Convective heat transfer model for determining quench recovery of high temperature superconducting YBCO in liquid nitrogen

Abstract

Stability of a superconducting magnet is critical for reliable operation of a device in which the magnet plays a role. With the advent of high temperature superconductors (HTS), liquid nitrogen may be used to cool HTS devices. Yttrium barium copper oxide (YBCO), an HTS with critical temperature of 93 K, is a promising HTS for transmission cables and electric power devices. However, before the coated YBCO conductor can be used, stability of the superconductor must well understood. One important component for the superconductor to be used in these power devices is highly conductive normal metal such as copper that electrically shunts the superconductor when it is driven to the normal state, intentionally or during a fault mode. In this thesis work, stability of coated YBCO conductor samples were studied both experimentally and analytically. Each test sample, 10-cm or 15-cm long and cooled directly by boiling liquid nitrogen, was investigated for its stability by means of an over- current pulse that exceeded the sample's nominal critical current at 77.3 K. Variables of the investigation include: 1) presence or absence of copper layer incorporated into the sample; 2) thickness of copper layer; 3) nominal operating current before and after the application of a current pulse; 4) pulse current amplitude, and 5) pulse current duration. Recorded signals were sample voltages, measured by two sets of voltage taps, and sample temperatures, measured by three thermocouples placed at the center and two ends. The experimental and analytical results both demonstrated that for coated YBCO conductor to operate stably under operating conditions expected in the real device, it requires copper lamination whose thickness nearly doubles the original conductor thickness.