Time-frequency distribution of acoustic emission signals induced from a rotating NbTi superconducting winding

Experimental analyses using the time-frequency distribution of acoustic emission (AE) signals were studied in terms of quench propagation along a NbTi superconducting winding in a rotor cooled by liquid helium. Data acquisition of the AE signals was carried out with a sampling frequency of 2 MHz to cover almost the full frequency range of the signals. Short-time spectrum analyses were performed for the AE signals when the voltage between taps at the superconducting winding rose after applying heat energy to the winding. A similar test with a heat pulse of lower energy, which did not cause quench propagation, were also carried out. The spectra in this case are equivalent to the background noise of the AE signals in the quench case, and so the AE components resulting from the quench propagation can be estimated.