High-temperature superconductor (HTS) current leads are important components of the EAST and CFETR tokamaks, which are responsible for operating the high parametric current. HTS current leads are made of Bi-2223/Ag-Au alloy tapes, which have the characteristics of slow quench propagation speed and weak quench signal. Traditional thermometers are easily damaged by the high voltage from the current leads, and the terminal voltage signal cannot reflect the hot spot changes of current leads in real time. In this paper, a novel quench detection method based on optical frequency domain reflection technology is proposed. Temperature variations of HTS can be obtained in real time by demodulating the Rayleigh scattered spectrum from the distributed optical fiber attached to the surface of HTS stacks. This paper describes a quenching experiment for one pair of 1-kA small current leads. The external thermal disturbance is increased to explore the quench propagation of HTS current leads under the condition of a self-field, 77 to 80 K air and conduction combined cooling method. From the experimental results, the temperature distribution map of the whole HTS lead is obtained. Compared with the quench voltage, the spectral shift of the fiber appeared to be about 2 s ahead, and the hot spot position can be located with 1-cm accuracy.

Optical strain sensors (OFSs) based on optical frequency domain reflection (OFDR) are an optical sensor with great application value and the ability to achieve distributed detection. However, both the external ambient temperature and the strain contribute to the frequency shift of the Rayleigh scattering spectrum in the optical fiber during the measurement. This means that the problem of temperature compensation (TC) must be considered when detecting strain in the environment with temperature field, especially in high temperature environment. In this article, a method of double-wire-based single optical fiber was proposed to aim at this problem, which makes one single fiber have two functions of TC and strain sensing at the same time. The contact strain sensing experiment was carried out on the conventional standard single-mode SMF-128 polyimide-coated (PI) distributed optical fiber strain sensor (DOFSS) under high temperature conditions (0-300 °C). The temperature compensation for the strain measurement of optical fiber sensor based OFDR was effectively realized by performing data fusion operation on the measurement results of TC and strain sensing. And the relationship between the temperature change rate, the compensation error rate (ER) was obtained. In addition, the non-contact fiber stretching sensing experiment under ultra-high temperature (50-600 °C) conditions was carried out for the first time, which further verified the effectiveness of the method for the compensation of the fiber strain measurement error. Feasibility, especially provides a simple and effective method for the application of strain measurement technology in high temperature environment.

The real-time monitoring of strain processes that can simultaneously realize a wide range and high spatial resolution has always been a huge challenge and cannot be satisfied by traditional electrical and non-contact measurement methods. In this article, the OFDR collocated with weak reflection FBG sensor (WRFBGs) was proposed to actively carry out multiple experiments on the non-standard reinforcement samples with complex strain processes in order to overcome this challenge. According to the result analysis, the complex strain variation law of the surface of the sample and the large span strain of adjacent locations during the whole process of tensile can be obtained by this method. The yield point (YP) can be accurately captured and positioned with mm-level horizontal precision. At the same time, real-timemonitoring can be realized for the step process when the strain ofthe point increases to more than 25000με instantly. The methodcan also realize the strain detection capability with a responsespeed of 38 Hz and a 1.28 mm high spatial resolution withmonitoring the real-time high-density and large straindistribution.

As the basic application of fiber optic sensing technology, strain measurement accuracy as a key index needs to be further calibrated and analyzed. In this paper, accuracy calibration experiments and the related analyses of two fiber-optic sensing technologies, the fiber-optic grating (FBG) and optical frequency domain reflectometry (OFDR), are carried out using a standard beam of equal strength and a mature resistive strain gauge (ESG). The fiber-optic single-point strain data for loading and unloading changes of the beams of equal strength show good continuity and linearity, with good cyclic stability, and the error in the strain test data is less than 2% after repeated loading. At the same time, using finite element theory to analyze the data and using the measured data error within 5%, a good strain test curve linearity is achieved and R2 is better than 0.998. After repeated loading and unloading tests, it is verified that the fiber grating and the distributed optical fiber in the strain test have good stability in repeatability accuracy. The calibration experiments and data analysis in this paper further illustrate the three sensing technologies in determining the strain test accuracy and the advantages and disadvantages of the indicators, and the development of the fiber optic sensing technology application provides basic technical support.

Insulated high temperature superconducting (HTS) magnet has extremely high upper critical field and excellent current carrying capacity, which serves as preferred option for CFETR CS, the fu-sion device in the future. The paper makes use of distributed op-tical fiber sensing sensor (DOFS) to study propagation rule of hot-spot temperature of HTS insulated double-pancake coil un-der different steady operating current by imposing thermal dis-turbance to local sections. Select two pieces of flexural flexible op-tic fibers with spatial resolution of 1 cm and sampling frequency of 1 Hz to detect the temperature of the insulated double-pancake coil, and proceed with quench propagation experiment under the condition of self-field and air cooling. The results show that hot-spot temperature propagates along the axial direction of the coil, and passes through the turn-to-turn insulation for radial propa-gation. It is discovered that as compared with quench voltage signal, variation to spectral offset of optic fiber is approximately 10 s earlier than voltage signal, and is available for positioning of hot spot. It is also verified that DOFS can be used for quench de-tection of HTS insulated coil.