The Magnetic Characterization of a MgB2 Superconducting Hollow Cylinder, at Variable Temperatures up to Tc

DC magnetic fields have been applied to a superconducting hollow MgB₂ cylinder of relatively large dimensions (d_ext/int = 48/44 mm, h = 31 mm), produced by reactive Mg liquid infiltration (RLI) process. Such a kind of full dense cylinder has no discontinuity in its shape, at difference with other HTS textured cylindrical objects, and it represents a prototype of a larger one that can be applied in many electromechanical applications, like bearing, or pure magnetic application, like shielding. The aim of the experiments is to verify its shielding ability at low fields and the maximum trapped fields, when higher field are applied. The measurements are performed at variable temperatures from 13 K up to T_c . The magnetic field values, measured by Hall probe located at the center of the cylinder, dependent on the various cooling regimes (either zero field cooling (ZFC) and field cooling (FC)) and on the presence of a background external magnetic field. The trapped fields dependence from the temperature and background field is reproduced by a simple model of the current distribution and with an analytical expression of J_c (B,T). It was possible also to describe the transition of the superconducting cylinder to the normal state, induced by flux jumping, on a base of a phenomenological model with two process having different time scale.