Study Of Guidance Force Charactericstics Of YBCO Bulk Exposed To Time_vary External Magnetic Field

Since the first discovery of high-temperature superconductors(HTSs) in 1986, Many scientists on the world have expressed a great interesting in it and done a lot of research.So the performance of the HTS is improved constantly,and it makes its application in engineering became easier.Because the high-temperature materials can operate at temperatures within the cooling capabilities of liquid nitrogen and have strong flux pinning effect,the stable levitation can be achieved without any complex control system out of superconducting materials.Bulk superconductors have great potential for various engineering applications,such as non-contact bearings,flywheels,momentum wheels,passive vibration free platforms and Maglev vehicles.Our group did a lot of research on HTS maglev vehicle system and successfully developed the first man-loading HTS maglev test vehicle on Dec.31,2000.In order to promote the application of the HTS maglev vehicle system in engineering,guidance force characteristics of a HTS exposed to time-vary magnetic field over a permanent magnetic guideway(PMG)were investigated in this dissertation,whose aim was to understand the interaction between the time-vary magnetic field and the guidance force decay of the HTS bulk and to provide corresponding experiment bases and theoretical direction so as to investigate the dynamical characteristics of the HTS maglev vehicle system.The magnetic inhomogeneity over the permanent magnet guideway used in the high-temperature superconducting maglev vehicle system was investigated.The surface magnetic field of the PMG was measured by a HTS maglev measuring system and simulated by a computer.From the experimental and the simulation results,it was found that the surface magnetic field was did non-uniform. Moreover,we investigated the influence of the magnetic inhomogeneity of the PMG on levitation force of the bulk superconductor in a quasi-static state.The guidance force is a characteristic parameter reflecting guidance characteristics of a HTS,also one of the key parameters for the design of HTS maglev vehicle.The influence of the time-vary external magnetic field perturbation on the guidance force of HTS bulk over the PMG was investigated by experimental methods in this dissertation.Firstly,the hysteresis curve and the ralaxation of the guidance force in the time-vary external magnetic field were studied.Based on the Bean critical model,we discussed the influence of the time-vary external magnetic field perturbation on the performance parameters, such as ac losses,trapped magnetic field,temperature,critical current density and so on,and explained the interaction between the time-vary external magnetic field and the decay of the guidance force.Secondly,we compared the guidance fore decay of the bulk exposed to the time-vary external magnetic field under the conditions of different field-cooled height or different work height.Finally,the effect of the time-vary external magnetic field on the guidance force of HTS bulk, which was covered with ice and was not cooled totally,was investigated.Based on the Bean critical model,a simulation method was adopted to calculate the guidance force decay of the bulk exposed to the time-vary magnetic field for the current HTS maglev system.Compared the experimental results with the simulation ones,it was found that the simulation method was feasibility and could qualitatively reveal the characteristics of guidance force of HTS bulk in this situation.Furthermore,some researches that were difficult or unable to be done systemically by experiments were carried out by this simulation method. The influence of relevant parameters,such as critical current density,thermal conductivity,thermal capacity,the dimension of a superconductor and so on,on the guidance force of the bulk exposed to time-vary magnetic field.With the help of the simulation method,we discussed the effect of the time-vary external magnetic field perturbation on the performance parameters,such as ac losses, trapped magnetic field,temperature,critical current density and so on.All the simulation results provide important scientific bases for the research and engineering practice of HTS Maglev vehicle.