Quench Behavior of Yttrium Barium Copper Oxide Coated Conductors

Superconducting magnet systems are the enabling technology for several research fields, e.g., experimental high-energy physics and fusion. Advanced superconducting magnet systems are strongly needed to achieve ever-higher beam energy in particle accelerators. They are also extensively used in plasma confinement for fusion. The energy stored in a magnet converts to heat when the magnet is quenching, i.e., a state change from superconducting to normal. The temperature increase and the high turn-to-turn voltage developed in a quench may degrade or damage the magnet. Thus, one of the key issues for the successful operation of superconducting magnets is the quench detection and protection. This thesis discusses the self-field quench behavior of YBa 2Cu3O7--delta (YBCO) coated conductors, one of the promising high-Tc conductors for superconducting magnets. The YBCO samples are provided by American Superconductor Corporation (AMSC) and SuperPower Incorporated (SPI). Samples are cryocooled and tested in self-field. A heat pulse generated by a heater fixed atop the sample is used to initiate a normal zone. Consecutive voltage taps are soldered along the sample to monitor the voltage development during a quench. Temperature profile is measured by type E thermocouples fixed along the sample. Minimum quench energy (MQE) and normal zone propagation velocity (NZPV) are measured as a function of operation temperature and transport current. It is found that the minimum quench energy (MQE) is on the order of 1 J and increases as the operation temperature decreases. MQE also increases with decreasing transport current. The normal zone propagation velocity (NZPV) is on the order of 10 mm/s and increases when the operation temperature decreases. It also increases with increasing transport current. Thus, an intrinsic trade-off exists between higher MQE (better stability) and higher NZPV (better protection performance). Lower operating temperature increases both MQE and NZPV, indicating the necessity of operating a YBCO magnet at a temperature as low as possible for better performance. Non-equipotential quench behavior is experimentally identified. When there is no obvious electrical connection between the substrate and the stabilizer in a conductor, the voltages on the substrate side rise in unison along the sample while distinct propagation and delay between the voltages traces on the stabilizer side is observed. Quench behaviors are compared between AMSC samples of similar architectures but with different stabilizers. The samples are stabilized by (1) Cu on both top and bottom sides of the sample (Cu-Cu); (2) Cu on one side and stainless steel (SS) on the other side of the sample (Cu-SS); and (3) SS on both sides of the sample (SS-SS). Quench-induced Ic degradation is observed. Quench experiments to induce degradation are conducted on AMSC's samples with three different stabilizers. A Cu-SS sample is tested at 60 K with a transport current of 30%Ic. A SS-SS sample is tested at 75 K with It = 26%Ic. Both samples buckled but no degradation is found. When the I t is low, the temperature increase in the middle part of the sample is uniform and hence low spatial temperature gradient (dT/d x). At the same time, the temporal temperature gradient is low due to the low Joule heating rate (dT/dt ∼ 50 K/s). A Cu-Cu sample is tested at 30 K with It = 90%Ic which is featured by a high d T/dt ∼ 1800 K/s, i.e., a thermal shock. Ic of the two middle sections of the sample is degraded slightly for ∼ 3%. The threshold values for the degradation of this specific Cu-Cu sample test case are Tpeak = 460 K and d T/dt =...