Pulse height analysis on the balance voltage and acoustic emission signals for the LHD superconducting coils

It has been observed in the superconducting helical coils of the Large Helical Device (LHD) that the balance voltage signals measured between the corresponding pairs of the coil blocks contain a number of spike signals during ramp-up and ramp-down processes of excitation. The spike signals might be generated by rapid changes of the self-inductances of the coil windings due to mechanical disturbances caused by large electromagnetic forces. Pulse height analysis (PHA) has been applied to analyze these signals in order to investigate the changes of mechanical properties of the coil windings as the excitation and cooling cycles proceed. In addition, acoustic emission (AE) sensors attached to the helical coil cans are also used to detect mechanical disturbances.