Theoretical Investigation Of Magnetic Flux Jumps And AC Losses In High-Temperature Superconductors Under Consideration Of Nom-Uniform Distribution Of Critical Current Density

As we all know, flux jump instability at the extreme low temperature and energylosses in alternating conditions in the high-temperature superconductors (HTSC) arefundamental subjects in application of superconductors, which are associated directlywith the security and the stability of the superconducting devices and systems, thusthey have been paid more attention in both theoretical and application research. Dueto that the effect of non-uniform distribution of flux-pinning potential andcritical-current density resulting from many types of lattice defects during thepreparation process of HTSC, flux jump instability and AC losses are arisen to be thekey issue in the design and optimization of the superconducting devices and systems.Based on the assumption of local adiabatic conditions and dynamic process ofmagneto-thermal interaction, this dissertation focuses on investigating the effects ofnon-uniform critical-current density distribution on flux jumps and AC losses inHTSC including thin film and many interesting theoretical results are obtained.Firstly, the effects of non-uniform distributions of critical-current density on fluxjumps in HTSC are discussed. From the point of view of the dynamic process ofmagneto-thermal interaction, the ambient temperature and sweep rate of the externalmagnetic-field are taken into account to develop the theoretical models of the effect ofdistribution of non-uniform critical-current density on flux jumps by solvingpredicting equations for predicting the first flux-jump field. In the framework of thecritical state model, the influences of the thermal distribution and temperatureresponse are considered to present the analytic expression of flux jumps of HTSC. Itis found that the predictions from the proposed models are good agreement with theexisting experimental data and numerical results. The theoretical results display thatthe flux jumps are seriously dependent on the effect of non-uniform critical-currentdensity distribution, which can suppress the occurrence of flux jumps, and the effectbecome more and more remarkabe with the increase of theparameterγ=J_cw/J_cs,which is characterized as the discrepancy of differentcritical-current density or pinning potential regions.Secondly, for the transport AC losses in superconducting cylinder, the effects ofnon-uniform distributions of critical-current density on the transport AC losses arediscussed theoretically by considering the self-field effects on the basis of Norrisequations. It can be found from the calculations that the non-uniform critical-currentdensity distribution increase from center outwards along radius could reduce thelosses efficiently. When the normalized transport current is relatively high or approach the critical current, the transport losses are affected by the effects ofnon-uniform critical-current density distribution, meanwhile the normalized transportcurrent is relative small the effects usually could be neglected. For the variation oflosses performance, the distribution of critical-current density increase linearly andquadratic from center outwards are different from critical-current density increasestepwise from center outwards. In particular, the distribution of critical-currentdensity around the cross-section edge of superconducting cylinder is comparablyimportant to reduce the transport losses.Finally, the effects of two-dimension superconductivity and distributioncharacteristic of magnetic field and current density are considered in superconductingthin film so as to predict the influence of non-uniform distributions of critical-currentdensity on AC losses. It can be noted from the theoretical calculations that there areessentially differences between the slab and thin film for the response to the externalmagnetic field and transport current. The slab and thin film are linearly and strongnon-linearly penetrated by field, respectively. In slab the current flow withcritical-current density in critical region and no current flow in field-free region, inthin film the current flow with critical-current density in critical region and the currentchanges continuously in field-free region, in which the current is not zero. The effectof non-uniform critical-current density distribution on transport losses in filmqualitatively is accord with other superconducting geometries. In addition, whenmagnetic field is relatively high, the effect of non-uniform critical-current densitydistribution on magnetization losses gradually disappears.After all, the theoretical investigations of the effect of non-uniform critical-currentdensity distribution on flux jumps and AC losses of HTSC materials in this papermust be important and useful for the precise measurements and exact predictions ofthe materials properties of HTSC materials as well as the design and application of thesuperconducting devices and thin film.