SIMPLE NUMERICAL-MODEL TO INTERPRET THE AC MEASUREMENTS ON TYPE-II SUPERCONDUCTORS

Classical a.c. magnetic measurements on superconductors, like a.c. susceptibility, magnetization cycles, shielded and trapped magnetic fluxes, are strongly related to the magnetic flux penetration inside the superconductors. In order to interpret the measurements performed on a wide class of superconductors (niobium, high critical temperature superconductors and multi-phase materials), we developed a simple numerical model based on the critical state model. As its main feature, the model takes into account the equilibrium magnetization as well as the bulk irreversible magnetization. The presence of irreversible surface currents is further included in the model. The transport critical current density J(c) is assumed to be a function of the local magnetic field B(i) according to different models depending on the superconductor properties and on the magnetic field amplitude. The model gives the flux profiles inside a superconducting infinite slab or cylinder placed in an a.c. + d.c. magnetic field. The model is used to explain the experimental results of a.c. magnetic measurements on samples of different materials.