Experimental results of current distribution in Rutherford-type LHC cables

Current distribution among the wires of multi-strand superconducting cables is an important item for accelerator magnets. A non-uniform distribution could cause additional field distortions in the magnet bore and can as well be one of the reasons of premature quenching. Since two years electrical measurements on superconducting Rutherford-type cables are performed at CERN as part of the reception tests for the Large Hadron Collider (LHC). Cable samples of 2.4 m length are tested at currents up to 32 kA, temperatures around 1.9 and 4.3 K, and fields up to 10 T, applied perpendicularly as well as parallel to the broad face of the cable. Last year, an array of 24 Hall probes was installed in the test set-up in order to measure the self-field of the cable samples along one cable pitch. Each of the probes measures the local field generated by the current in the strands close by, and the results of the all probes reflect therefore the distribution of the strand currents. Experiments are done varying the applied field and its direction, the temperature, the cable current ramp-rate, the strand-to-strand contact resistance and the cable joint resistances. In many cases a current imbalance is observed at lower currents, which at higher currents leads to either a current redistribution or to a premature quench. The main aim of the paper is to show the possibilities and importance of this type of measurements, and to present first experiments and results. Many tests are performed to understand the qualitative trends, but are often quantitatively not directly representative for the behavior of the cable in real magnets. (12 refs).