Tunable coaxial cavity resonator for linear and nonlinear microwave characterization of superconducting wires

We discuss experimental results obtained using a tunable cylindrical coaxial cavity resonator constituted by an outer Cu cylinder and an inner bulk Bi_1.8Pb_0.26Sr_2Ca_2Cu_3O_10+x cylinder. We have used this device to investigate the microwave response of the superconducting wire, both in the linear and nonlinear regimes. In particular, by tuning the different modes of the cavity to make them resonant at exactly harmonic frequencies, we have detected the power emitted by the superconductor at the second- and third-harmonic frequency of the driving field. The results obtained in the nonlinear regime, whether for the microwave surface impedance or the harmonic emission, are discussed in the framework of the models reported in the literature for granular superconductors. The use of this kind of device could be of great interest to investigate and characterize wires of large dimensions, to be used for implementing superconducting-based microwave devices.