Progress on the conductor for the superconducting toroids of the ATLAS experiment

The production of the conductor for the superconducting toroids of the ATLAS experiment at LHC (CERN) is under way. The conductor is obtained by coextruding a big Rutherford cable with very pure aluminium. All toroids, one Barrel Toroid (BT) and two End Cap Toroids (ECT), require a conductor with a critical current of about 60 kA at 5 T, 4.2 K and a RRR in excess of 800 (at zero field), together with a very good bonding between cable and aluminum: 20 MPa for the BT and 15 MPa for the ECT. So far four long lengths, approximately 7 km out of the 90 km of the total need, have been successfully produced, thus demonstrating the feasibility of the process. The paper describes the results of the R&D phases and the results obtained in this first production. The critical current degradation due to cabling and coextrusion was found quite moderate (7-8% in total), despite the low copper content of the strands (Cu:NbTi=1.09-1.18). The bonding was also very good with values well in excess of the specification. A special control system based on continuous ultrasonic inspection was used in the coextrusion line to monitor the bonding quality in real time during production.