Study On The Relationship Between Levitation Force Of High Temperature Superconducting Bulks And External Magnetic Field Characteristics

As the key material of the high temperature superconducting(HTS)maglev train,the levitation force produced by the high temperature superconducting maglev bulk is directly related to the load capacity and economy of the HTS maglev train.In the previous studies,the relationship between levitation force and the external magnetic field intensity and gadient were mainly experimentally studied above the permanent magnet guideway(PMG).However,because the magnetic field is relatively fixed in PMG,it is difficult to adjust the magnetic field distribution,and the linear relationship obtained by many research conclusions can only be applied to the specific experimental PMG.Secondly,the distribution and variation of the vertical magnetic field(B_z)and transverse magnetic field(B_r)in the rail magnetic field are complex,so it is hard to determine the precise influence of the B_z and B_r on the levitation force of the bulk.Moreover,due to the magnetic field strength limitation of the PMG,there is a lack of research on the levitation force characteristics of the bulk in the strong magnetic field environment.In this thesis,based on the existing superconducting magnet platform in the group,the relationship between the levitation force of YBCO and Gd BCO two kinds of bulk materials in different magnetic intensity and field variation condition and the external magnetic field characteristics were studies systematically by utilizing the properties of adjustable and strong magnetic field of superconducting magnets.Initially,the experiment measured the distribution of the vertical magnetic field characteristics inside the superconducting magnet,including the vertical magnetic field(B_z),the transverse magnetic field(B_r),and the vertical magnetic field gradient(d B_z/dz),which were generally considered as the main factors affecting the levitation force of the bulk in the previous studies.Then,the variation trend of the levitation force along the z-axis of the bulk in the magnetic field of the superconducting magnet 0～3 T under the cooling field condition was measured.By comparing the levitation force curve with the distribution curve of magnetic field characteristics,it was found that although the distribution curve and peak position of B_zand B_r in the superconducting magnet does not change with the change of magnetic field intensity,the peak point of the levitation force of bulks changes from the peak point of theΔB_z×B_r curve to the peak point of the B_r curve with the increase of the magnetic field intensity.And the levitation force curve of bulks changes from close to theΔB_z×B_r curve to close to the B_r curve with the increase of magnetic field intensity.In order to study the changing factors of the levitation force curve of the bulk,this thesis studied the relationship between the levitation force and the magnetic field changeΔB that affects the internal current of the bulk.Because the transverse magnetic field(B_r)inside the superconducting magnet is very small,ΔB_z can be approximately substituted forΔB.And the magnetic field change of the bulk can be changed by controlling its cooling height and movement distance of the bulk,so as to study the influence of magnetic field change on the levitation force of the bulk.The study found that the levitation force of the superconducting bulk has the saturation effect,namely with the increase of magnetic field variation,the levitation force gain amplitude decrease slowly and gradually tends to zero,and then even if the magnetic field change value continues to increase,the value of levitation force no longer increases.And it was also found that the magnetic variation corresponding levitation force saturation of YBCO bulks are all about 0.720 T.Subsequently,Gd BCO bulks were studied in this thesis,and it was found that the variation of Gd BCO bulks levitation force saturation magnetic field are significantly greater than that of YBCO bulks,and the levitation force of Gd BCO is significantly improved in a strong magnetic field environment,and the maximum levitation force measured in the experiment is 111.8%higher than that of YBCO bulks.In addition,the levitation relaxation of the bulk was also studied in this thesis.The results showed that the increase of the magnetic field variation and the increase of the magnetic field strength will aggravate the levitation relaxation loss of the bulk.The COMSOL Multiphysics simulation software is used to build a simulation model of long distance motion of bulk in the superconducting magnet.In this thesis,the simulation results are used to reproduce the above experimental results,and the ultimate condition simulation research is carried out to verify the applicability of the experimental conclusion in the ultimate condition.In order to study whether the levitation force and relaxation attenuation of YBCO and Gd BCO bulks are derived from the critical current density J_c value inside them,this thesis studied influence of critical current density J_c on the saturation magnetic field point and the relaxation of levitation force of bulk.The results showed that the larger J_c value of the bulk in the strong magnetic field environment is,the larger the magnetic field variation of the bulk to reach the saturation of levitation force is,and the smaller the relaxation attenuation of levitation force will be.