Superconductor for the coils of the modular stellarator Wendelstein 7-X

The magnet system of the planned stellarator Wendelstein 7-X (W 7-X) consists of 50 nonplanar and 20 planar coils. Nonplanar coils require a winding process with compound bending (simultaneous bending in two planes). The use of a stiff conductor would lead to technical difficulties and requires very expensive tools for the winding of these coils. Therefore a conductor was industrially developed which is relatively soft and therefore of good workability during the winding process. Once the winding is completed, a heat treatment gives the conductor a final strength being high enough to withstand all mechanical loads which might be expected during the operation of W 7-X. The conductor consists of a four stage cable with 192 superconducting strands and a special Al-alloy jacket extruded around the cable. Properties of this conductor and test results attained up to now are given