Protection and Quench Detection of YBCO Coils Results With Small Test Coil Assemblies

This paper presents the results of an experimental and theoretical study of the stability and protection of small test coil assemblies wound with second-generation (2G) high-temperature superconducting (HTS) wires. The study, conducted as a part of the efforts to improve the design and applicability of YBCO wire technology to real-life power devices, such as magnets, focused on two specific issues: 1) internal voltage developed within the winding resulting from a spatially nonuniform normal zone in a coil that is effectively short-circuited across its terminals; and 2) detection of a localized ldquohot spotrdquo with acoustic emission (AE) signals to complement the usual resistive voltage technique. Each test coil assembly, a stack of four single YBCO pancakes electrically connected in series and shunted by a ldquopersistent-moderdquo switch, simulated a power-supply driven magnet a moment after a quench zone appeared within the winding. A heater was incorporated into each pancake coil to drive 25% (one pancake), 50%, 75%, or 100% (four pancakes) of the test coil assembly. Measured internal voltages arising from different quench sizes and distributions are compared with the results of the simulation. ldquoHot-spotrdquo induced AE signals are also presented on a single coil to determine if AE signals may be useful to facilitate the early detection of a normal zone in HTS windings.