As a typical example to see the effects of thermally-activated flux creep at liquid-Nitrogen temperatures on the superconducting apparatus using high Tc superconductors, the decrease of the transport current in the persistent-current mode is discussed for a multi-layer superconducting solenoid with an infinite length wound by a superconducting tape. It is pointed out that the normalized decay rate of the persistent current is much smaller than that of the magnetization of the superconducting film inside the tape, so far as the initial creep process is concerned. The ratio between the above two kinds of normalized decay rates is roughly given by η, which is defined as the thickness of the superconducting film to the bore radius of the solenoid. The period of the initial creep process, during which the total flux inside the solenoid is conserved, increases exponentially with the increase of the effective pinning potential, Uc. As a numerical example, the normalized decay rate of the persistent current is increased to the order of 10-4% at 77K after one year for the superconducting tape with Uc-0.6eV at the set-up flux density inside the solenoid, when the thickness of the superconducting film is chosen as 10μm for winding the solenoid with the bore radius of 0.5m. On the other hand, the period of the initial creep process at 77K is of the order of 10 hours for Uc-0.2eV, while it becomes to be of the order of 1010 years for Uc-0.6eV. After the initial creep process is over, the normalized decay rate of the persistent current becomes of the same order as that of the magnetization of the superconducting film. Thus, we may need a further effort to increase the value of Uc of superconducting wires in high fields at 77K.
In order to find a guiding principle for increasing the value of Uc, a theory of flux creep developed recently is reviewed briefly. Some features of the theory are: (1) averaged volume of flux bundles, Vb, characterizing the thermal hopping motion of pinned fluxoids is related to observable physical quantities through the correlation lengths of the pinned fluxoids, (2) the effective pinning potential for the flux creep, Uc, is given by Vb times a half of the effective pinning potential per unit volume, Up, the gradient of which gives the macroscopic pinning force density, Fp=JcB, where Jc is the critical current density and B is the flux density, and (3) the B dependence of Uc is given by Uc(B)∝Jc(B)1/2B-1/4 for strongly pinning samples. It is shown that this theory can explain quantitatively the observed data of the decay rate of magnetization for conventional superconductors as well as for strongly pinning high Tc superconductors.
To decrease the effects of flux creep on the superconducting apparatus operating at liquid-Nitrogen temperature, the present reviews suggests that we need a further effort for developing the superconducting wires with a high critical transport current density as well as with a thin film thickness or small filament diameters.
