Design, Production and QA Test Results of the NbTi CIC Conductors for the W7-X Magnet System

The magnet system of the W7-X Stellarator consists of 50 non-planar and 20 planar coils. The superconductors for both types have the same design, which is a Cable-in-Conduit (CIC) conductor of 243 NbTi strands. Namely the non-planar coils asked for a conductor, which can be easily wound to a complex shape, but has high mechanical strength to resist the large magnetic forces acting during magnet operation. This has been achieved by jacketing the cable into an Aluminium conduit by a co-extrusion process. The used 6063 Aluminium alloy is very soft in the extruded state, but gets high strength properties after winding during a precipitation hardening treatment at about 160°C. The production of the needed 390 conductor lengths (including spares) and the related QA tests are nearly completed and a large number of coils are fabricated. Some of them were already subjected to a cold test at CEA Saclay, where the conductor behaved as expected from short sample measurements.