Acoustic Emission Measurements for Locating High-Voltage Breakdowns in Large Superconducting Magnet Systems
A disappointingly low withstand voltage capability was found during high-potential testing of an electrical system consisting of a large superconducting coil and the equipment connected as it was installed in the International Fusion Superconducting Magnet Test Facility (IFSMTF)— two superconducting buses, two vapor-cooled leads, and 120 sensor cables with ambient temperature and cryogenic vacuum feedthrough connectors. DC and transmission line techniques were unsuitable for finding the location of the dielectric breakdowns. An acoustic emission (AE) measurement system was developed with which to determine the location of breakdowns in large coils after installation in IFSMTF. Using triangulation with AE sensors, the system measures the difference in time-of-arrival of transient waveforms caused by the dc voltage discharge. The system was calibrated on a stainless steel surface representing the coil case, and its accuracy was found to be better than 5 cm. This paper describes both the acoustic emission measurements and the high-voltage testing system used concurrently. Also presented are the experimental results of a series of high-voltage tests that led to the determination of the exact location of one breakdown in the GD/C coil system of the Large Coil Task (LCT) and the results of the successful repair.
KeywordsAcoustic Emission Calibration Test Stainless Steel Surface Coil System Acoustic Emission Technique
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