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

Acoustic emission (AE) induced from an alternating current (ac) superconduc ting coil was studied during operation in liquid helium, The operating curr ent of the coil was increased from zero with a constant ramp rate at a comm ercial frequency of 50 Hz until quenching in the coil. The AE signals induc ed from the coil around the time of the quenching initiation were compared with the operating current, the coil voltage, and the phase difference betw een the current and the voltage in order to estimate the changes in those s ignals which resulted from the quenching process. The AE signals continued almost at very low levels before the quenching. At the moment of the quench ing initiation, the AE signals clearly and drastically rose approximately 1 0-15 ms earlier than the decrease in the operating current or the phase dif ference, The AE signals induced by the wire motion during 50-Hz ac operatio n were also compared with the operating current.