Magnetic Instabilities in High Field Superconductors

In the process of magnetizing cylindrical specimens of a typical high field superconductor Nb-50 at.%Ti, flux jumps were induced by magnetic disturbances. The stability limit field H fj increased steadily with increasing temperature, and no magnetic instability occurred for temperatures in excess of about 6.5 K. The calculation of H fj was performed taking into account the cylindrical sample geometry and the critical state equation J c B 1-γ=α. According to the relative magnitudes of the magnetic diffusivity D m and the thermal diffusivity D t, the expression of H fj was derived for two cases; (1) D m>D t, and (2) D m ≪D t. Good agreement between experiment and theory was obtained on the stability limit field H fj and the temperature above which magnetic instabilities do not take place.