Stability and quench behavior of YBa₂CU₃O _7-X coated conductor at 4.2 K, self-field

YBa₂CU₃O_7-X (YBCO) coated conductors (CCs) are now capable of carrying very high transport critical current density J _c over a broad range of magnetic field and temperature space, and as a result, they are receiving significant interest for a wide range of applications. While many of these applications take advantage of the high-temperature performance of YBCO CCs, because the YBCO CC is typically produced on a high-strength substrate and carries very high J_c at very high magnetic field, there is now growing interest in using YBCO CCs at 4.2 K to generate very high magnetic fields. The transition from high-field conductor to high-field superconducting magnet, however, requires that some challenging issues be addressed. One of the most important challenges remaining is to better understand the stability and quench behavior at 4.2 K, so that an effective quench protection system can be developed. Here, we report on measurements of the stability and quench behavior of short YBCO CC at 4.2 K by inducing a quench via a heat pulse from a heater mounted on the conductor surface. Through gradually increasing heater pulse amplitude, the transition from stable to unstable (i.e., recovery to quench) is observed through voltage and temperature measurements. Using these data, the minimum quench energy (MQE) and normal zone propagation velocity (NZPV) are determined. It is found that, for the same fraction of critical current (I/I_c), YBCO CCs have similar MQE and NZPV as Ag-alloy-clad Bi₂Sr₂CaCu ₂O_x wires and significantly higher MQE and lower NZPV than those of MgB₂ round wires of similar I_c (4.2 K). Furthermore, the voltage and temperature versus time data are correlated to better understand the quench onset behavior at 4.2 K. It is determined that a normal temperature gradient exists from the CC surface to the YBCO layer within the conductor, as well as a directly measured longitudinal temperature gradient. After the heater pulse has ended but while the transport current continues, the temperature gradient along the length becomes dominant. Nevertheless, voltage and temperature measurements remain problematic for quench detection in large magnets because of the slow longitudinal propagation velocity. Thus, new approaches to quench detection and/or protection of high-field YBCO magnets are needed.