cooling, one for the cold end of the CL (both at about 5 K), and one for the CL's heat exchanger (entrance temperature about 50 K). After intensive tests of two prototypes the series production has been established and completed. The current leads were tested at room temperature, and with electrical currents up to the maximum current at cryogenic temperatures.