Quench Studies of YBCO Insulated and Noninsulated Pancake Coils

As a result of extremely high upper critical fields B_c2, high temperature superconductors (HTSs) have the potential to be used as high field insert coils in magnet systems where the background field is provided by low temperature superconductors (LTS). However, due to low quench propagation velocity in HTS as compared to LTS, the issue of developing a fast and reliable quench detection and protection scheme for such magnet systems remains a serious challenge. In order to provide a stable operation for HTS inserts, quench studies on YBCO coils were performed. Three single pancake coils were wound with 1.5 m of YBCO coated conductor provided by Superpower. They were equipped with quench initiation heaters and instrumented with type K thermocouples and voltage taps to monitor the development of the quench. Two of the coils were additionally insulated with a thin layer of polyimide film laid on top of YBCO CC. The heat propagation with no transport current was studied at 77 K and 4.2 K without an external magnetic field. Further tests included quench propagation at 77 K. The measurements involved voltages developed around the quenched region, hot spot temperature, and determining the threshold voltage to ensure the successful operation of the quench protection system.