Research On The Magnetic Field Analysis Method Of The Large-scale Facility For Nearly Zero Magnetic Field Environment

The nearly zero magnetic field environment means that the amplitude of the magnetic field is extremely low,to fulfill the requirements for some frontier scientific research and aerospace industry.This subject comes from the simulation of the weak magnetic field in the space,one task in the planed Large Research Infrastructure “Space Environment Simulation Research Infrastructure” in Harbin Institute of Technology.The subject is also one part of the magnetic environment system layout in China which includes the weak field,the middle field,and the strong field.The nearly zero magnetic field facility shields the external magnetic field and multiple disturbances in the environment.The nearly zero magnetic field is the prerequisite of the realization of high precise controllable weak magnetic field.The research object in this thesis is the large-scale zero magnetic field facility with the room-size inner space.The thesis focuses on the analysis method of magentic field property in zero magnetic field facility,in order to support the construction of the facility in international advanced level.The magnetic properties of the shielding material are the necessary input data for analyzing the shielding performance of the zero magnetic field facility.The DC and AC magnetic properties of permalloy,a typical shielding material,are tested according to the standard test method.By improving the present test system,the initial magnetization curve in lower DC magnetic field and hysteretic curves in lower frequency AC magnetic field are obtained.The static and frequency dependent Jiles-Atherton model for permalloy is built,and the parameters are confirmed by the experiments of the material samples.The shielding factor which is defined as the ratio of the magnetic field magnitude without and with shielding for the same location,is one of the most important characteristics of a magnetic shield.The mechanism of shielding in static and alternating magnetic fields are presented,leading to the analytic formulas for shielding factor.The shielding factors of more complicated shapes can be calculated by the finite-element method.However,the FEM method has a key meshing problem that the length of the shield is in the order of meters while the thickness is only a few millimeters.The improved FEM method use a surface boundary condition instead of a domain to describe the thin shielding layer,which is able to simulate the influence of edges and holes efficiently.Furthermore,the shielding wall are made up of permalloy strips and sheets,as a result of which,the airgaps between permalloy cause the reduction of the effective permeability.A method to calculate the equivalent permeability of the nonlinear discontinuous layered ferromagnetic medium is proposed and demonstrated by the experiments of ring samples.The residual static field is the amplitude of the field inside the room,which is another important specification of zero magnetic field facility.The residual static field is related to the shielding factor,but more dominated by the magnetization state of the shielding material.Therefore,the zero magnetic field facility has to be demagnetized.The calculation method for the dynamic process of demagnetization are built base on the Jiles-Atherton model.The method is able to calculate the field strength and the field density of 3d object during and after the demagnetization for the first time.The method is demonstrated by the experiment of a cylindrical sample and a snake sample with a complex bending magnetic path.Using the method,the influences of the demagnetization current on the residual field of material,the residual static field inside shields under some certain coils configurations are studied,presenting the effects of above demagnetization parameters.Combining the analysis of the shielding performance and demagnetization process,the thesis gives the quantitative explanation of why the demagnetization determines the residual static field.During the demagnetization,the materials reach to the equilibration when the combination result in the room of residual field from the material and the damped external field is a lowest state.A new shape of shield is proposed,to optimize the shielding factor and not reduce the residual field while the shield has big holes for demagnetization coils.Through the experiments in BMSR-2 in Physikalisch-Technische Bundesanstalt(PTB)and the magnetically shielded laboratory in Technische Universit?t München,the proposed calculation methods in this thesis are confirmed.In conclusion,the magnetic properties of permalloy are measured and the calculation method of the shielding factor based on the equivalent permeability is proposed,to calculate the shielding performance of the facility and also the coupling effect between shielding materials and the field from coils inside room accurately.The calculation method of the demagnetization or equilibration are proposed and demonstrated,to calculate the residual static field precisely.A new structure of the zero magnetic field facility is proposed to optimize the shielding factor without reducing the residual static field.The systematic analysis method of zero magnetic field facility are obtained,supporting the optimized design and the construction of the world's leading facility for zero magnetic field and weak magnetic field environment.