Optical Measuring Techniques In Extreme Environment And Their Applications On Superconductor Properties

Superconducting materials have a wide application foreground in fields of power,transportation and electronic devices due to its special physical properties such as zero-resistance,Meissner effect,and Josephson effect.Superconducting quantum interference devices(SQUID),fault current limiters and high field magnets manufactured by superconductor films and tapes based on Bi or Y have greatly advanced the scientific investigation and improved people's living and production.However,these applications are always in the complex environments of extreme low temperature,high current and strong magnetic field,deformations are usually induced in these superconducting materials because of thermal mismatch,exerted electromagnetic force and manufacturing.The previous studies have shown that deformations have effects on three fundamental parameters of superconductors,i.e.critical temperature,critical current and critical magnetic field.Take critical current as an example,in1976 researcher found tensile deformation can significantly lower the critical current density of 3Nb Sn,the following studies show that all kinds of deformations(tension,bending,torsion)can reduce the critical current density.Nevertheless most of the research focus on the relation between surface deformations(measured by strain gages)and critical current density,there are few works concern the generation and evolution of the damage during deformation and their effects on critical current density.The reason is that it is difficult to implement the material inner damage detecting techniques such as X-ray or CT-scanning methods in extreme environment.Thus,to find a new way e.g.magneto-optical technique to observe the generation and evolution of damage in superconductors during deformation will be a key to reveal the mechanics of the critical current reduction that caused by deformation,which are long-stading problems in past years.This thesis contains two parts,the first is about the full-field thermal and magnetic stress measurements in superconductor films,their relevant techniques and theory frames,the second is about implementation of a real-time and in situ innet damage observation technique in superconductor tapes during deformations.The achievements are as followed(1)First,the effects of cryogenic air medium on CGS curvature measurements and error factors are given theoretically.Second,a modifying factor is induced when CGS is applied in multiple media measurements and verified by comparison of experimental results in media of air,water and silicone soil.Lastly,in some scenarios,CGS measurements encounter sparse interferometric fringe image due to the large Yong's modulus of materials or low loads.This would cause a problem to fringe analysis and make a low-accuracy result,a fringe multiplication method which regards the phase as a parameter is presented.Comparing the common methods,this method not only provides arbitrary integral-multiple fringe image but also separates the DC from main components.Its reliability is also verified experimentally by an optical-elastic technique.(2)New models that contain the magnetic body force between the curvatures and film stresses are built,the relation and difference between the presented models and Stony theory framework are discussed in the case of axisymmetry and non-axisymmetry.The results show that new model cannot degrade into Stony model which is based on misfit strain.After that,an optical system that contains glass window(diameter is 50mm),refrigerator cooling(lowest temperature is 30K),temperature control(the accuracy is 0.1K)and magnetic field excitation(the accuracy is 0.1mT)is constructed.The full-field curvatures and stresses measurements of films are achieved during the process of cooling and magnetizing by applying the CGS system extendly in complex environments of cryogenic temperature and magnetic field.Besides,it is the first time that the temporal evolutions of full-field film stresses during the pulse of magnetic field are obtained with an improved method.These provide reference for assessing the reliability of superconductor films that work in pulsed magnetic field.(3)The optical focusing technique is developed to solve the problem caused by differential pressure in vacuumizing and material deformation during cooling,which is a part of work of building a device that used to study superconductor properties in low temperature,electric,magnetic field and stress multi-field environments.Meanwhile,a self-adaption method for intensity-magnetic field calibration is proposed to solve the problem caused by non-uniform illumination.After that,two kinds of experiments about YBCO coated conductors are implemented as followed: firstly,the influence of tensile strain to stability of the critical state is investigated.When the superconductor tapes are zero-field cooled,the peripheries of tapes form the critical state in an applied magnetic field.The flux starts to nucleate and move in the area of mixed state when the strain attains 0.6%,then penetrates into Meissner area with gradually increased strain untill the whole Meissner area destroyed.The experimental results show that the penetration depth versus strain follows the exponential law and the magnitude of penetration velocities are between 1 um/s and 1 mm/s.Secondly,the effect of tensile strain on superconductor tape with currents in self-field is also researched.When the superconductor tape is zero-field cooled,the constant currents of 100 A is applied,then the critical state of flux generates in self-field.It is found that the flux start to move and penetrate into Meissner area with attained strain of 0.62%,a little more increased strain would quickly turn the tapes into quench.