Application Of High-accuracy Magneto-optical Technology In Magnetic Field Measurement Of Superconducting Thin Film

Superconducting materials have unique zero resistance effect,meissner effect and Josephson effect,and three interrelated critical parameters,namely critical temperature,critical magnetic field and critical current density,which have wide application market in both strong and weak current fields.Functional devices represented by superconducting thin films have important engineering application value and research prospect in microwave communication,micromagnetic measurement and quantum technology.These superconducting thin film devices usually operate in extreme environments such as extremely low temperature,strong magnetic field and high vacuum,and their electromagnetic characteristics are difficult to observe directly.The magneto-optical imaging technology based on Faraday effect for magnetic field measurement has the advantages of real-time,full-field,high resolution,wide temperature application range and non-destructive advantages.It has been widely used in the popular fields such as high-angle grain boundaries,current density distribution and magnetic flux movement.And a series of important research results have been achieved.Since there is no Faraday effect in the superconducting materials,magneto-optical measurements of the superconducting materials are made by placing the magneto-optical medium with Faraday magneto-optical effect(magneto-optical film)represented by Bi-YIG on the surface of the superconducting film to observe the magnetic field.It can be seen that the distance between the magneto-optical film and the superconducting layer directly determines the spatial resolution of the magnetic field measurement.The magneto-optical film with a reflective layer is used in magneto-optical measurement experiments now,which obviously reduces the accuracy of the experiment.In order to further improve the precision of magnetic field measurement,a method of using the magneto-optical film without reflective layer is proposed in this dissertation,and a method to eliminate the interference fringe caused by using magneto-optical film without reflective layer is given.In addition,since the accuracy of the magnetic field measurement in the magneto-optical experiment is determined by the characteristic parameter of the magneto-optical film,the Verdet constant,the relationship between the Verdet constant and temperature has not been considered in existing experiments.However,the research in this paper found that the change of the experimental temperature will affect the size of the Verdet constant,so the three-image calibration experiment with different temperatures gives the law of the influence of the temperature on the Verdet constant and eliminates the impact for final magnetic field measurement.Finally,due to the extreme service environment of superconducting materials,it is difficult for existing experiments to give in-situ electromagnetic characteristics and deformation measurement of superconducting materials.This paper proposes an organic combination of digital image correlation method and magneto-optical imaging technology to achieve in-situ measurement of magnetic field and deformation of superconducting materials.The main work of this paper is summarized as follows:First,this paper proposes to use a transparent magneto-optical film without a reflective layer to replace the traditional magneto-optical film,which shortens the distance between the magneto-optical layer and the surface of the measured object,and qualitatively improves the spatial resolution of magneto-optical imaging technology.At the same time,it was found that the method using this transparent magneto-optical film would bring some bright and dark stripes that affect the analysis of the experimental results,and the continuous wavelength shooting method is used to eliminate the fringes caused by interference during the test.Then,a new calibration method is proposed to solve the problem that the Verdet constant changes with temperature.First,some fixed experimental temperatures were selected,with a wide range of coverage,including both the cases above and below the critical temperature of the superconducting material.At each selected experimental temperature,the influence of temperature rise and fall on the magneto-optical film magnetic field calibration is given by the three-image calibration method,and a unified magnetic field-gray calibration method is obtained to eliminate the influence of temperature on the test results.Finally,the magneto-optical imaging technology without reflective layer is combined with digital image correlation technology to build an experimental device for in-situ measurement of the tensile strain and surface magnetic field of the superconducting tapes,and the YBCO superconducting tapes at different temperatures is observed under the conditions of low temperature stretching and applied magnetic field.