Measurement Method For Temperature/Strain Under Low-temperature And Multi-field Environment Based On Distributed Optical Fibers And Its Application In Superconducting Magnets

Due to the advantages of high current carrying,strong magnetic field,low energy consumption and compact structure,superconducting magnets have important applications in the field of high-tech electromagnetic equipment such as thermonuclear fusion power generation,high-speed maglev trains,high-energy particle accelerators and ultra-high-resolution nuclear magnetic resonance imagers,and will bring about extensive and profound technological changes.Superconducting magnets operate in extreme multi-field environments such as low temperatures,high currents and strong magnetic fields.Its electromagnetic properties will be affected by the mechanical deformation caused by the strong electromagnetic force,which is a typical forcethermal-electric-magnetic multi-physics coupling problem.The characterization and measurement of multi-field mechanical properties under extreme multi-field has become an important basic subject for the functional realization and safe operation of high-field superconducting magnets.At present,the mechanical measurement methods and technologies under extreme multi-field are still extremely scarce,and the existing traditional mechanical testing methods(such as resistance strain gages)are seriously limited due to the defects of performance degradation,difficult compensation and large electromagnetic interference under extreme conditions.Optical fiber has the advantages of small size,light weight,high sensitivity,anti-electromagnetic interference,and distributed measurement.It is expected to become an ideal measurement method in environments such as low temperature and strong electromagnetic interference.Focusing on a series of basic problems such as low-temperature calibration,strain transfer,and multi-field signal decoupling measurement of optical fibers in lowtemperature and multi-field environments,this paper studies the mechanical experimental method of Rayleigh scattering OFDR distributed optical fiber sensors in low-temperature and multi-field environments.And a series of important research progress has been made.The main research work and the basic conclusions obtained in this paper are as follows:(1)Obtaining distributed optical fiber sensing characteristics at low temperature is the premise and basis for accurate temperature/strain measurement of superconducting magnets.Through theoretical and experimental research,this paper carried out the calibration experiment of optical fiber strain and temperature sensitivity coefficient in the temperature range from room temperature(293K)to liquid nitrogen temperature region(77K),and obtained high-precision low-temperature sensing parameters.The experimental results show that the temperature sensitivity coefficient of the optical fiber has significant temperature dependence and nonlinear characteristics;the temperature dependence of the strain sensitivity coefficient is negligible,and can be extrapolated to the low temperature region below 77 K.Furthermore,based on the calibration results,a temperature compensation method for elastic strain and thermal strain measurement at low temperature and large temperature change is established,and its effectiveness has been experimentally verified.(2)OFDR sensing fibers are usually composed of glass fibers and polymer coatings.When used for magnet strain test,it needs to be supplemented by epoxy resin,paraffin,solder and other bonding or filling materials,so it involves the problem of deformation transmission of multi-layer structure.Considering that the mechanical and thermal properties of materials have significant temperature dependence under the condition of low temperature and large temperature change,we established a generalized "fiber-interlayer-matrix" strain transfer shear lag model to characterize the deformation relationship between the fiber and the matrix.By solving the boundary value problem of the strain transfer differential equation,the strain transfer characteristics of the optical fiber sensing structure at low temperature are obtained,and related experiments are carried out.The results show that reducing the thickness of the interlayer,increasing the elastic modulus of the interlayer and increasing the fiber buried length can effectively improve the strain transfer rate;the hardening effect of the interlayer at low temperature can significantly improve the strain transfer rate,so the applicability is better;load Configurations(including uniform,linear,and gradient)also significantly affect strain transfer effects.(3)During the testing process of cooling,quenching,and rewarming,a large temperature change and thermal stress will appear inside the superconducting magnet.Therefore,OFDR fibers face the problem of cross-sensitivity of stress and temperature.Inside the magnet,the narrow space and strong electromagnetic interference make traditional decoupling or compensation methods severely limited.To solve this problem,we propose a new method for decoupling measurement based on distributed optical fiber ring configuration and wiring.Through harmonic analysis of the mixed signal of the fiber ring,the real-time decoupling measurement of temperature and field strain field can be realized,and is also valid for non-uniform fields.The effectiveness of the established new method is verified by experiments with non-uniform mechanical and temperature combined loads on the thin-plate structure.This new measurement method can also realize high-precision decoupling measurement of large-area,high-gradient non-uniform strain and temperature fields through the array deployment of small-radius fiber rings,which has great potential application value.Its significant advantages are currently difficult to achieve with other measurement methods.(4)In view of the current situation of difficult HTS quench detection,based on the self-built OFDR distributed optical fiber sensing multi-field signal acquisition and quench detection system,we carried out the thermally triggered quench experiment of HTS tape and uninsulated HTS coil.Research.The real-time distributed temperature measurement is realized by the optical fiber attached to the surface of the hightemperature superconducting tape,and the spatiotemporal evolution results of the whole process of the superconducting tape from thermal triggering to quench propagation are obtained.The results clearly show the information of heat generation,accumulation and temperature distribution during the quench process.Compared with voltage and thermocouple measurement signals,optical fiber signals have significant advantages such as fast response,anti-interference,high sensitivity,and large-scale distributed measurement.Furthermore,by embedding the optical fiber inside the hightemperature superconducting non-insulated coil,the whole process monitoring of the coil overcurrent quench and thermally triggered quench is realized.The 2D full-field information and images of the temperature evolution inside the coil during the quench process are reconstructed based on the continuous distributed measurement signals.The whole process of transformation,expansion and extinction of the quench region in the coil is completely presented.The concept of a smart superconducting coil with an internal full-field temperature sensing capability is realized.(5)We have extended the developed low-temperature multi-field distributed optical fiber testing method to the testing of three new types of accelerator superconducting magnet prototypes,namely DCT,CCT and FECR,which are being developed by the Institute of Modern Physics of the Chinese Academy of Sciences.The real-time strain monitoring of the whole process of the complex superconducting magnet was successfully carried out under the extremely low temperature of 4.2K and the strong magnetic field.The 1D and 2D strain field characteristics of large-scale regions during the whole process of DCT quasi-static excitation and CCT pulsed excitation were obtained in the fiber.Especially in the multi-step prestress application process such as pressing and cooling of the FECR magnet,the real-time strain full-field information provided by the optical fiber greatly improves the monitoring and control level of the prestress in the entire complex assembly process.At present,this test method has been verified and applied in many tests of a variety of new accelerator superconducting magnets developed by the Chinese Academy of Sciences.Distributed optical fibers exhibit measurement advantages that are difficult to achieve with traditional test methods,and play a key role in guiding the structural design,fabrication,and testing of new magnets.At present,this method has been applied to the testing of more superconducting magnets for more than two months,and has become an important test method in the development of superconducting magnets.In this paper,the basic theory,basic experiment and application research of distributed optical fiber measurement method in low temperature and multi-field environment are systematically carried out,and the distributed optical fiber temperature/strain measurement method in low temperature multi-field environment is preliminarily established.And realized the quench detection and strain measurement of high temperature superconducting material and magnet structure.The relevant research results fully demonstrate the effectiveness and superiority of this method in continuous,long-term,real-time,and high-precision measurements in large spaces,and will provide an important means for the characterization and measurement of mechanical properties under extreme multi-field conditions.