AE measurement of the LHD helical coils

The authors have measured AE signals from the helical coils of the LHD (Large Helical Device) system at NIFS (National Institute of Fusion Science, Gifu, Japan) in order to monitor the state of the superconducting coils. Four AE sensors are attached on the surface of the vessel containing the helical coils, and preamplifiers are installed close to the sensors in the area with a leakage magnetic field of about 0.02 T. The measuring system is remotely controlled by PC´s via a LAN system. The AE signals are recorded and analyzed. Their relation with the balance voltage signal of the coil has been investigated. The obtained experimental result shows that (1) the AE signals have been successfully recorded despite the leakage magnetic field on preamplifier, (2) the AE signal shows a good correlation with the balance voltages, (3) the AE signal is observed only when the excitation current is changing, (4) the pattern of the observed AE signal is stable for each excitation pattern.