Acoustic emission and normal components in an alternating current superconducting coil

The relationship between acoustic emission (AE) signals and normal components that appeared in a superconducting winding in an alternating current superconducting coil cooled in liquid helium was investigated around the time of quench initiation. The coil was impregnated with epoxy to prevent wire motion of the superconducting winding. The operating current of 50 Hz at the coil was ramped from zero to a level that caused quenching of the coil, where external heat energy was not applied to the superconducting winding. AE envelopes were used for the investigation because the envelopes provide higher time resolution and higher dynamic range than the method of using AE counting rates that were conventionally used in monitoring direct current superconducting magnets and coils. AE envelopes appeared immediately after the appearance of normal components in the superconducting winding before and after quench initiation. In the cases before quench initiation, normal components appeared several tens of milliseconds...