AE technique is expected to be an useful monitoring and diagnosing technique for superconducting magnet. Already, it has been proven that AE technique is a powerful tool to identify and understand causes of magnet quenches. In the paper, the developement of AE technology as it is applied to superconducting magnet is reviewed. Future directions of research are also discussed.
Origins of AE are investigated. It is shown that wire motions and other mechanical disturbances are dominant AE sources and that flux motion, except flux jumping, produces no descernable AE signals. Papers concerning AE sources are reviewed and the results are compared.
The monitoring superconducting magnet by its acoustic emission has been found effective during the operation of the magnet. We have observed various AE from the magnet. Sources of AE generation are roughly classified and ordered according to their strength of AE, i.e., fracture of conductor destruction, conductor movement, fracture of support material, magnetization of conductor and S/N transition. Here, acoustic emission during the magnetization of the conductor and the magnet are mainly explained by showing the experimental results and theoretical discussions. The AE due to magnetization has been found to be related to the magnetic flux movement by Lorentz force.
Application of acoustic emission (AE) measurement to the superconducting magnet is reviewed. Four kinds of applications are available at the moment: (a) Simultaneous measurement of AE and coil voltage, which is used to identify the quench origin. (b) Measurement of AE count rate during coil charging, appropriate to recognize the general trend of mechanical disturbance occurence in the magnet. (c) Spectrum analysis of an AE signal generated by a vibrator which is suitable for long range diagnosis of the magnet system. (d) Location of electrical breakdown of instrumentation wires. Each of the techniques is briefly reviewed and compared in this paper.
The objective of acoustic emission measurement at Japan Atomic Energy Research Institute is an establishment of a general diagnostic method for superconducting magnet systems. Monitoring of acoustic emission will be one of the methods to predict a serious failure of magnet systems in a vacuum vessel. For this purpose, several sensors were installed on the Japanese LCT coil and the Test Module coil (TMC). Some of acoustic activity was similar as seen in these coils. The correlation between voltage spikes and acoustic events is excellent during single coil charging mode, but poorer during out of plane force mode. The acoustic emission of the Poloidal Unit Pancake (PUP) was monitored in the first case of the pulse poloidal coil for fusion. The localization of electrical insulation damage with the acoustic emission technique is one of its most useful applications.
According to a pulse operation of SCM, there appears high voltage which makes difficult to distinguish the resistive voltage from inductive and fluctant potential. We have monitored 3MJ SCM by acoustic emission with applying 44 sensors on the magnet. 4 sensors were located symmetrically between each double pancake. Counts and amplitude of acoustic bursts and also the acoustic epicenter were observed. The monitoring of burst counts continuously during the operation has been found effective for getting reliable pulsed SCM, by knowing fatigue properties of the magnet and the preferable duty cycle of the magnet chargings.
Three superconducting magnets for hybrid magnets and a number of independent superconducting magnets have been installed in the High Field Laboratory for Superconducting Materials. Most type of these superconducting magnets are pancake wound coils. AE event counting rate of pancake wound coils were measured as a function of a coil current. The typical AE patterns of pancake wound coils were observed. The results indicated that the pancakes were moved along the coil axis. The difference of AE patterns between just before and after recompression of magnet were discussed. The usefulness of supervising pancake wound magnets with AE technique were also discussed.
A method to detect a quench of superconducting magnets by acoustic resonance is presented. This technique utilizes the change of propagation characteristics of acoustic wave which is injected by a piezoelectric element (driver) into a superconducting magnet. The injected acoustic wave travels through the magnet, and is received by another piezoelectric element (receiver) attached to the magnet. The amplitude and the phase of the receiver voltage are affected by induced stress due to a local temperature rise. The experiment shows that a local temperature rise of about 2-3K can be detected by this method.
Fundamental mechanisms of winding motions during charging superconducting test coils are discussed under coil simulation conditions. The winding motion is detected by AE techniques and plane strains of the thin layer test coils. The plane strains can be divided into the tensile strain which is along a circumferential direction of the coil and the compressive strain which is in the axial direction of it. Mechanical dissipation energy densities in both the directions are estimated and compared with AE energy. It is found that the compressive strain is an influential source on the AE signal and the coil instability.
Acoustic emission (AE) techniques have been applied to the development of GFRP dewar vessel. The monitoring systems were thought to be inevitable in developing GFRP dewar vessel in order to estimate the durability and safety of the dewar. Furthermore, systems are important to investigate the causes and to locate the troubles for the repairment. GFRP dewar made by adhesive bonding method has been developed and the durability has been monitored by means of AE method during the transfering the coolant and pressurise tests. It is found that the condition of coolant and vacuum leakage could be detected in terms of AE method.
Stress induced dissipative energies were measured at 4.2K for epoxy cracking in a copper epoxy bonding and an epoxy-impregnated superconducting coil with an AE sensor and a newly developed high-resolution energy transducer. The dissipative energies have been correlated to AE energy. The correlation indicated that it is possible to use AE energy to quantify these dissipative energies. The dissipated crack energies in the coil were determined to be -2mJ at the maximum. Difference in AE waveforms associated with cracking and friction was also observed.