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