Fluorescent thermal imaging method for investigating transient effects in high-temperature superconductor tapes and coils

Second generation high-temperature superconductors (HTS) are an enabling technology for high field magnets and also a promise for higher efficiency and reduced mass in electrical applications. A technological challenge that remains and limits in many cases the more widespread application of HTS is the stability and protection of these devices. It is known from low-temperature superconductors (LTS) that disturbances can cause a local loss of superconductivity and a quick reversal into a normal-conducting, dissipative state. This transition is referred to as quench. Due to large engineering current densities a quench can have severe consequences including the total destruction of the superconducting devices. While HTS are inherently more stable than LTS, their normal zone propagation velocity (NZPV) is also inherently orders of magnitude slower. This means that localized hot spots can still form, which may lead to damaging the superconducting apparatus. A deeper understanding of the quench is crucial for better understanding the stability of both HTS tapes and devices as well as for designing protection systems. Thermal imaging of this transient effect is expected to give a deeper insight into the mechanism of quench as well as thermal runaway in HTS applications. Such a measurement technique does not exist for the required cryogenic temperatures and time resolution, however it would prove most valuable as a novel measurement technique for the superconducting community. This work focused on developing a thermal imaging method, based on fluorescent microthermographic imaging, to be used for investigation of HTS tapes and coils in transient states. By up-scaling the fluorescent thermal imaging method to work with objects outside of the microscopic domain as well as implementing it at vastly higher speeds using a commercial high-speed camera, a new measurement method is presented for 2D temperature mapping of superconducting applications at cryogenic temperatures. The introduced high-speed fluorescent thermal imaging method uses europium tris[3-(trifluoromethylhydroxymethylene)-(+)- camphorate] (EuTFC) as the fluorescent dye with a precisely measured temperature dependent fluorescent light emission. The dye is applied as a coating on a surface to be measured using droplet deposition, followed by a 30 min heat treatment at 175° C for stabilization. After cooling the sample to 77K, UV LEDs were used for excitation, causing the coating to emit visible light as a function of its temperature. Over a temperature range of 77 K to 260 K the fluorescent light intensity decays linearly, while the colour of the emitted light remains unchanged. By normalizing both a thermal imaging measurement and the temperature calibration of the fluorescent dye around a known reference temperature (for example 77 K, the boiling temperature of liquid nitrogen at atmospheric pressure) in the post-processing, the temperatures can be calculated using the initial calibration curve within this range. The result is a sequence of images showing a surface temperature map of the imaged object. For demonstration the method was used for qualitative quench measurement in HTS coated conductor tapes as well as to determine their normal zone propagation velocity in liquid nitrogen. Short current pulses around the critical current were applied to several tapes with different architectures and showed effects previously also presented in the literature, however from a thermal perspective. The forming of "banding quench" was visible on several occasions with short current pulses 30% to 50% above the critical current of the tapes. This indicates that the NZPV may be more complex than a single zone propagating at a given speed and that the current amplitude drastically changes the behaviour of the quench and not only the NZPV itself. It was also shown that in tapes with a metallic stabilization layer currents significantly above the critical current transfer quasi instantaneously into the shunt layer and cause a uniform heating and film boiling over the surface. Current pulses around the critical current in both stabilizer-free and stabilized tapes caused a local, propagating zone, that quickly exceeded the temperature limit of the thermal imaging. The results of NZPV calculated from the thermal imaging were compared with electrical measurement data provided by Polytechnique Montr´eal, Canada for validation. While the two methods define quench from different physical approaches, the results indicated that at high current amplitudes the NZPV were similar. It is hypothesised that since the heating scales with the second power of the current, in tapes with a high critical current the electric field and heating develop closely simultaneously. In tapes with a lower critical current, however, the thermal imaging reported consistently slower propagation speeds due to the significantly lower heating power. The method was also implemented beyond single HTS tapes on two demonstrator pancake coils, wound with and without turn-to-turn insulation. In these experiments the coils' finished surface was coated with the fluorescent dye and were measured in a conduction-cooled assembly at 77 K. In the non-insulated coil the self-protecting behaviour was observed in an experiment where a sudden change in current paths was recorded together with a quick redistribution of the heating on the thermal imaging. In an overcurrent measurement at 110% of the critical current a thermal runaway was also captured. Over a duration of #approx#60 s a slow yet steady voltage rise indicated current transfer into the radial path, however no local hot spots or distinct heating were identified. A rapid temperature rise became apparent only at the point of the superconducting transition, where the coil's centre windings heated to approximately 100 K, indicating this as the weakest point. While the coil was manually protected to avoid damage, the heating was concentrated around the coil's centre and encompassed merely the innermost few windings. Propagation in the radial path was minimal. In the insulated coil no heating was detected during normal operation. To create a localized disturbance and determine the stability, a resistive surface heater was retrofitted on the upper side of the coil. A thermal runaway was detected in the coil after a 7 s long 3.2 W heater pulse, where the coil did not stabilize after the heater was turned off. The temperature rise was localized around the heater where it was spreading along the windings in contact with the heater, but not in the transverse direction. Temperatures reached up to 150 K at which point the coil was rapidly discharged. Both the insulated and non-insulated coils have retained their critical currents after the measurements and have shown no signs of damage.