Research Of Quench Detection In The HTS Current Leads Using Distributed Optical Fibers Based On Rayleigh Scattering

High Temperature Superconducting(HTS)current lead is connected with the superconducting magnets and power supply.Compared with pure metal current lead,it can reduce heat leakage by more than two-thirds.It is an indispensable and important component in large superconducting device.Using HTSCL can greatly reduce the operation cost of superconducting devices,but it also brings the risk of quench.The HTSCL quench detection systems in EAST device consist of a primary(resistive voltage based)and back-up(temperature based)detection systems,which meet the basic quench detection requirements,but it is unable to carry out quench early warning and positioning,and it is difficult to carry out quench analysis.Distributed Optical Fibers Quench Detection(DOFQD)based on Rayleigh Scattering(RS)can be a solution.Distributed Optical Fiber(DOF)adopts the special glass fiber as the sensor,can track temperature excursions with the sensitivities of 0.5-2k with the gap of 5mm.At the same time,the method has the advantages of high-voltage resistance and immunity from electromagnetic interference.But the biggest drawback of this method is that the Optic Fiber sensor is very brittle in transverse direction resulting in very difficult installation.Therefore,no large superconducting device in the world has used this method.In order to apply DOF to HTSCL quench detection,this paper focuses on the following key technologies:1.Process design of optical fiber sensor directly laid on current lead.The package is designed and manufactured to protect the optical fiber and remove the influence of external stress on the optical fiber signal.The optical fiber vacuum flange circuit is used to cross the two air pressure environments of vacuum and atmosphere.The optical fiber signal is led out from the dewar tank,and has passed the electrical and vacuum leakage rate test.The successful installation and signal extraction of optical fiber sensor is the key process to ensure the implementation of DOF.2.Design and construction of DOFQD system.The hardware system including laser,photodetector and data acquisition card is used to obtain the Rayleigh scattering frequency shift information,and the functions of data remote communication,interface display,judgment and alarm are realized based on Java language.3.Modeling analysis of HTSCL quench.Based on the finite element analysis method,the quench simulation of conduction cooling current lead and immersion cooling current lead are carried out respectively as the theoretical basis of subsequent experiments.In order to verify the function of DOF system,the following verification experiments are carried out in this paper:1.Minimum resolution experiment of temperature measurement using optical fiber:using vacuum refrigeration device to manufacture low-temperature environment,the functional relationship between spectral offset and temperature in the temperature range of 25K-285k is obtained.2.Quench positioning experiment:the optical fiber installed inside the insulating material of the current lead can monitor the temperature distribution of the current lead in the liquid nitrogen immersion environment,quickly capture and locate the local hot spots,with an error of ± 5mm.3.Experiment of quench early warning:soak the high-temperature current lead sample in liquid nitrogen,apply current,manufacture quench conditions,and synchronously monitor the changes of optical fiber,temperature and voltage signals.The experimental results show that DOFQD can quickly and accurately detect the temperature change on the current lead,and realize effective quench early warning and positioning.