Transport current and magnetization of Bi-2212 wires above liquid Helium temperature for cryogen-free applications

Since the discovery of high temperature superconductors, a possible cryogen-free scenario has always been wished. Nowadays, liquid Helium is running out, and it is likely that the cooling by will be a large part of the costs of any superconducting system. Bi-2212 wires at temperature higher than 4.2 K still show a very high irreversibility field and thus a deep investigation of their properties in such a range of temperature is very useful in order to assess the applicability in high field cryogen-free magnets. Here electrical transport and magnetic properties characterization at variable temperature and magnetic field on our “GDG—processed” wires are reported together with a well-described original approach to calculate the irreversibility field Hirr. This study is devoted to provide reference data on the behaviour of the only isotropic wire for high field application with an eye to the performances at temperatures above 4.2 K.

Abnormal behaviour of the resistive transition to the normal state in superconducting Nb-Ti tapes just below Hc2

The results of studying the resistive properties of Nb-Ti tapes in high magnetic fields close to the upper critical field (Hc2) are reported. The voltage-current curves (VCCs) and resistance-current cures (RCCs) show an “abnormal” hysteresis which is opposite in sign to a common thermal one caused by self-heating. Several features were noted: the existence of threshold current below which VCCs are reversible and the presence of excessive voltage noise in the hysteresis region. Earlier the similar behaviour was observed in granular superconductors. The nature of the “granular” properties of Nb-Ti tapes in high magnetic field and the relation with other observed effects, such as the irreversibility field and the apparent anisotropy of the upper critical field are discussed.

Inhomogeneity and irreversibility field of superconducting Nb-Ti tapes

Magnetizations and current-voltage characteristics of superconducting tapes from Nb-50% wt.Ti alloy are studied. In the region of external magnetic fields close to the upper critical field, there is a so called irreversibility field, which is often associated with the phase transition of vortex matter. However, in this study it is shown that the whole volume of experimental data can be described if we abandon the hypothesis of a phase transition and go over to the hypothesis of macroscopic inhomogeneity. This hypothesis implies the existence of two superconducting components with different upper critical fields, which naturally arise as a result of the production process.