Stability and Quench Protection of High-Temperature Superconductors

In the design and operation of a superconducting magnet, stability and protection are two key issues that determine the magnet's reliability and safe operation. Although the high-temperature superconductor (HTS) is considered much more stable than the low-temperature superconductor (LTS), it is susceptible to damage caused primarily by three events that can occur in large-scale "real" devices: 1) overheating; 2) high voltage; and, 3) overstressing. In this thesis, we have investigated the first two issues as well acoustic emission (AE) technique as a possible mean for an early detection of a quench. For most of the experimental work reported here, we used "pancake" coils wound with coated YBCO conductor, the HTS of choice by those currently developing HTS-based electric power devices, though, YBCO itself to date is still in the development phase. For protection against overheating, an HTS magnet assembled with pancake coils may be made self-protecting through speedy 2-D or even 3-D normal zone propagation (NZP) within its winding, aided by good thermally-diffusive turn-to-turn spacers. We have found experimentally that good thermal diffusivity alone, however, does not guarantee fast 2-D NZP: thermal contact resistance between winding layers plays a crucial role in NZP in the transverse direction. For high internal voltage, a small test "magnet" consisting of two pancake coils was studied to investigate the internal voltage distributions within the magnet when one of the pancakes was driven normal with a heater. Measured voltage distributions were compared with those of simulation. Finally, to complement standard resistive voltage technique, an acoustic emission (AE) technique was investigated for detection of a quench at an instance earlier than that possible with a resistive voltage technique. With improved understanding of these issues, we should be able to develop protection techniques that ensure reliable and safe operation of HTS devices.