Application of DSPI to detect inhomogeneous heating on superconducting ceramics

Abstract

This paper presents the first application of digital speckle pattern interferometry (DSPI) to detect inhomogeneous heat generation on a superconducting ceramic at cryogenic temperatures. The light scattered by the object is recorded with a CCD camera at the same time as a smooth reference beam. Comparison of two non-simultaneous frames provides information about the out-of-plane deformation field. Spatial phase shifting is used in order to get a good quality fringe pattern. The technique has been applied as a non-destructive evaluation of the performance of ceramic high temperature superconducting materials. DSPI allows the determination of the point where a hot spot will be generated with heating levels that do not deteriorate the sample properties. An excellent agreement between DSPI hot spot location and the position of the melting point that appeared in a destructive experiment has been obtained.

Analysis of Quench Initiation in YBCO Coated Conductors Using Optical Interferometric Techniques

“…DSPI was able to predict the location of a hot spot without damaging the sample. This was confirmed by inducing a quench that caused the melting of the sample, in which the melt was always located at the position previously predicted by DSPI experiments [1], [2]. One of the main advantages of this technique is that it allows visualizing the origin of a hot spot, without damaging the sample, and, as a result, it is possible to perform additional microstructural studies to determine the origin of these inhomogeneities [2].…”

“…One limitation of DSPI experiments is that measurements were performed with the sample cooled in vapor [1]- [3] because when the sample was cooled in liquid nitrogen coherence between two images is lost due to the changes induced in the laser optical path due to the refraction index variations produced by liquid nitrogen movements. However, for some applications, as for instance the development of fault current limiters, it is important to analyze quench generation when samples are immersed in liquid nitrogen.…”

“…In DSPI measurements, the interference of the light coming from the object and that coming from a reference wave is recorded. A fringe pattern is obtained by comparing this interference in two different states; an initial non-deformed one and a later state, after some deformation has taken place [1]. The analysis of this fringe pattern provides information about the surface deformation between these two states.…”

“…Manuscript This technique was successfully applied to observe the quench transition of superconducting materials. In particular, it was used to visualize inhomogeneous heating in bulk Bi-2212 textured ceramics associated to hot spot generation [1], [2]. In these studies, DSPI detected deformations due to dilatation caused by the heat generated during the transition from the superconducting state to the normal state.…”

Superconductor ceramics behavior analyses during service by speckle metrology

“…This technique produces fringe patterns by comparing two different specklegrams and has some interesting properties such as, non-contact nature, digital recording, high sensitivity and full-field analysis. This technique has already been tested in a high temperature superconductor during service demonstrating the possibilities of the technique for hot spot detection [4], without changing the sample properties because only a small current is required. These studies are important because once the hot spot is located, microstructural analysis can be performed in order to correlate this position with the appearance of a given microstructural defect on the sample.…”