Acoustic emission occurrence induced from a NbTi superconducting coil under alternating current operation

Abstract

Acoustic emission (AE) induced from an alternating current (ac) superconducting coil was studied during operation in liquid helium. The operating current of the coil was increased from zero with a constant ramp rate at a commercial frequency of 50 Hz until quenching in the coil. The AE signals induced from the coil around the time of the quenching initiation were compared with the operating current, the coil voltage, and the phase difference between the current and the voltage in order to estimate the changes in those signals which resulted from the quenching process. The AE signals continued almost at very low levels before the quenching. At the moment of the quenching initiation, the AE signals clearly and drastically rose approximately 10-15 ms earlier than the decrease in the operating current or the phase difference. The AE signals induced by the wire motion during 50-Hz ac operation were also compared with the operating current.

Fabrication of a high performance acoustic emission (AE) sensor to monitor and diagnose disturbances in HTS tapes and magnet systems

“…These studies demonstrated that the AE technique is a very useful diagnostic tool for superconductors and superconducting magnets . In the 1980s, mechanical events, such as conductor motion (permanent deformation) and epoxy fracture, could be detected using the AE technique [4][5][6][7][8][9][10][11][12][13][14][15][16] and it was demonstrated that the AE technique is useful for detecting the presence of transient heating, and hence the existence of hot spots within low temperature superconductor (LTS) magnets [17,18]. The AE technique is also suitable for detecting the presence of heated regions in a high temperature superconductor (HTS) magnet immersed in a bath of LN2 boiling at 77 K, where a temperature difference of only few Kelvin within the HTS magnet would be sufficient to generate AE signals when localized heat is applied [19][20][21].…”