AC losses and critical current in an aluminum stabilized mixed matrix NbTi superconductor composite

AC losses were measured for a twisted (L = 1.57 cm) superconductor wire (0.16 cm O.D.) consisting of 54 NbTi tubular filaments which were filled with high purity aluminum stabilizer and imbedded in a high strength aluminum alloy matrix. The measurements were made at 4.2 ° K in bias fields of zero and five Tesla as a function of ac field amplitude and frequency (1 Hz kHz). AC field amplitudes which are less than necessary to fully penetrate the filaments were used throughout the experiment. At frequencies lower than about 10 Hz the losses are dominated by partial penetration hysteresis effects while at higher frequencies eddy current losses dominate. The eddy current losses were calculated using the anisotropic continuum model developed by Carr and co-workers and good agreement between theory and experiment was obtained over the entire frequency range from 10 Hz to 20 kHz. Critical current vs bias field measurements were also carried out. At ρ = 10^-11Ωcm the Jcin the NbTi was found to be 1.17 × 10⁹A/m²and 1.40 × 10⁹A/m²in fields of 5 T and 4 T respectively. At lower fields the sample quenched before showing a measurable resistive onset. The value of Jcmeasured at 5 T is in excellent agreement with that calculated from the partial penetration hysteresis losses observed at low frequencies.