Test results for an advanced conductor for the Wendelstein 7-X magnet system

Wendelstein 7-X is a stellarator plasma experiment currently under construction in Greifswald, Germany. Its magnet system consists of 50 modular (non planar) superconducting main field coils and 20 superconducting planar auxiliary coils. In order to develop a windable conductor for the very complicated shape of the modular coils, a development program was initiated to find a conductor which is relatively soft during the winding process or, better, forming process of the winding. Afterwards, the winding block will be vacuum impregnated with filled epoxy resin and hardened by a heat treatment. The goal to find such a conductor is reached with a cable-in-conduit (CIC) conductor. The cable has 192 strands, and a jacket of an aluminum alloy was co-extruded. Several pieces of approx. 200 m length were produced by two companies and coiled to test solenoids. They were tested at the Forschungszentrum Karlsruhe in the test facility STAR (Superconducting Test Arrangement), where they were placed into the gap between two pairs of coils used as background field coils. The test results were very encouraging. However, in order to enhance the stability of the conductor for the W 7-X plasma experiment, the number of strands was raised to 243 and the Cu/SC ratio was enhanced from 2 to the range of 2.5 to 3.0. The results of this test for the so-called advanced conductor are reported here and compared to the earlier tests.