Development of Aluminum-Stabilized Superconducting Cables for the Mu2e Detector Solenoid

The Mu2e experiment at Fermilab is designed to measure the rare process of direct muon-to-electron conversion in the field of a nucleus. The experiment comprises a system of three superconducting solenoids, which focus on secondary muons from the production target and transport them to an aluminum (Al) stopping target, while minimizing the associated background. The detector solenoid (DS) is the last magnet in the transport line, and its main functions are to provide a graded field in the region of the stopping target as well as a precision magnetic field in a volume that is large enough to house the tracker downstream of the stopping target. The DS coils are designed to be wound using NbTi Rutherford cables conformed to high purity Al for stabilization and then cold-worked for strength. Two types of Al-stabilized conductor are required to build the DS coils, i.e., one for the gradient section and one for the spectrometer section of the solenoid. The dimensions are optimized to generate the required field profile when the same current is transported in both conductors. The conductors contain NbTi Rutherford cables with 12 (DS1) and 8 (DS2) strands, respectively, and are manufactured by two different vendors. This paper describes the results of the manufacturing of production lengths of the Al-stabilized cables needed to build the Mu2e DS as well as the testing campaigns and main results. The main cable properties and results of electrical and mechanical tests are summarized and discussed for each stage of the cable development process. Results are compared with design values to show how the production cables satisfy all the design criteria starting from the NbTi wires to the Al-stabilized cables.