Research On Key Technology Of Bitter-like High-Temperature Superconducting Magnet Excited By Flux Pump

Superconducting magnets are widely used in the field of strong magnetic fields due to their low energy consumption,high stability,light weight and small size.The technology of fully superconducting high field magnets has made important progress in recent years,which has important application in the fields of large scientific instruments,national defense,material science and biomedicine.In the high magnetic field,high-temperature superconductors can achieve higher magnetic field capture function.Common high-temperature superconducting(HTS)materials for high magnetic fields above 25 T are Bi2212,Bi2223 and REBCO(RE represents rare metals).In a high magnetic field environment,the irreversible electric field of REBCO can reach 7 T at 77 K,so the comprehensive performance of REBCO superconductor is better than that of Bi2212 and Bi2223.However.superconducting tapes can be produced long enough by means of superconducting tape lapping,the welding technology of HTS tapes is far from the condition of closed-loop operation of superconducting magnets,which has become one of the technical bottlenecks of closed-loop operation.Bitter-like HTS magnet is a superconducting magnet stacked by REBCO superconducting plates.The magnet can realize the closed-loop operation of HTS magnet and avoid the problem of non-resistance welding of HTS tape.By controlling the flux pump excitation technology of the superconducting switch to excite the superconducting magnet,the Bitter-like HTS magnet can trapped the magnetic flux density by the principle of electromagnetic induction,which can realize the closedloop operation of the HTS magnet in the persistent current mode(PCM).Bitter-like HTS magnet is proposed in this dissertation and the structure design of the magnet is studied.The excitation mechanism of the magnet is introduced,which is controlled by flux pump with different superconducting switches.At the same time,the electromagnetic characteristics,critical current and AC loss of the magnet are studied.The electromagnetic thermal coupling algorithm is established to study the temperature distribution and the thermal stability of the magnet.In this dissertation,th e topology of switch-controlled flux pump to excite the two HTS sub-loops is proposed,which excitation mode and structure design of different types of switches are studied.According to the number of switches,they can be divided into single switch and double switch.According to the control principle,they can be divided into thermal switch and magnetic switch.At the same time,according to the H-formulation method combined with the power exponential model,the system analysis is carried out,and the feasibility of the simulation method is verified by experiments.The excitation principle of single and double two HTS sub-loops under different excitations is adopted,and the corresponding experiments are carried out.It is proved that the double-layer superimposed two HTS sub-loops can capture double the magnetic flux density compared with the two HTS sub-loops under the same conditions.The preparation method of REBCO plate with two holes is designed and discussed in this dissertation.The principle of switch-controlled flux pump excitation is analyzed by qualitative analysis and quantitative analysis using the proposed REBCO plate with two holes.The magnetic switch and thermal switch control flux pump are designed to excite the single REBCO plate with two holes,and the excitation principle of the quantitative analysis method is verified by experiment.At the same time,the magnetic density of the center of the hole is calculated according to the three-dimensional(3D)H-formulation method.The feasibility of the simulation method is verified by experiments,and the closed-loop operation of the REBCO plate with two holes is realized.Based on the finite element calculation method and the geometric characteristics of the REBCO superconducting tape,the electromagnetic characteristics of the REBCO plate with two holes are analyzed.The analysis of the last periodic magnetic field distribution and the current change on the superconducting tape for the two earliest cycles of excitation further prove the switch-controlled flux pump can realize closed-loop operation of the double-hole superconducting plate excitation.Bitter-like HTS magnet designed in this dissertation is superimposed by REBCO plate with two holes.According to the excitation principle of the flux pump controlled by the thermal switch and the magnetic switch to the Bitter-like HTS magnet,the experimental platform and the experimental process are designed.The T-A formulation method is used to compare the experimental and simulation results,and the accuracy of the T-A method is proved by experiments.At the same time,the electromagnetic characteristics of the magnet are analyzed by simulation.Based on the coupling model of electromagnetic thermal simulation,the thermal stability of the qubit-like high temperature superconducting magnet is calculated and analyzed.The temperature distribution during the operation of the qubit-like high temperature superconducting magnet is studied,which lays a foundation for the stable operation of the qubit-like high temperature superconducting magnet.According to the special structure of the slit superconducting loop,as an extension of the closed-loop operating magnet,an 8-shaped magnetic flux density amplifier is proposed,which can be used as a sensor in the weak current field.The principle of 8shaped magnetic density amplifier is experimentally verified by field cooling and zero field cooling.Four groups of transmission current and flux density calibration experiments were carried out to analyze the influence of the amplitude of the excitation source and four groups of magnetic density amplifiers with different sizes on the induced current.The electromagnetic characteristics of the magnetic density amplifier are calculated and analyzed by H-formulation method.The AC loss of the magnetic density amplifier is calculated by analytical method and finite element method.The thermal stability of the magnetic density amplifier is calculated.The influence of frequency on temperature is analyzed,which lays a foundation for the application of magnetic density amplifier in alternating field.