Structural Diagnostics of Superconducting Magnets Using Diffuse Field Ultrasound

Among various diagnostics, acoustic techniques are valuable for characterizing mechanical instabilities associated with lengthy training, premature quenching and other performance limitations of high-field accelerator magnets. While acoustic emissions from magnets were studied since early 80 s, techniques involving external acoustic excitation have so far been of limited use. As such techniques are already well-developed in materials science and geoscience, they offer a great potential for the superconducting magnet applications. In a typical magnet coil an acoustic pulse emitted by a coupled ultrasonic transducer would experience multiple scatterings from boundaries and interfaces along its propagation path, in resemblance to a diffusion process. The resulting diffuse ultrasonic field uniquely samples geometrical boundaries, scatterer locations and sound velocity distribution in the propagation medium. Variation of these properties can be tracked in real time and with high accuracy by monitoring shape distortions and temporal shift of the ultrasonic waveforms acquired by sensors coupled to the coil. We discuss key physical principles and capabilities of diffuse field ultrasound, and give examples of using it for detecting interface instabilities in Canted Cosine Theta (CCT) Nb3Sn superconducting accelerator magnets built by the US Magnet Development Program.