Experimental Study On AC Loss Of Superconducting Complex Structure

Since its discovery in 1911,superconductivity has always been a frontier discipline in the fields of materials and physics.It has been widely used in the fields of high-energy physics,energy,medical treatment and information after more than100 years of research and development and it has become a high-tech frontier technology seized by developed countries in the world.China has also listed this field as "Made by China 2025",which is a major national demand.We know that superconducting materials have no loss when transmitting direct current lower than its critical current,which is the "zero resistance effect" of superconducting materials.However,in the application process of superconducting materials,it is inevitable that superconducting materials transmit time varying current or operate in an alternating magnetic field environment,the magnetic flux vortex in superconducting materials will move due to this alternating electromagnetic field,overcome the pinning force to do work,and then produce energy loss,which is the AC loss of superconducting materials.On the one hand,the AC loss increases the temperature of superconducting materials,reduces its superconducting performance,and brings huge potential safety hazards to superconducting devices.On the other hand,the temperature rise caused by AC loss requires more cooling investment,which will significantly increase the operation cost of superconducting devices.Therefore,the AC loss of superconducting materials has become one of the key parameters in the development and design of superconducting devices.The accurate measurement of AC loss has always been a difficult problem faced by superconducting electrical academia because the superconducting materials is in the environment of extremely low temperature and strong electromagnetic field.In particular,the existing research shows that the measurement results of the most widely used electric method for AC loss measurement of superconducting materials are significantly affected by phase difference,which has not been effectively solved.In addition,with the increasing demand for the research of high field magnet,a novel superconducting complex structure CORC cable has become the first choice for the winding of high field superconducting magnet because of its excellent mechanical properties,high current carrying capacity and very low anisotropy.Therefore,accurately measuring the AC loss of this CORC cable structure has become a key problem that must be solved before its large-scale application.This doctoral dissertation focuses on the key problems existing in the electric method of AC loss of superconducting materials and the basic problems of AC loss measurement of CORC cable structure.The main research results are as follows:Firstly,the superconducting material is equivalent to pure resistance and pure inductance in series,and a non-phase voltage difference extreme value measurement scheme based on compensation coil is proposed.The theoretical derivation shows that there is a critical compensation voltage,which can eliminate the induced voltage caused by inductance.The theoretical result was verified by superconducting samples,the experimental measurement shows that there is indeed a minimum voltage in the process of adjusting the compensation coil,which proves the reliability of the theoretical method.On this basis,a concept of "cycle resistance" independent of frequency is proposed,which can calculate the loss results at other frequencies only by one measurement in the process of AC loss test.It not only saves the test time,but also solves the problem that the measurement results are sensitive to phase difference in the existing methods.Secondly,a mechanical loading device is designed based on the advantage that the above AC loss measurement method is not affected by mechanical loading.On the one hand,the transverse compression of superconducting tape is realized,on the other hand,the AC loss of superconducting material caused by mechanical loading can be measured.With this device,the laws of critical current degradation and AC loss change of different specifications of YBCO superconducting tape and Bi2223/Ag tape under compressive load are tested.The results show that the AC loss begins to increase sharply at the same normalized current under the corresponding critical compressive stress value,but the normalized AC loss gradually decreases under the same normalized current,it shows a different law from that of superconducting tape under axial tensile load.Thirdly,for single-layer superconducting CORC cable,according to its structure,the superconducting tape and metal core are regarded as parallel circuit branches,the theoretical model of the mutual inductance between the superconducting strip and metal core and its AC loss is established,and the analytical expression is given.CORC cables with copper rod core and epoxy resin core covered with single-layer superconducting tape with the same parameters were wound respectively,and their AC losses at different frequencies and amplitudes of current were systematically measured.The results show that the AC loss of epoxy resin rod core CORC cable is consistent with that of a single tape,and the loss per unit length per cycle is frequency independent;The loss per unit length per cycle of copper rod core CORC cable decreases with the increase of frequency at the same current,but the value is greater than that of epoxy core CORC cable.In addition,the experimental results show that the measured voltage at the same position of copper rod and superconducting tape of copper core CORC cable is basically the same,which proves the correctness of establishing the theoretical model as a parallel circuit.Finally,based on the theoretical and experimental results,the mutual inductance of metal core and superconducting tape in copper core CORC cable is given,which lays a foundation for the subsequent AC loss analysis of this structure.Finally,the AC loss of a new type of CORC cable with thin-walled copper tube or stainless steel solenoid as the central support instead of copper rod is preliminarily measured,and the basic characteristics of AC loss of copper tube core CORC cable and stainless steel solenoid core CORC cable are obtained.It is found that the size and material of this thin-walled structure will have a significant impact on the AC loss of CORC cable.The specific performance is as follows: under the same current conditions,the AC loss of thin-walled copper tube core CORC cable is significantly higher than that of copper rod core CORC cable,and the AC loss of stainless steel solenoid core CORC cable is the lowest.Based on the established model of mutual inductance and AC loss of CORC cable,the mutual inductance between superconducting tape and copper tube with different wall thickness is given.