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 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.
Highlights
Coils wound from second generation high temperature superconductors (HTS) are being researched for use in high field magnets [1, 2], as well as in electrical motors and generators [3]
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
Our group has previously reported on using a high-speed fluorescent thermal imaging method, where with the help of a fluorescent, temperature-sensitive coating and a high-speed camera, the surface temperature of HTS tapes can be mapped during a quench [14]
Summary
Coils wound from second generation high temperature superconductors (HTS) are being researched for use in high field magnets [1, 2], as well as in electrical motors and generators [3]. Non-insulated (NI) coils [4], where the HTS tapes are wound without turn-to-turn insulation, are actively researched [5,6,7,8,9,10,11] as an alternative. The fluorescent thermal imaging’s obvious requirement is that a surface has to be optically visible. This puts significant constraints on both the coil’s and the cryostat’s design. Ideally the pancake coil should have a large innerto-outer diameter distance (winding thickness), in order to allow the observation of any meaningful thermal effect. To solve these problems, a simple conduction plus gas cooled experimental setup was constructed
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