Study On Suppression Of Flux Avalanche Behaviors For YBCO Superconducting Thin Films

Superconducting electronics based on Josephson effect has become one of the frontier directions of superconducting applications,The successful development of superconducting quantum interferometers,superconducting filters,resonators and couplers has strongly advanced the development of information and communication fields.By using epitaxial growth and biaxial substrate technology,it is possible to create high-quality YBa₂Cu₃O_7-x（YBCO）superconducting films with high critical temperatures and currents,which are the first choice for high-temperature superconducting electronics applications.For YBCO superconducting materials,when the external magnetic field is higher than its lower critical field value,the magnetic field enters the interior of the superconductor in the form of flux quanta.If the flux is subjected to equal Lorentz force and pinning force,the flux inside the superconductor has a sub-stable distribution,also called the critical state,and a small perturbation like a laser hot spot or magnetic field variations can cause the flux avalanche and reach a new equilibrium state.Flux avalanche is the kinetic behavior of rapid penetration of magnetic flux into the interior of the material caused by the positive feedback of magnetic thermal instability within the superconductor.Once the flux avalanche occurs,it can cause the destruction of the electromagnetic properties of the superconducting material or even cause irreversible damage to the superconducting material.The problem of how to successfully suppress the flux avalanche behavior in superconducting thin films has thus become a persistent problem in the field of superconducting electronics.In recent years,researchers have found that the deposition of metals with excellent electrical and thermal conductivity in superconducting thin films can significantly suppress the occurrence of flux avalanche behavior,so that two mechanisms of flux avalanche behavior suppression emerge,namely,eddy current suppression and thermal conduction suppression in the metal layer.Until now,there is no effective way to separate these two suppression mechanisms,so that there is no quantitative analysis pathway for either eddy current suppression or heat conduction suppression.In this master’s thesis,a single eddy current suppression flux avalanche behavior was achieved by setting up an insulation layer on the surface of YBCO superconducting thin film and then depositing a metal layer.In particular,the flux avalanche velocity characteristics before and after vortex suppression were systematically investigated by the constructed double-exposure optical path,and it was found that the vortex effect can significantly reduce the flux avalanche velocity of superconducting materials.In addition,for the existing theoretical model of flux motion with infinite boundary and without considering the velocity of flux motion,a new theoretical model is established to effectively predict the trajectory of flux motion on the one hand,and the predicted velocity suppression results agree with the experiment on the other hand.The main results as following1.An in-situ online optical observation system for laser-triggered flux avalanche behavior has been constructed,and a systematic study on the suppression of flux avalanche behavior of YBCO superconducting thin films by Ag metal layers has been carried out.The avalanche characteristics were quantified and analyzed by calculating the area and fractal dimension of the flux avalanche region,and the variation of the flux avalanche characteristics of YBCO superconducting films with the thickness of the metal coating was investigated.2.The eddy current effect in metal plating was separated by setting up a heat insulation layer,and the suppression of flux avalanche in YBCO films by both eddy current effect and heat conduction was studied in comparison.The results show that both eddy current effect and heat conduction can effectively suppress the flux avalanche behavior,and the eddy current effect is significantly effective in suppressing the flux avalanche of superconducting films at lower magnetic fields under the same sample conditions.3.The classical magneto-optical imaging method is combined with the double exposure method to build an in-situ measurement system for the velocity of flux avalanche behavior of superconducting materials,and the flux avalanche velocity of YBCO superconducting films at single eddy current suppression is measured experimentally and compared with that of the initial YBCO films.The results show that the presence of the eddy current effect causes a sharp reduction in the rate of flux collapse,with a reduction amplitude of about 70%.4.A theoretical model of the suppression effect of vortice effect in coating-metal on flux avalanche behavior is established.The superconducting vortex or flux is simplified to a magnetic monopole,and the problem that the existing model is only for semi-infinite plane is solved by Schwaz-Christoffel（SC）transformation,while the equation of motion of the superconducting flux is established considering its velocity of motion,and the trajectory and force characteristics of the magnetic monopole are given analytically.The results show that the results predicted by using the model are in good agreement with the experiment...