Recent Progress of the Quench Detection in the EAST HTS Current Leads Using Distributed Optical Fibers

High-temperature superconducting (HTS) current leads (CL) made of Bi-2223 superconducting tapes in Experimental and Advanced Superconducting Tokamak (EAST) are connected with the superconducting magnets and power supplies. The quench detection systems consist of primary (resistive voltage-based) and backup (temperature-based) detection systems; due to the required fast response time, the quench detection system for the EAST HTSCL relies primarily on direct voltage measurement. The traditional temperature and voltage detection methods have two main defects. First, to prevent the temperature sensor from high-voltage breakdown, it has to be installed in the rear section of the insulators, leading to its sensitivity decreasing significantly. Second, because the average zone propagation velocity of HTS is around 1/100∼1/1000 of low-temperature superconducting (LTS) materials, the small resistive voltage during quench is very difficult to detect. Distributed optical fibers quench detection (DOFQD) based on Raleigh scattering (RS) can be a solution, which adopts the special communication glass fiber as the sensor. It can continuously monitor temperature changes (0.5 to 2 K) at intervals of 5 mm. 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 optical fiber (OF) sensor is very brittle in the transverse direction resulting in complicated installation. Recently, the OF sensor has been directly laid inside the insulation layer of the HTSCL and led out of the vessel successfully. Meanwhile, the Parsons discharge test, the thermal shock test, and the vacuum leakage rate test can come up with the technical requirements. The power- on test was shown that DOFQD could detect a quench ahead of voltage measurement when the HTSCL was soaked in the 77 K liquid nitrogen. The next plan is to apply this quench detection method in the EAST HTSCL.