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

Abstract

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> 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 “hot spot” 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 “persistent-mode” 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. “Hot-spot” 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. </para>