Fluorescent thermal imaging of a non-insulated pancake coil wound from high temperature superconductor tape

Abstract

We have wound a 157-turn, non-insulated pancake coil with an outer diameter of 85 mm and we cooled it down to 77 K with a combination of conduction and gas cooling. Using high-speed fluorescent thermal imaging in combination with electrical measurements we have investigated the coil under load, including various ramping tests and over-current experiments. We have found found that the coil does not heat up measurably when being ramped to below its critical current. Two over-current experiments are presented, where in one case the coil recovered by itself and in another case a thermal runaway occurred. We have recorded heating in the bulk of the windings due to local defects, however the coil remained cryostable even during some over-critical conditions and heated only to about 82-85 K at certain positions. A thermal runaway was observed at the center, where the highest magnetic field and a resistive joint create a natural defect. The maximum temperature, ∼100 K, was reached only in the few innermost windings around the coil former.

Fluorescent thermal imaging of a quench in insulated and non-insulated REBCO-wound pancake coils following a heater pulse at 77 K

“…This experiment revealed a remarkable resemblance to a thermal runaway when the coil was operated in an overcurrent regime, as presented previously [10]. We believe this is due to the fact that no larger heating power than ∼3.2 W could be applied in the current measurement approach.…”

“…A finished image of the non-insulated coil is shown in figure 1. A more in-depth explanation of the experimental setup, coil winding method, cryostat and instrumentation of the non-insulated coil was presented in a previous publication [10]. The insulated coil was wound similar to the non-insulated coil, the only difference being a 50 μm thick Kapton sheet co-wound to serve as electrical insulation.…”

“…The value was determined at an overall coil terminal voltage, calculated using the 0.1 μV cm −1 and 1 μV cm −1 criteria, respectively, multiplied by the total conductor length. The coils were cooled to 77 K in a custom-made cryostat assembly, using conduction cooling on an aluminium surface in a 77 K nitrogen gas atmosphere [10]. The camera was positioned above the coil and provided a top-down look as indicated in figure 1.…”

“…Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. thermal behaviour of pancake coils and the exact development of a thermal runaway, fluorescent thermal imaging was implemented on two distinct pancake coils [9,10]. One of the coils was wound using the conventional insulated coil winding technique while the other coil was wound without turn-to-turn insulation [11].…”

Understanding quench in no-insulation (NI) REBCO magnets through experiments and simulations

“…However, simulation work by Markiewicz et al [21] shows the variation of temperature across pancake is very small compared to the rise in the temperature during quench. Experimental fluorescent thermal imaging by Gyuraki et al shows that hot spot is not concentrated around the local point defect, but warmer regions of relatively uniform temperature spanning a few turns exist near the local defect [35]. The maximum temperature was only about 22 K higher than the initial temperature of the coil.…”