n-Values of commercial YBCO tapes before and after irradiation by fast neutrons

Abstract

The n-value is an important superconducting parameter, which represents the homogeneity of characterized superconductor as well as thermally activated depinning. In addition n-values are important for the evaluation of pinning mechanisms and pinning forces. n-values are crucial input parameters for the numerical simulations of superconducting tapes, coils and other complicated superconducting applications where E-J power law applies. In this publication, complex measurement data of n-values from different 2nd generation of high temperature superconducting (2G HTS) tapes are presented and analysed. In addition, 2G HTS tapes were step by step irradiated by fast neutron fluences up to 1x1022 m-2 . n-values of the irradiated tapes, containing additional randomly distributed pinning centres, are presented, analysed and compared with unirradiated samples. Special attention is placed on the underlying physics resulting in power-law part of the I-V curve and on the correlation between critical currents and n-values. The measurements are performed within the temperature range of 50 K-85 K and magnetic fields up to 15 T.