Session: 01-13-01: Congress-Wide Symposium on NDE & SHM: Computational Nondestructive Evaluation and Structural Health Monitoring Count

Paper Number: 91393

91393 - Acoustic Emission Measurement and Location Analysis of Acoustic Emission Source for Superconducting Coil Quench During Training 

Superconducting magnet coils are an essential that are component used in many fields, including heavy ion therapy in medicine, magnetic levitation railways in infrastructure, and silicon single-crystal pullers in semiconductor equipment manufacture. These coils provide an extremely intense magnetic field by running a large current through superconducting wire that has no electrical resistance in a cryogenic environment. Measures to prevent quenching are also essential for safe and reliable operation. Quench is an uncontrollable and irreversible phenomenon that occurs when part of a coil changes back to the normal state from the superconducting state. The large current causes enough Joule heating to rapidly raise the temperature at that point and the surroundings, and may cause the coils to burn out.

 During manufacturing of superconducting coil products, coils are tested using current above the specification to intentionally induce quenching before shipment. This protects the coils from quenching during operation, and is called training. Without training, quenching may occur at currents within the specifications. Training is repeated at higher and higher currents until finally quenching does not occur. However, convergence of quenches is unpredictable.

 Acoustic emission (AE) measurement has been applied to training to determine the mechanism of the quench phenomenon with the aim of minimizing quenches.

 One known cause of quenches is mechanical disturbances, such as movement of the conductors in the coil during operation. Interactions between the large current and the intense magnetic field causes strain or movement of the wires, and then cryogenic environment also causes thermal stress between the conductor and the epoxy. These two stresses may cause strain energy to be released at the stressed point.

 AE is a non-destructive testing method that can detect high-frequency elastic waves generated by mechanical events inside an object, and is able to localize the wave source by using multiple sensors. It offers the unique advantage that it can endure the cryogenic environment at 4K without disturbing the electrical insulation.

 A small ellipse-shaped coil was made to evaluate AE measurement. The coil had a long diameter of 222 mm, a short diameter of 162 mm, and a height of 110 mm. Three AE sensors with a resonance frequency of 70 kHz were installed on each side of the coil at equal spacing around the circumference. AE signals were acquired from 6 channels simultaneously at a sampling rate of 10 MHz, and were recorded in order of time of arrival. Localization of the AE source was calculated from time differences between sensors based on the wave velocity and the position of each sensor.

A transmission-reception test was performed in advance by using the AE sensor as a transmitter to calibrate and measure the velocity of the waves that propagate axially and circumferentially inside the coil when there is no current.

In the training experiment, the coil was cooled to approximately 4 K by mechanical refrigeration in a vacuum vessel. The current to the coil was controlled to produce a gradual change, and the current rise was occasionally paused for several minutes to observe the change in AE events. Each training cycle terminated when a quench occurred. After refrigeration, training sequences were repeated continuously. The entire training ended after over 10 quenches and all AE events that occurred in each training cycle were measured.

　High-amplitude AE waves were generated just before each quench. Location analysis results were plotted on an expansion plan of the coil. Most of the events appeared in the CT image projection at the points of coil winding crossover. The crossover parts are assumed to have high enough internal stress to cause concentration of strain energy.

At the first excitation, the AE event occurred in proportion to the excitation current. After several quench training cycles, the number of AE events declined and the amplitude decreased.

 These results indicate the possibility that AE measurement may be useful for determining the origination points of quenching caused by mechanical disturbances.

Presenting Author: Junko Hirokawa Toshiba Corporation

Presenting Author Biography: Junko Hirokawa is a research scientist of mechanical systems at Toshiba Corporation corporate research and development center. Her work focuses on non-destructive testing using acoustic emission technology. She received Master of Science in Engineering degree in 2002 from Keio University.

Authors:

Junko Hirokawa Toshiba Corporation
Osamu Nishimura Toshiba Corporation
Yousuke Hisakuni Toshiba Corporation
Akira Kano Toshiba Corporation
Hideaki Uehara Toshiba Corporation
Tomoko Monda Toshiba Corporation
Kenji Hirohata Toshiba Corporation
Toshinobu Ito Toshiba Energy Systems and Solutions Corporation
Shohei Takami Toshiba Energy Systems & Solutions Corporation
Tomofumi Orikasa Toshiba Energy Systems and Solutions Corporation
Kiyokazu Sato Toshiba Energy Systems & Solutions Corporation