Experimental Researche On Levitation Characteristics Of High Temperature Superconductors In Different Applied Magnetic Fields

Owing to its unique performances of zero resistance, meissner effect and flux pinning, a high temperature superconductor (HTS) can levitate self-stably in applied magntic fields without energy supply. Magnetic field is an important implement to explore the physical phenomenon and mechanism. In the magnetic fields generated by permanent magnet (PM) and permanent magnetic guideway (PMG), many experimental and theoretical researches on static and dynamic levitation characteristics of high temperature superconductors (HTSs) have been carried out. Based on those research results, the main mechanical performance and dynamic response of the magnetic levitation system have been opened out to a certain extent. Under the premise of unclear superconducting mechanism, it is necessary to study the levitation characteristics of HTSs in different applied magnetic fields. These works will promote to more profoundly understand the superconducting mechanisms of HTSs and interaction mechanisms between HTSs and outer magnetic fields. At present, PM and PMG are usually used as the magnetic source (MS) in most traditional magnetic levitation systems. The levitation performances of HTSs in those systems are limited by the magnetic field intensity and shape of PM and PMG. In recent years, with the development of superconducting magnet (SM) techniques, the magnetic field intensity which is applied to scientific reaearches becomes larger and larger. The study of levitation characteristics of HTSs in high magnetic field generated by SM is becoming to be a new research orientation.This dissertation experimentally investigated the levitation characteristics of HTSs in different applied magnetic field which is produced by PM, PMG and SM. All these works were carried out on the single-bulk levitation force measurement system, the high temperature superconducting maglev vehicle system and the controllable conduction-cooling high magnetic field SM sysem respectively. Firstly, the levitation force and relaxation characteristics of a HTS in a PM magnetic field were studied under different original cooling conditions, different work heights (WHs) and field-cooling heights (FCHs). The levitation forces of HTSs with different arrangements over a PMG were also researched. Then, a magnetized high temperature superconductor (MHTS) was applied to the magnetic levitation system. The levitation force and levitation force relaxation of the MHTS in a PM magnetic field were also presented in this thesis. The influences of demagnetization rate and magnetization intensity on a field-cooling (FC) MHTS were mainly researched. Finally, extreme condition of the high magnetic field was used to investigate the levitation characteristics of a HTS. The levitation forces of a HTS in high and low gradient varying magnetic fields generated by SM were studied. Moreover, the levitation force and relaxation characteristics of a HTS in intensity varying magnetic fields of SM were experimentally investigated in more detail in this dissertation. Some new physical phenomena and charactiersitcs were obtained which can not be observed in traditional PM magnetic fields in order to provide some experimental foundations for more wide applications of high temperature superonducting magnetic levitation systems in the future.From the above experiments, it was found that the poles of FC superconductor and PM had great influence on the forces acted on the superconductor. In the same pole condition, the maximum levitation forces acted on the superconductor and the hysteresis both increased with decreasing the FCH. When the poles were contrary, the attractive forces had been acted on the superconductor. The forces at the WH decreased with decreasing the FCH, while the hysteresis increased at the same time. It was also discovered that the relaxation of the FC superconductor increased with decreasing the FCH. From the research about levitation forces of HTSs with three different arrangements in the magnetic field of PMG, it denoted that the factors of arrangement of HTSs, WH, cooling method, FCH and the magnetic field distribution of PMG would effected the levitation characteristics of the magnetic levitation system. We also found that demagnetization rate and magnetization intensity both had influence on levitation performances of MHTS. The maximum levitation forces of the MHTS decreased with increasing the demagnetization rates, however, the levitation forces of the MHTS almost obtained saturation when the exciting currents exceeded 30A. The relaxation of MHTS increased with increasing the exciting currents and decreased with increasing the WH. From the experiments carried out in the SM varying magnetic fields, it was obviously shown that the levitation forces of the HTS were more dependent on magnetic field gradient than magnetic field intensity. In the low magnetic field, the effect of motion times on levitation forces could be neglected. In addition, a series of novel levitation characteristics of the HTS were obtained in the intensity varying magnetic field of the SM. When the applied magnetic fields were not fully penetrated the HTS, the HTS exhibited quite different levitation characteristics in the process of magnetic field up and down. The levitation characteristics of the HTS displayed anti-symmetric properties along Y axis. The studies of relaxations in different intensity SM magnetic field, it was found that the method which added an extral magnetization procedure in pre-loading process would improve the relaxation characteristics of the THS effectively and the effective levitation forces could not be suppressed highly.