Research On The Key Techniques Of The Flat-top Pulsed Magnetic Field With High Stability Driven By Battery Bank

The flat-top pulsed magnetic field（FTPMF）,which combines the advantages of high stability of steady-state magnetic field and high strength of pulsed magnetic field,is the significant development direction of pulsed magnetic field technology.With the further development of scientific experimental system such as specific heat measurement and Nuclear Magnetic Resonance（NMR）,the magnetic field environment requirements with magnetic field stability better than 100 ppm,ripple free,and flat top duration of more than 100 ms are proposed by scientist.At present,although many countries have adopted different technical means to achieve a variety of FTPMF with different performances,the FTPMFs generated have more or less defects in ripple,stability,or flat-top duration,which cannot fully meet the requirements of scientific experiments.In order to realize the regulation of the long flat-topped pulsed magnetic field with high stability and free ripple,and application of the FTPMF,this paper will be carried out in the following three aspects:（1）Study the suppression technology of pulse high-current flat-top ripple and real-time feedback control strategy to realize the regulation of FTPMF with high stability.（2）Study the occurrence mechanism and improvement measures of Direct-Current Current Transformer（DCCT）false balance,expand dynamic measurement range of DCCT,and provide technical support for the realization the ripple detection and analysis of FTPMF.（3）The high stability FTPMF based on the battery power supply is achieved.An experimental platform for the specific heat measurement under FTPMF is constructed,and exploratory research is carried out.To realize high-precision regulation of FTPMF,in this paper,the battery-type pulsed high magnetic field discharge system is taken as the research object.In order to solve the problem that the FTPMF stability is hard to be further improved caused by the switching ripple,a linear regulation bypass based on the active region of an insulated gate bipolar transistor（IGBT）is proposed.It is an unconventional experimental method to make IGBT work in the active region,so the relevant application data cannot be obtained.For this reason,the current control stability of the IGBT in the active area is thoroughly studied using the IGBT small signal model,and concludes that to avoid the output current oscillation of the IGBT module model used is that the gate resistance is greater than 1Ω.The causes of the Miller effect are analyzed in detail,and the influence of the Miller effect when the IGBT is working in the active region is verified through experiments,and the threshold conditions for avoiding the Miller effect are obtained.The parallel current sharing characteristics of the active area of the IGBT module are analyzed,and the linear drive circuit for the IGBT active area is designed.The step response test is performed on the drive circuit and the IGBT as a whole.The results show that the performance of drive circuit and the IGBT are good.Based on the working conditions in IGBT active area,the calculation formula for selecting the device parameters in the linear bypass is obtained,which lays the foundation for the construction of the FTPMF system.According to the requirement of scientic research,the construction goal of 40 T/100 ms is proposed,on account of the limit of IGBT power,the number of IGBTs needed in the linear bypass increases sharply when the magnetic field increases.To this end,this paper proposes a scheme to improve the efficiency of IGBTs based on the current injection method to reduce the number of parallel IGBTs in the linear bypass.Four interleaved Buck circuits are designed as power supply for current injection,and of which the output ripple is controlled within 1 V to avoid the influence of the injected power ripple on the characteristics of IGBT active region.On this basis,a battery-type FTPMF system of 40 T/100 ms was designed and verified by simulation.The results of simulation research show that this scheme can reduce the number of IGBTs used from 35 to 8,which greatly saves the cost and reduces the difficulty of implementation,and makes pioneering research for the realization of 40 T high-performance FTPMF.In FTPMF ripple detection,current is a physical quantity directly controlled by the FTPMF closed-loop control system,therefore the measurement accuracy of the current sensor is one of the key factors to achieve high stability FTPMF.In order to realize the high accuracy detection and analysis of FTPMF and further improve its stability,this paper proposes a technical solution for high-precision measurement of flat-topped pulse current using a direct-current current transformer（DCCT）.The flat-topped pulsed high current has the characteristics of wide range and high dynamic range,so how to avoid the false balance of DCCT during the rapid rise of the current which makes DCCT impossible to measure becomes the application bottleneck.To this end,this paper uses a simplified three-fold line magnetization model,theoretically obtained that the peak excitation magnetic core and the core saturation magnetic potential are the determinants of the static linear range of the magnetic modulator.Through the simulation study of the magnetic modulator based on the JA hysteresis model,the static linear range estimation formula of the magnetic modulator is summarized,and the upper limit of the unbalanced current of the DCCT in normal operation state is obtained.At the same time,to avoid the occurrence of false balance caused by excessive unbalanced current when the measured current changes dynamically in a large range,a DCCT of feedforward desaturation was proposed in the paper,and the superiority of the scheme in steady-state performance and dynamic performance is analyzed theoretically.On this basis,a principle prototype of 30 k A was designed,and the improvement of dynamic response and the self-recovery of false balance were verified through experiments.The prototype was calibrated at the National Center for High Voltage Measurement and its measurement accuracy turn out to be better than 10 ppm,which proved the superiority and feasibility of the proposed scheme.Based on the FTPMF high-precision control method and ripple detection technology mentioned above,this paper carried out the design and implementation of FTPMF system.For the large inertia characteristics of FTPMF,the intermittent sampling PI control method is selected.Based on the small signal transfer function model of the FTPMF system,the robustness of the control system when the resistance of the magnet changes is analyzed,and the theoretical basis for the selection of control parameters is determined.It is proved that the selected PI control parameters are compatible with the change of the resistance of the magnet to ensure the control accuracy of FTPMF.On this basis,a 23 T high stability ripple-free FTPMF control system was built,and detailed performance tests were conducted.A FTPMF of 23.37 T/100 ms/64.2 ppm has been achieved,whose stability is better than the other existing FTPMF and the flat top duration can reach 100 ms,and the magnetic field adjustment resolution can reach 7×10^-4 T,which can realize the precise adjustment of the magnetic field strength.After that,the technical of specific heat measurement is researched initially.The specific heat measurement system composition and thermal pulse specific heat measurement principle are introduced in the paper,and the design of calorimeter and sample rod are introduced in detail.A data acquisition system is developed,and the specific heat measurement technology research under the FTPMF was established.The specific heat test experiment of material Ba₃Mn_1.9Cr_0.1O₈ under FTPMF is carried out.Comparing with related literature,it proves the feasibility of the specific heat measurement platform established,which enriches the types and research scope of scientific experimental platforms under the pulse field.Finally,all the works have done in the paper is summarized and the further research in the future is prospected.