Measurement Method And Application Investigations On The AC Loss In High Temperture Superconductors

The notable difference between superconductor and the traditional conductor is that the superconductor can transport direct current without joule heat,when it is in the superconducting state.Due to the excellent properties of superconductors,a great deal of researches have been studied.In recent years,researchers carried out many important research results in the field of preparation,physical mechanism and practical application of the new superconducting materials.In the case of a non-ideal type II superconductor with a practical prospect,there is no energy loss inside the superconductor when it carries a DC current,and not exceeding the critical current,and the superconductor is stable in operation.However,during the practical application of the superconductor,the change of the current-carrying or the environment electromagnetic field will cause energy loss,which is usually defined as the AC loss.The generation of AC losses can cause the local temperature raise of the superconductor with the decreasing of the pinning performance and critical current,which brings great security risk to the superconducting device.Such as the explosion of European Hadron Collider giant superconducting magnet in 2008,it is due to liquid helium evaporation caused by the local quench of superconducting materials,and it took 14 months to shut down and cost $60 million dollars.Therefore,the AC loss characteristics of superconducting materials have become one of the most important parameters for the safety design of superconducting devices,and the progresses on the testing methods of AC losses of superconducting materials have attracted many attentiones of scholars.In addition,for the high temperature superconducting materials of oxide ceramic whose critical temperatures are higher than the liquid nitrogen boiling point,it is inevitable that micro cracks occur in the process of preparation and application.Therefore,the AC loss characteristics of the cracked superconducting material are also one of the primary problems,and the difficulties lies in the complexity of the shielding current distribution at the crack tip,it is hard to leads to use the original method to test the energy loss in this region.In this thesis,based on the research of the key parameters and test errors of the local calorimetric measurement method,the AC loss characteristics and variation of internal current and resistance in the high temperature superconducting joints are obtained for the first time.The previous calorimetric measurement method was improved and the AC losses of the cracked superconducting samples were tested.Finally,the AC losses of these kinds of samples are also tested by the electrical measurement method.The comparison results show the reliability of the local calorimetric measurement method proposed in this paper.The main work done and its results are as follows:Firstly,the test systems of local calorimetric and electric test built by ourself are introduced in detail.Then the AC losses of the YBCO coated conductors are measured by these two test methods,and the experimental results were compared with Norris theoretical calculation results,verified the reliability of the two systems.The experimental factors of the local calorimetric measurement method were also studied in detail,and the optimum DC load speed and load interval required to test the critical current and calibration curve are obtained.The optimal heating time is 47 s by analyzing the experimental error of the method.Next,the transport AC losses in solder joint of the YBa2Cu3O7-xcoated conductor(YBCO CC)and original superconducting samples in different arrangement modes at liquid nitrogen temperature were measured by the local calorimetric measurement method.By comparing the AC losses of the superconducting joints with the original sample,it is found that the joint length has little effect on its loss,and the heat generation at the superconducting joint does not affect the stability of the whole operation.At the same time,the experimental test method and the theoretical model for testing the current distribution of the sample with joint are proposed.The theoretical calculationes are in agreement with the experimental results,which indicate the correctness of the theoretical model proposed in this paper.The original local calorimetric measurement method is modified by the study of the experimental principle,and the AC loss characteristics of the cracked superconductor are obtained.The experimental results show that with the increase of the crack length,the effect of the crack on the power loss of the original superconducting sample increases sharply,which seriously affects the stability of the superconducting device.Finally,The AC losses of the samples subjected to lateral compression and cracked superconducting samples were measured by electrical measurement method.The influence of transverse compressive stress on the AC loss of superconducting samples was clarified,and compared with the influence of axial tension on the AC loss.Then the AC losses of the superconductor with an artificial transverse crack are compared with the results of the improved local calorimetric measurement method.The results show that the AC loss obtained by these two methods is consistent with the variation of the loading current and the crack length,so the reliability of the improved local calorimetric measurement method is proved.